COMP284 Scripting Languages Handouts

COMP284 Scripting Languages
Lecture 1: Overview of COMP284

Handouts

Ullrich Hustadt

Department of Computer Science
School of Electrical Engineering, Electronics, and Computer Science

University of Liverpool

Contents

1 Introduction
Motivation
Scripting languages

2 COMP284
Aims
Learning outcomes
Delivery
Assessment

COMP284 Scripting Languages Lecture 1 Slide L1 1

Introduction Motivation

How many programming languages should you learn?

1 Academic / Educational viewpoint:
Learn programming language concepts and
use programme languages to gain practical experience with them
imperative / object-oriented C, Java
functional Maude, OCaml, Haskell
logic/constraint Prolog, DLV
concurrent
then all (other) programming languages can be learned easily

2 An employers viewpoint:
Learn exactly those programming languages that the specific employer
needs

3 Compromise: Spend most time on 1 but leave some time for 2 to
allow more than one language from a class/paradigm to be learned

4 Problem: Which additional language do you cover?
; Look what is used/demanded by employers

COMP284 Scripting Languages Lecture 1 Slide L1 2

Introduction Motivation

Programming languages: Job ads

Software Developer
(Digital Repository)
University of Liverpool University Library

31,020 35,939 pa

To work as part of a small team based in the University Library, working closely

with the Universitys Computing Services Department on the institutional digital

repository, recommending and developing technical solutions, tools and

functionality to integrate the repository with other internal systems and to enable

research outputs to be shared externally. You will be an experienced Software

Developer with knowledge of LAMP technologies such as XML, XSLT, Perl and

Javascript. You will hold a degree in Computer Science or a related discipline

and/or have proven industrial experience of software development. The post is

full time, 35 hours per week.

Job Ref: A-576989

COMP284 Scripting Languages Lecture 1 Slide L1 3

Introduction Motivation

Programming languages: Job ads

Senior Software Development Manager
IMDb Video and Recommendations (Seattle, WA)

IMDb (a wholly-owned subsidiary of Amazon) is recruiting for a Senior Software

Development Manager to lead our What to Watch team. Youll be charged

with transforming IMDb from a reference site to a place where hundreds of

millions of people find and discover what to watch across a variety of video

providers, and seamlessly connect them with watching the movies and TV shows

best suited for them, wherever and whenever they may be.

Basic qualifications:
Bachelors degree in Computer Science, Computer Engineering or

related technical discipline
10+ years of experience as a software developer
5+ years experience managing people
Software development experience in OOP, Java, Perl, HTML, CSS,

JavaScript, Linux/UNIX, AJAX, MySQL

COMP284 Scripting Languages Lecture 1 Slide L1 4

Introduction Motivation

Programming languages: Job ads

Full-time Remote Worker
AOL Tech (Engadget, TUAW, Joystiq, Massively)

AOL Tech is looking for a great front-end developer who can help us take
Engadget and our other blogs to new levels.

The ideal candidate is highly proficient in JavaScript/jQuery, comfortable with

PHP / mySQL and experienced in web design, optimization and related

technologies for desktop and mobile. A solid understanding of mobile-first design

is a must.

Requirements:
High proficiency in JavaScript/jQuery
Familiar with spriting, lazy loading, and other general

performance-optimized techniques
Mac access for compatibility with current tools
HTML5/CSS3
Git, SSH

COMP284 Scripting Languages Lecture 1 Slide L1 5

Introduction Motivation

Websites and Programming Languages

Website Client-Side Server-Side Database

Google JavaScript C, C++, Go, Java,
Python, PHP

BigTable, MariaDB

Facebook JavaScript Hack, PHP, Python,
C++, Java, . . .

MariaDB, MySQL,
HBase Cassandra

YouTube Flash,
JavaScript

C, C++, Python, Java,
Go

BigTable, MariaDB

Yahoo JavaScript PHP MySQL, PostgreSQL

Amazon JavaScript Java, C++, Perl Oracle Database

Wikipedia JavaScript PHP, Hack MySQL, MariaDB

Twitter JavaScript C++, Java, Scala MySQL

Bing JavaScript ASP.NET MS SQL Server
Wikipedia Contributors: Programming languages used in most popular websites. Wikipedia, The Free Encyclopedia,
20 October 2017, at 11:28. http://en.wikipedia.org/wiki/Programming_languages_used_in_most_popular_websites
[accessed 23 October 2017]

COMP284 Scripting Languages Lecture 1 Slide L1 6

http://en.wikipedia.org/wiki/Programming_languages_used_in_most_popular_websites

Introduction Scripting languages

Scripting languages

Script

A user-readable and user-modifiable program that performs simple
operations and controls the operation of other programs

Scripting language

A programming language for writing scripts

Classical example: Shell scripts

#!/bin/sh

for file in *; do

wc -l $file

done

Print the number of lines and name for each file in the current directory

COMP284 Scripting Languages Lecture 1 Slide L1 7

Introduction Scripting languages

Scripting languages: Properties

Program code is present at run time and starting point of execution
compilation by programmer/user is not needed
compilation to bytecode or other low-level representations

may be performed behind the scenes as an optimisation

Presence of a suitable runtime environment is required for the execution
of scripts

includes an interpreter, or just-in-time compiler, or bytecode compiler plus
virtual machine

typically also includes a large collection of libraries

Executation of scripts is typically slower then the execution of code that
has been fully pre-compiled to machine code

#!/bin/sh

for file in *; do

wc -l $file

done

COMP284 Scripting Languages Lecture 1 Slide L1 8

Introduction Scripting languages

Scripting languages: Properties

Rich and easy to use interface to the underlying operating system,
in order to run other programs and communicate with them

rich input/output capabilities, including pipes, network sockets, file I/O,
and filesystem operations

Easy integration within larger systems
often used to glue other systems together
can be embedded into other applications

#!/bin/sh

for file in *; do

wc -l $file

done

COMP284 Scripting Languages Lecture 1 Slide L1 9

Introduction Scripting languages

Scripting languages: Properties

Variables, functions, and methods
typically do not require type declarations
(automatic conversion between types, e.g. strings and numbers)

Some built-in data structures
(more than in C, fewer than in Java)

Ability to generate, load, and interpret source code at run time
through an eval function

JavaScript:
var x = 2;

var y = 6;

var str = if (x > 0) { z = y / x } else { z = -1 };

console.log(z is , eval(str )); // Output: z is 3

x = 0;

console.log(z is , eval(str )); // Output: z is -1

COMP284 Scripting Languages Lecture 1 Slide L1 10

Introduction Scripting languages

Scripting languages: Properties

The evolution of a scripting language typically starts
with a limited set of language constructs for a specific purpose

Example: PHP started as set of simple functions
for tracking visits to a web page

The language then accumulates more and more language constructs
as it is used for a wider range of purposes

These additional language constructs may or may not fit well together
with the original core and/or may duplicate existing language constructs

During this evolution of the language, backward compatibility
may or may not be preserved

; Language design of scripting languages is often sub-optimal

COMP284 Scripting Languages Lecture 1 Slide L1 11

COMP284 Aims

Aims

1 To provide students with an understanding of
the nature and role of scripting languages

2 To introduce students to some popular scripting languages
and their applications

3 To enable students to write simple scripts using these languages
for a variety of applications

COMP284 Scripting Languages Lecture 1 Slide L1 12

COMP284 Learning outcomes

Learning Outcomes

At the end of the module students should be able to

1 compare and contrast languages such as JavaScript, Perl and PHP
with other programming languages

2 document and comment applications witten using a scripting language

3 rapidly develop simple applications, both computer and web-based,
using an appropriate scripting language

COMP284 Scripting Languages Lecture 1 Slide L1 13

COMP284 Delivery

Delivery of the module (1)

1 Lectures

Structure:
16 to 18 lectures

Schedule:
1 or 2 lectures per week spread over 9 weeks

See your personal timetable and e-mail announcements for details

Lecture notes and screencasts are available at
cgi.csc.liv.ac.uk.uk/~ullrich/COMP284/notes

Revise the lectures before the corresponding practical
Additional self study using the recommended textbooks

and the on-line material is essential

COMP284 Scripting Languages Lecture 1 Slide L1 14

cgi.csc.liv.ac.uk.uk/~ullrich/COMP284/notes

COMP284 Delivery

Delivery of the module (1)

2 Practicals

Structure:
7 practicals with worksheets (3 Perl, 2 PHP, 2 JavaScript)

; gain understanding via practice
; get answers to questions about the lecture material

Up to 3 additional practicals for questions about the assignments

Schedule:
1 practical per week for about 10 weeks

Practicals start in week 2

Practicals assume familiarity with Linux and departmental Linux systems
; To recap, use the worksheets available at

cgi.csc.liv.ac.uk.uk/~ullrich/COMP284/notes

Practicals assume familiarity with the related lecture material

COMP284 Scripting Languages Lecture 1 Slide L1 15

cgi.csc.liv.ac.uk.uk/~ullrich/COMP284/notes

COMP284 Delivery

How to learn a new programming language

Once you know how to program in one programming language,
additional programming languages are best learned by a process of
enquiry and practice guided by existing experience

Typically, the questions that guide you are
What kind of . . . are there?

Example: What kind of control structures are there?

What is the syntax for . . . ?
Example: What is the syntax for conditional statements?

What happens if . . . ?
Example: What happens if 1 is divided by 0?

How do I . . . ?
Example: How do I catch an exception?

Talk to other people who are currently trying to learn the same
language or have already learned it
; Ask what has surprised them most

COMP284 Scripting Languages Lecture 1 Slide L1 16

COMP284 Delivery

How to learn a new programming language

Once you know how to program in one programming language,
additional programming languages are best learned by a process of
enquiry and practice

The best kind of learning is learning by doing
; The questions posed on the previous slide are often best explored

by experimenting with small sample programs (toy programs)

Work on substantive programs
; You need to convince employers that you have worked on programs

more substantive than toy programs

; The assignments are pretend substantive programs
but in reality are too small

Employers value experience, in particular, the experience that you get
from overcoming challenges
; Assignments that are not challenging are of limited value

COMP284 Scripting Languages Lecture 1 Slide L1 17

COMP284 Delivery

Delivery of the module (3)

3 Office hours

Monday, 16:00 Ashton, Room 1.03

but always arrange a meeting by e-mail first
([email protected])

4 Announcements will be send by e-mail

You should check you university e-mail account at least every other day
Always use your university e-mail account

if you want to contact me or any other member of staff

COMP284 Scripting Languages Lecture 1 Slide L1 18

COMP284 Delivery

Recommended texts

Core reading
R. Nixon:

Learning PHP, MySQL, & JavaScript. Learning PHP. . . , 4th edition.
OReilly, 2009. OReilly, 2014.
Harold Cohen Library: 518.561.N73 or e-book

R. L. Schwartz, brian d foy, T. Phoenix:
Learning Perl. Learning Perl, 7th edition.
OReilly, 2011. OReilly, 2016.
Harold Cohen Library: 518.579.86.S39 or e-book

Further reading
M. David:

HTML5: designing rich Internet applications.
Focal Press, 2010.
Harold Cohen Library: 518.532.D24 or e-book

N. C. Zakas:
Professional JavaScript for Web Developers.
Wiley, 2009.
Harold Cohen Library: 518.59.Z21 or e-book

COMP284 Scripting Languages Lecture 1 Slide L1 19

COMP284 Assessment

Assessment

This is a coursework-based module
(no exam)

Three assessment tasks need to be completed throughout the semester:
Perl Deadline: Friday, 2 March, 17:00
PHP Deadline: Monday, 9 April, 12:00
JavaScript Deadline: Friday, 27 April, 17:00

Effort required: about 10 hours each
Available at: http://cgi.csc.liv.ac.uk/~ullrich/COMP284/

COMP284 Scripting Languages Lecture 1 Slide L1 20

http://cgi.csc.liv.ac.uk/~ullrich/COMP284/

COMP284 Assessment

Attendance and Performance

Average Average Average
Lecture Practical Module

Students Attendance Attendance Mark

2011-12 33 76.0% 70.0% 63.1
2012-13 58 82.0% 69.0% 64.5
2013-14 107 80.0% 60.0% 59.1
2014-15 119 71.3% 65.2% 54.5
2015-16 76 67.4% 46.8% 57.9
2016-17 114 43.8% 38.3% 53.0

From 2014-15, screencasts of the lectures were available to students
From 2015-16, the requirement to write a report on each program was dropped
Hypothesis 1:

Lecture Attendance > 75% and Practical Attendance > 65% Module Mark > 62
Hypothesis 2:

Screencasts Available Module Mark < 59COMP284 Scripting Languages Lecture 1 Slide L1 21COMP284 AssessmentAcademic Integrity Plagiarism occurs when a student misrepresents, as his/her own work,the work, written or otherwise, of any other person (including anotherstudent) or of any institution Collusion occurs where there is unauthorised co-operation between astudent and another person in the preparation and production of workwhich is presented as the students own Fabrication of data occurs when a student enhances, exaggerates, orfabricates data in order to conceal a lack of legitimate dataIf you are found to have plagiarised work, colluded with others, orfabricated data, then you may fail COMP284Serious offenders may be excluded from the UniversityDo not try to take a shortcutYou must do the work yourself!COMP284 Scripting Languages Lecture 1 Slide L1 22COMP284 AssessmentAcademic Integrity: Lab rules Do not ask another student to see any part of their code for aCOMP284 assignment; contravention of this leads to collusion Do not show or make available any part of your code relating for aCOMP284 assignment to any other student; contravention of this leads to collusion Do not share (links to) on-line material that might help with aCOMP284 assignment; contravention of this leads to collusion Lock your Lab PC when you leave it alone Where you use any material/code found on-line for a COMP284assignment, you must add comments to your code indicating its originby a proper academic reference; contravention of this is plagiarism; acknowledged code re-use may still result in a lower markCOMP284 Scripting Languages Lecture 1 Slide L1 23COMP284 Scripting LanguagesLecture 2: Perl (Part 1)HandoutsUllrich HustadtDepartment of Computer ScienceSchool of Electrical Engineering, Electronics, and Computer ScienceUniversity of LiverpoolContents3 Perl: OverviewHistoryApplicationsJava vs Perl4 ScalarsDefinitionIntegers and Floating-point numbersStringsBooleansComparisons5 Variables, Constants, and AssignmentsVariablesConstantsAssignmentsVariable interpolationCOMP284 Scripting Languages Lecture 2 Slide L2 1Perl: Overview HistoryPerl Originally developed by Larry Wall in 1987Perl 6 was released in December 2015 Borrows features from Cimperative language with variables, expressions, assignment statements,blocks of statements, control structures, and procedures / functions Lisplists, list operations, functions as first-class citizens AWK (pattern scanning and processing language)hashes / associative arrays, regular expressions sed (stream editor for filtering and transforming text)regular expressions and substitution s/// Shelluse of sigils to indicate type ($ scalar, @ array, % hash, & procedure) Object-oriented programming languagesclasses/packages, inheritance, methodsCOMP284 Scripting Languages Lecture 2 Slide L2 2Perl: Overview ApplicationsPerl: Uses and applications Main application areas of Perl text processing; easier and more powerful than sed or awk system administration; easier and more powerful than shell scripts Other application areas web programming code generation bioinformatics linguistics testing and quality assuranceCOMP284 Scripting Languages Lecture 2 Slide L2 3Perl: Overview ApplicationsPerl: Applications Applications written in Perl Movable Type web publishing platformhttp://www.movabletype.org/ Request Tracker issue tracking systemhttp://bestpractical.com/rt/ Slash database-driven web application serverhttp://sourceforge.net/projects/slashcode/COMP284 Scripting Languages Lecture 2 Slide L2 4http://www.movabletype.org/http://bestpractical.com/rt/http://sourceforge.net/projects/slashcode/Perl: Overview ApplicationsPerl: Applications Organisations using Perl Amazon online retailerhttp://www.amazon.co.uk BBC TV/Radio/Online entertainment and journalismhttp://www.bbc.co.uk Booking.com hotel bookingshttp://www.booking.com craigslist classified adshttp://www.craigslist.org IMDb movie databasehttp://www.imdb.com Monsanto agriculture/biotechhttp://www.monsanto.co.uk/ Slashdot technology related newshttp://slashdot.orgCOMP284 Scripting Languages Lecture 2 Slide L2 5http://www.amazon.co.ukhttp://www.bbc.co.ukhttp://www.booking.comhttp://www.craigslist.orghttp://www.imdb.comhttp://www.monsanto.co.uk/http://slashdot.orgPerl: Overview Java vs PerlJava versus Perl: Java1 /* Author: Clare Dixon2 * The HelloWorld class implements an application3 * that prints out “Hello World”.4 */5 public class HelloWorld {6 // —————METHODS ————-7 /* Main Method */8 public static void main(String [] args) {9 System.out.println(“Hello World”);10 }11 }Edit-compile-run cycle:1 Edit and save as HelloWorld.java2 Compile using javac HelloWorld.java3 Run using java HelloWorldCOMP284 Scripting Languages Lecture 2 Slide L2 6Perl: Overview Java vs PerlJava versus Perl: Perl1 #!/usr/bin/perl2 # Author: Ullrich Hustadt3 # The HelloWorld script implements an application4 # that prints out “Hello World”.56 print “Hello World
“;Edit-run cycle:1 Edit and save as HelloWorld2 Run using perl HelloWorld1 Edit and save as HelloWorld2 Make it executable chmod u+x HelloWorldThis only needs to be done once!3 Run using ./HelloWorldCOMP284 Scripting Languages Lecture 2 Slide L2 7Perl: Overview Java vs PerlPerl Perl borrows features from a wide range of programming languagesincluding imperative, object-oriented and functional languages Advantage: Programmers have a choice of programming styles Disadvantage: Programmers have a choice of programming styles Perl makes it easy to write completely incomprehensible code; Documenting and commenting Perl code is very importantCOMP284 Scripting Languages Lecture 2 Slide L2 8Perl: Overview Java vs PerlPerl Perl makes it easy to write completely incomprehensible code; Documenting and commenting Perl code is very important #!/usr/bin/perl # Authors: Schwartz et al. / Ullrich Hustadt # Text manipulation using regular expressions # # Retrieve the Perl documentation of function atan2 @lines = perldoc -u -f atan2 ; # Go through the lines of the documentation , turn all text # between angled brackets to uppercase and remove the # character in front of the opening angled bracket , then # print the result foreach (@lines) { s/w<([^ >]+) >/ U$1/g;
print;
}

In the example, there are more lines of comments than there are lines of code
COMP284 Scripting Languages Lecture 2 Slide L2 9

Perl: Overview Java vs Perl

Perl for Java programmers

In the following we will consider various constructs of the Perl
programming language

numbers, strings
variables, constants
assignments
control structures

These will often be explained with reference to Java
(like Java, unlike Java)

Note that Perl predates Java
; common constructs are almost always inherited by both languages

from the programming language C

COMP284 Scripting Languages Lecture 2 Slide L2 10

Perl: Overview Java vs Perl

Perl scripts

A Perl script consists of one or more statements and comments
; there is no need for a main function (or classes)
Statements end in a semi-colon
Whitespace before and in between statements is irrelevant

(This does not mean its irrelevant to someone reading your code)

Comments start with a hash symbol # and run to the end of the line
Comments should precede the code they are referring to

COMP284 Scripting Languages Lecture 2 Slide L2 11

Perl: Overview Java vs Perl

Perl scripts

Perl statements include
Assignments
Control structures

Every statement returns a value

Perl data types include
Scalars
Arrays / Lists
Hashes / Associative arrays

Perl expressions are constructed from values and variables using
operators and subroutines

Perl expressions can have side-effects
(evaluation of an expression can change the program state)

Every expression can be turned into a statement by adding a semi-colon

COMP284 Scripting Languages Lecture 2 Slide L2 12

Scalars Definition

Scalar data

A scalar is the simplest type of data in Perl
A scalar is either
an integer number
0 2012 -40 1_263_978

a floating-point number
1.25 256.0 -12e19 2.4e-10

a string
hello world hello world

Note:
There is no integer type, string type etc
There are no boolean constants (true / false)

COMP284 Scripting Languages Lecture 2 Slide L2 13

Scalars Integers and Floating-point numbers

Integers and Floating-point numbers

Perl provides a wide range of pre-defined mathematical functions
abs(number) absolute value
log(number) natural logarithm
random(number) random number between 0 and number
sqrt(number) square root

Additional functions are available via the POSIX module
ceil(number) round fractions up
floor(number) round fractions down
Note: There is no pre-defined round function

use POSIX;

print ceil (4.3); // prints 5

print floor (4.3); // prints 4

Remember: Floating-point arithmetic has its peculiarities
David Goldberg: What Every Computer Scientist Should Know About Floating-Point
Arithmetic. Computing Surveys 23(1):548.

http://perso.ens-lyon.fr/jean-michel.muller/goldberg.pdf

COMP284 Scripting Languages Lecture 2 Slide L2 14

http://perso.ens-lyon.fr/jean-michel.muller/goldberg.pdf

Scalars Integers and Floating-point numbers

Mathematical functions and Error handling

Perl, PHP and JavaScript differ in the way they deal with applications
of mathematical functions that do not produce a number

In Perl we have

log(0) produces an error message: Cant take log of 0
sqrt(-1) produces an error message: Cant take sqrt of -1
1/0 produces an error message: Illegal division by zero
0/0 produces an error message: Illegal division by zero

and execution of a script terminates when an error occurs

A possible way to perform error handling in Perl is as follows:
eval { run the code here . # try

1;

} or do { handle the error here using $@ # catch

};

The special variable $@ contains the Perl syntax or routine
error message from the last eval, do-FILE, or require command

COMP284 Scripting Languages Lecture 2 Slide L2 15

Scalars Strings

Strings

Perl distinguishes between

single-quoted strings and
double-quoted strings

single-quoted strings double-quoted strings
(taken literally) (interpreted/evaluated)

hello ; hello
dont ; dont
hello ; hello
backslash\ ; backslash
glass\table ; glasstable
glasstable ; glasstable

hello ; hello
dont ; dont
hello ; hello
backslash\ ; backslash
glass\table ; glasstable
glasstable ; glass able

In Java, single quotes are used for single characters and
double quotes for strings

COMP284 Scripting Languages Lecture 2 Slide L2 16

Scalars Strings

Double-quoted string backslash escapes

In a single-quoted string t is simply a string consisting of and t
In a double-quoted string t and other backslash escapes have the

following meaning

Construct Meaning
Logical Newline (actual character is platform dependent)
f Formfeed
r Return
t Tab

l Lower case next letter
L Lower case all following letters until E
u Upper case next letter
U Upper case all following letters until E
Q Quote non-word characters by adding a backslash until E
E End L, U, Q

COMP284 Scripting Languages Lecture 2 Slide L2 17

Scalars Strings

UTF-8

Perl supports UTF-8 character encodings which give you access to
non-ASCII characters

The pragma

use utf8;

allows you to use UTF-8 encoded characters in Perl scripts

The function call

binmode(STDIN , :encoding(UTF -8));

binmode(STDOUT , :encoding(UTF -8));

ensures that UFT-8 characters are read correctly from STDIN and
printed correctly to STDOUT

The Unicode::Normalize module enables correct decomposition
of strings containing UTF-8 encoded characters

use Unicode :: Normalize;

COMP284 Scripting Languages Lecture 2 Slide L2 18

Scalars Strings

UTF-8

Example:

binmode(STDOUT , :utf8);

print x{4f60}x{597d}x{4e16}x{754c}
; # chinese

print x{062d}x{fef0}
; # arabic

For further details see Schwartz et al., Appendix C

COMP284 Scripting Languages Lecture 2 Slide L2 19

Scalars Strings

String operators and automatic conversion

Two basic operations on strings are
string concatenation.

hello . world ; helloworld
hello . . world ; hello world
Uhello . LWORLD ; HELLO LWORLD

string repetition x:
hello x 3 ; hello hello hello

These operations can be combined
hello . world x 2 ; hello world world

Perl automatically converts between strings and numbers
2 . worlds ; 2 worlds
2 * 3 ; 6
2e-1 x 3 ; 0.20.20.2 (0.2 repeated three times)
hello* 3 ; 0

COMP284 Scripting Languages Lecture 2 Slide L2 20

Scalars Booleans

Booleans

Unlike Java, Perl does not have a boolean datatype
Instead the values

0 # zero

# empty string

0 # string consisting of zero

undef # undefined

() # empty list

all represent false while all other values represent true

COMP284 Scripting Languages Lecture 2 Slide L2 21

Scalars Booleans

Boolean operators

Perl offers the same short-circuit boolean operators as Java: &&, ||, !
Alternatively, and, or, not can be used

A B (A && B)

true true B (true)
true false B (false)
false true A (false)
false false A (false)

A B (A || B)

true true A (true)
true false A (true)
false true B (true)
false false B (false)

A (! A)

true (false)
false 1 (true)

Note that this means that && and || are not commutative, that is,
(A && B) is not the same as (B && A)

($denom != 0) && ($num / $denom > 10)

COMP284 Scripting Languages Lecture 2 Slide L2 22

Scalars Comparisons

Comparison operators

Perl distinguishes between numeric comparison and string comparison

Comparison Numeric String
Equal == eq
Not equal != ne
Less than < ltGreater than > gt
Less than or equal to <= leGreater than or equal to >= ge

Examples

35 == 35.0 # true

35 eq 35.0 # false

35 == 35.0 # true

35 < 35.0 # false35 lt 35.0 # trueABC eq “Uabc” # trueCOMP284 Scripting Languages Lecture 2 Slide L2 23Variables, Constants, and Assignments VariablesScalar variables Scalar variables start with $ followed by a Perl identifier A Perl identifier consists of letters, digits, and underscores,but cannot start with a digitPerl identifiers are case sensitive In Perl, a variable does not have to be declared before it can be used Scalar variables can store any scalar value(there are no integer variables versus string variables)COMP284 Scripting Languages Lecture 2 Slide L2 24Variables, Constants, and Assignments VariablesScalar variables A variable also does not have to be initialised before it can be used,although initialisation is a good idea Uninitialised variables have the special value undefHowever, undef actslike 0 for numeric variables andlike for string variablesif an uninitialised variable is used in an arithmetic or string operation To test whether a variable has value undef use the routine defined$s1 = “”;print $s1 eq undef: ,($s1 eq undef) ? TRUE:FALSE,”
“;print $s1 defined:,(defined($s1)) ? TRUE:FALSE,”
“;print $s2 defined:,(defined($s2)) ? TRUE:FALSE,”
“;$s1 eq undef: TRUE$s1 defined: TRUE$s2 defined: FALSECOMP284 Scripting Languages Lecture 2 Slide L2 25Variables, Constants, and Assignments VariablesSpecial Variables Perl has a lot of pre-defined variables that have a particular meaningand serve a particular purposeVariable Explanation$_ The default or implicit variable@_ Subroutine parameters$a, $b sort comparison routine variables$& the string matched by the last successful pattern match$/ input record separator, newline by default$ output record separator, undef by default$] version of Perl used For a full list seehttps://perldoc.perl.org/perlvar.html#SPECIAL-VARIABLESCOMP284 Scripting Languages Lecture 2 Slide L2 26https://perldoc.perl.org/perlvar.html#SPECIAL-VARIABLESVariables, Constants, and Assignments ConstantsConstantsPerl offers three different ways to declare constants Using the constant pragma:use constant PI => 3.14159265359;

(A pragma is a module which influences some aspect of the
compile time or run time behaviour of Perl)

Using the Readonly module:
use Readonly;

Readonly $PI => 3.14159265359;

Using the Const::Fast module:
use Const::Fast;

const $PI => 3.14159265359;

With our current Perl installation only constant works
; variable interpolation with constants does not work

COMP284 Scripting Languages Lecture 2 Slide L2 27

Variables, Constants, and Assignments Assignments

Assignments

Just like Java, Perl uses the equality sign = for assignments:

$student_id = 200846369;

$name = Jan Olsen;

$student_id = E00481370;

But no type declaration is required and the same variable can hold a
number at one point and a string at another

An assignment also returns a value,
namely (the final value of) the variable on the left
; enables us to use an assignment as an expressions

Example:

$b = ($a = 0) + 1;

# $a has value 0

# $b has value 1

COMP284 Scripting Languages Lecture 2 Slide L2 28

Variables, Constants, and Assignments Assignments

Binary assignments

There are also binary assignment operators that serve as shortcuts for
arithmetic and string operations

Binary assignment Equivalent assignment

$a += $b $a = $a + $b

$a -= $b $a = $a $b

$a *= $b $a = $a * $b

$a /= $b $a = $a / $b

$a %= $b $a = $a % $b

$a **= $b $a = $a ** $b

$a .= $b $a = $a . $b

Example:

# Convert Fahrenheit to Celsius:

# Subtract 32, then multiply by 5, then divide by 9

$temperature = 105; # temperature in Fahrenheit

($temperature -= 32) *= 5/9; # converted to Celsius

COMP284 Scripting Languages Lecture 2 Slide L2 29

Variables, Constants, and Assignments Assignments

Variable declarations

In Perl, variables can be declared using the my function
(Remember: This is not a requirement)

The pragma

use strict;

enforces that all variables must be declared before their use,
otherwise a compile time error is raised

Example:

use strict;

$studentsOnCOMP284 = 133;

Global symbol $studentOnCOMP284 requires explicit

package name at ./ script line 2.

Execution of ./ script aborted due to compilation errors.

use strict;

my $studentsOnCOMP281;

$studentsOnCOMP281 = 154;

my $studentsOnCOMP283 = 53;

COMP284 Scripting Languages Lecture 2 Slide L2 30

Variables, Constants, and Assignments Variable interpolation

Variable interpolation

Variable interpolation

Any scalar variable name in a double quoted string
is (automatically) replaced by its current value

Example:

$actor = Jeff Bridges;

$prize = Academy Award for Best Actor;

$year = 2010;

print 1: $actor won the $prize in $year
;

print 2: ,$actor , won the ,$prize , in ,$year ,
;

Output:

1: Jeff Bridges won the Academy Award for Best Actor in 2010

2: Jeff Bridges won the Academy Award for Best Actor in 2010

COMP284 Scripting Languages Lecture 2 Slide L2 31

Variables, Constants, and Assignments Variable interpolation

Revision

Read

Chapter 2: Scalar Data

of

R. L. Schwartz, brian d foy, T. Phoenix:
Learning Perl.
OReilly, 2011.
Harold Cohen Library: 518.579.86.S39 or e-book

COMP284 Scripting Languages Lecture 2 Slide L2 32

COMP284 Scripting Languages
Lecture 3: Perl (Part 2)

Handouts

Ullrich Hustadt

Department of Computer Science
School of Electrical Engineering, Electronics, and Computer Science

University of Liverpool

Contents

6 Control structures
Conditional statements
Switch statements
While- and Until-loops
For-loops

7 Lists and Arrays
Identifiers
List literals
Contexts
List and array functions
Foreach-loops

8 Hashes
Identifiers
Basic hash operations
Foreach

COMP284 Scripting Languages Lecture 3 Slide L3 1

Control structures Conditional statements

Control structures: conditional statements

The general format of conditional statements is very similar to that in
Java:

if (condition) {

statements

} elsif (condition) {

statements

} else {

statements

}

condition is an arbitrary expression
the elsif-clauses is optional and there can be more than one
the else-clause is optional but there can be at most one
in contrast to Java, the curly brackets must be present

even if statements consist only of a single statement

COMP284 Scripting Languages Lecture 3 Slide L3 2

Control structures Conditional statements

Control structures: conditional statements

Perl also offers two shorter conditional statements:

statement if (condition );

and

statement unless (condition );

In analogy to conditional statements
Perl offers conditional expressions:

condition ? if_true_expr : if_false_expr

Examples:

$descr = ($distance < 50) ? “near” : “far”;$size = ($width < 10) ? “small” :($width < 20) ? “medium” :”large”;COMP284 Scripting Languages Lecture 3 Slide L3 3Control structures Conditional statementsBlocks A sequence of statements in curly brackets is a block; an alternative definition of conditional statements isif (condition) blockelsif (condition) blockelse block Instatement if (condition );statement unless (condition );only a single statement is allowedbut do block counts as a single statement,so we can writedo block if (condition );do block unless (condition );COMP284 Scripting Languages Lecture 3 Slide L3 4Control structures Switch statementsControl structures: switch statement/expressionStarting with Perl 5.10 (released Dec 2007), the language includes aswitch statement and corresponding switch expressionBut these are considered experimental and need to be enabled explicitlyExample:use feature “switch”;given ($month) {when ([1,3,5,7,8,10,12]) { $days = 31 }when ([4 ,6 ,9,11]) { $days = 30 }when (2) { $days = 28 }default { $days = 0 }}Note: No explicit break statement is neededCOMP284 Scripting Languages Lecture 3 Slide L3 5Control structures While- and Until-loopsControl structures: while- and until-loops Perl offers while-loops and until-loopswhile (condition) {statements}until (condition) {statements} A proper until-loop where the loop is executed at least oncecan be obtained as followsdo { statements } until (condition );The same construct also works for if, unless and whileIn case there is only a single statement it is also possible to writestatement until (condition );Again this also works for if, unless and whileCOMP284 Scripting Languages Lecture 3 Slide L3 6Control structures For-loopsControl structures: for-loops for-loops in Perl take the formfor (initialisation; test; increment) {statements}Again, the curly brackets are required even if the body of the loop onlyconsists of a single statement Such a for-loop is equivalent to the following while-loop:initialisation;while (test) {statements;increment;}COMP284 Scripting Languages Lecture 3 Slide L3 7Lists and Arrays IdentifiersLists and Arrays A list is an ordered collection of scalars An array (array variable) is a variable that contains a list Array variables start with @ followed by a Perl identifier@identifierAn array variable denotes the entire list stored in that variable Perl uses$identifier[index]to denote the element stored at position index in @identifierThe first array element has index 0 Note that$identifier@identifierare two unrelated variables (but this situation should be avoided)COMP284 Scripting Languages Lecture 3 Slide L3 8Lists and Arrays List literalsList literals A list can be specified by a list literal, a comma-separated list of valuesenclosed by parentheses(1, 2, 3)(“adam”, “ben”, “colin”, “david”)(“adam”, 1, “ben”, 3)( )(1..10 , 15, 20..30)($start ..$end) List literals can be assigned to an array:@numbers = (1..10 , 15, 20..30);@names = (“adam”, “ben”, “colin”, “david”); Examples of more complex assignments, involving array concatenation:@numbers = (1..10 , undef , @numbers , ( ));@names = (@names ,@numbers ); Note that arrays do not have a pre-defined size/lengthCOMP284 Scripting Languages Lecture 3 Slide L3 9Lists and Arrays List literalsSize of an array There are three different ways to determine the size of an array$arraySize = scalar(@array);$arraySize = @array;$arraySize = $#array + 1; One can access all elements of an array using indicesin the range 0 to $#array But Perl also allows negative array indices:The expression $array[-index]is equivalent to $array[scalar(@array)-index]Example:$array[-1] is the same as $array[scalar(@array)-1]is the same as $array[$#array]that is the last element in @arrayCOMP284 Scripting Languages Lecture 3 Slide L3 10Lists and Arrays List literalsArray index out of bound Perl, in contrast to Java, allows you to access array indices that areout of bounds The value undef will be returned in such a case@array = (0, undef , 22, 33);print $array [1] = ,$array [1], , which ,(defined($array [1]) ? “IS NOT” : “IS”, “undef
“;print $array [5] = ,$array [5], , which ,(defined($array [5]) ? “IS NOT” : “IS”, “undef
“;$array [1] = , which IS undef$array [5] = , which IS undef The function exists can be used to determine whether an array indexis within bounds and has a value (including undef) associated with itprint $array [1] exists: ,exists($array [1]) ? “T”:”F”,”
“;print $array [5] exists: ,exists($array [5]) ? “T”:”F”,”
“;$array [1] exists: T$array [5] exists: FCOMP284 Scripting Languages Lecture 3 Slide L3 11Lists and Arrays ContextsScalar context versus list context Scalar contextwhen an expression is used as an argument of an operation that requiresa scalar value, the expression will be evaluated in a scalar contextExample:$arraySize = @array;; @array stores a list, but returns the number of elements of @arrayin a scalar context List contextwhen an expression is used as an argument of an operation that requiresa list value, the expression will be evaluated in a list contextExample:@sorted = sort 5;; A single scalar value is treated as a list with one element in alist contextCOMP284 Scripting Languages Lecture 3 Slide L3 12Lists and Arrays ContextsScalar context versus list contextExpressions behave differently in different contexts following these rules: Some operators and functions automatically return different values indifferent contexts$line = ; # return one line from IN

@lines = ; # return a list of all lines from IN

If an expression returns a scalar value in a list context, then by default
Perl will convert it into a list value with the returned scalar value being
the one and only element

If an expression returns a list value in a scalar context, then by default
Perl will convert it into a scalar value by take the last element of the
returned list value

COMP284 Scripting Languages Lecture 3 Slide L3 13

Lists and Arrays List and array functions

List functions

Function Semantics

grep(expr,list) in a list context, returns those elements of
list for which expr is true;
in a scalar context, returns the number of
times the expression was true

join(string,list) returns a string that contains the elements
of list connected through a separator
string

reverse(list) returns a list with elements in reverse order

sort(list) returns a list with elements sorted in
standard string comparison order

split(/regexpr/,string) returns a list obtained by splitting string
into substring using regexpr as separator

(list) x number returns a list composed of number copies
of list

COMP284 Scripting Languages Lecture 3 Slide L3 14

Lists and Arrays List and array functions

Array functions: push, pop, shift, unshift

Perl has no stack or queue data structures,
but has stack and queue functions for arrays:

Function Semantics

push(@array1,value)

push(@array1,list)

appends an element or an entire list to the
end of an array variable;
returns the number of elements in the
resulting array

pop(@array1) extracts the last element from an array
and returns it

shift(@array1) shift extracts the first element of an array
and returns it

unshift(@array1,value)

unshift(@array1,list)

insert an element or an entire list at the
start of an array variable;
returns the number of elements in the
resulting array

COMP284 Scripting Languages Lecture 3 Slide L3 15

Lists and Arrays List and array functions

Array operators: push, pop, shift, unshift

Example:

1 @planets = (earth);
2 unshift(@planets ,mercury,venus);
3 push(@planets ,mars,jupiter,saturn);
4 print Array@1: , join( ,@planets),
;
5 $last = pop(@planets );
6 print Array@2: , join( ,@planets),
;
7 $first = shift(@planets );
8 print Array@3: , join( ,@planets),
;
9 print @4: ,$first , ,$last ,
;

Output:

Array@1: mercury venus earth mars jupiter saturn

Array@2: mercury venus earth mars jupiter

Array@3: venus earth mars jupiter

@4: mercury saturn

COMP284 Scripting Languages Lecture 3 Slide L3 16

Lists and Arrays List and array functions

Array operators: delete

It is possible to delete array elements
delete($array[index])

removes the value stored at index in @array and returns it
only if index equals $#array will the arrays size shrink to the

position of the highest element that returns true for exists()

@array = (0, 11, 22, 33);

delete($array [2]);

print $array [2] exists: ,exists($array [2])?T:F,
;

print Size of $array: ,$#array+1,
;

delete($array [3]);

print $array [3] exists: ,exists($array [3])?T:F,
;

print Size of $array: ,$#array+1,
;

$array [2] exists: F

Size of $array: 4

$array [3] exists: F

Size of $array: 2

COMP284 Scripting Languages Lecture 3 Slide L3 17

Lists and Arrays Foreach-loops

Control structures: foreach-loop

Perl provides the foreach-construct to loop through the elements of a
list

foreach $variable (list) {

statements

}

where $variable, the foreach-variable, stores a different element of the
list in each iteration of the loop

Example:

@my_list = (1..5 ,20 ,11..18);

foreach $number (@my_list) {

$max = $number if (! defined($max) || $number > $max);

}

print(Maximum number in ,join(,,@my_list), is $max
);

Output:

Maximum number in 1,2,3,4,5,20,11,12,13,14,15,16,17,18 is 20

COMP284 Scripting Languages Lecture 3 Slide L3 18

Lists and Arrays Foreach-loops

Control structures: foreach-loop

Changing the value of the foreach-variable changes the element of the list
that it currently stores

Example:

@my_list = (1..5 ,20 ,11..18);

print Before: .join(, ,@my_list ).
;

foreach $number (@my_list) {

$number ++;

}

print After:.join(, ,@my_list ).
;

Output:

Before: 1, 2, 3, 4, 5, 20, 11, 12, 13, 14, 15, 16, 17, 18

After: 2, 3, 4, 5, 6, 21, 12, 13, 14, 15, 16, 17, 18, 19

Note: If no variable is specified, then the special variable $_ will be
used to store the array elements

COMP284 Scripting Languages Lecture 3 Slide L3 19

Lists and Arrays Foreach-loops

Control structures: foreach-loop

An alternative way to traverse an array is

foreach $index (0..$#array) {

statements

}

where an element of the array is then accessed using $array[$index] in
statements

Example:

@my_list = (1..5 ,20 ,11..18);

foreach $index (0..$# my_list) {

$max = $my_list[$index] if ($my_list[$index] > $max);

}

print(Maximum number in ,join(,,@my_list), is $max
);

COMP284 Scripting Languages Lecture 3 Slide L3 20

Lists and Arrays Foreach-loops

Control structures: foreach-loop

In analogy to while- and until-loops, there are the following variants of
foreach-loops:

do { statements } foreach list;

statement foreach list;

In the execution of the statements within the loop, the special variable
$_ will be set to consecutive elements of list

Instead of foreach we can also use for:

do { statements } for list;

statement for list;

Example:

print Hello $_!
foreach (Peter,Paul,

Mary);

COMP284 Scripting Languages Lecture 3 Slide L3 21

Lists and Arrays Foreach-loops

Control structures: last and next

The last command can be used in while-, until-, and foreach-loops
and discontinues the execution of a loop

while ($value = shift($data)) {

$written = print(FILE $value );

if (! $written) { last; }

}

# Execution of last takes us here

The next command stops the execution of the current iteration
of a loop and moves the execution to the next iteration

foreach $x ( -2..2) {

if ($x == 0) { next; }

printf(10 / %2d = %3d
,$x ,(10/ $x));

}

10 / -2 = -5

10 / -1 = -10

10 / 1 = 10

10 / 2 = 5

COMP284 Scripting Languages Lecture 3 Slide L3 22

Hashes Identifiers

Hashes

A hash is a data structure similar to an array but it associates scalars
with a string instead of a number

Alternatively, a hash can be seen as a partial function mapping strings
to scalars

Remember that Perl can auto-magically convert any scalar into a string

Hash variables start with a percent sign followed by a Perl identifier

%identifier

A hash variable denotes the entirety of the hash

Perl uses

$identifier{key}

where key is a string, to refer to the value associated with key

COMP284 Scripting Languages Lecture 3 Slide L3 23

Hashes Identifiers

Hashes

Note that

$identifier

%identifier

are two unrelated variables (but this situation should be avoided)

An easy way to print all key-value pairs of a hash %hash is the following

use Data:: Dumper;

$Data :: Dumper ::Terse = 1;

print Dumper %hash;

Note the use of %hash instead of %hash
(%hash is a reference to %hash)

Data::Dumper can produce string representations for
arbitrary Perl data structures

COMP284 Scripting Languages Lecture 3 Slide L3 24

Hashes Basic hash operations

Basic hash operations

Initialise a hash using a list of key-value pairs

%hash = (key1 , value1 , key2 , value2 , );

Initialise a hash using a list in big arrow notation

%hash = (key1 => value1 , key2 => value2 , );

Associate a single value with a key

$hash{key} = value;

Remember that undef is a scalar value

$hash{key} = undef;

extends a hash with another key but unknown value

COMP284 Scripting Languages Lecture 3 Slide L3 25

Hashes Basic hash operations

Basic hash operations

One can use the exists or defined function to check
whether a key exists in a hash:

if (exists $hash{key}) { }

Note that if $hash{key} eq undef, then exists $hash{key} is true

The delete function removes a given key and its corresponding value
from a hash:

delete($hash{key});

After executing delete($hash{key}), exists $hash{key} will be
false

The undef function removes the contents and memory allocated to
a hash:

undef %hash

COMP284 Scripting Languages Lecture 3 Slide L3 26

Hashes Basic hash operations

Basic hash operations

It is also possible to assign one hash to another

%hash1 = %hash2;

In contrast to C or Java this operation creates a copy of %hash2
that is then assigned to %hash1

Example:

%hash1 = (a => 1, b => 2);

%hash2 = %hash1;

$hash1{b} = 4;

print $hash1{b } = $hash1{b }
;

print $hash2{b } = $hash2{b }
;

Output:

$hash1{b} = 4

$hash2{b} = 2

COMP284 Scripting Languages Lecture 3 Slide L3 27

Hashes Foreach

The each, keys, and values functions

each %hash returns a 2-element list consisting of the key and
value for the next element of %hash, so that one can
iterate over it

values %hash returns a list consisting of all the values of %hash,
resets the internal iterator for %hash

keys %hash returns a list consisting of all keys of %hash,
resets the internal iterator for %hash

Examples:

while ( ($key ,$value) = each %hash ) {

statements

}

foreach $key (sort keys %hash) {

$value = $hash{$key};

}

COMP284 Scripting Languages Lecture 3 Slide L3 28

Hashes Foreach

Example: Two-dimensional hash as a database

1 use List::Util sum;
2 $name{200846369 } = Jan Olsen ;
3 $marks{200846369 }{COMP201 } = 61;
4 $marks{200846369 }{COMP207 } = 57;
5 $marks{200846369 }{COMP213 } = 43;
6 $marks{200846369 }{COMP219 } = 79;
7
8 $average = sum(values($marks{200846369 }))/
9 scalar(values($marks{200846369 });

10 print(avg: $average
);

Output:

avg: 60

COMP284 Scripting Languages Lecture 3 Slide L3 29

Hashes Foreach

Example: Frequency of words

1 # Establish the frequency of words in a string
2 $string = peter paul mary paul jim mary paul;
3
4 # Split the string into words and use a hash
5 # to accumulate the word count for each word
6 ++ $count{$_} foreach split (/s+/,$string );
7
8 # Print the frequency of each word found in the
9 # string

10 while ( ($key ,$value) = each %count ) {
11 print($key => $value; );
12 }

Output:

jim => 1; peter => 1; mary => 2; paul => 3

COMP284 Scripting Languages Lecture 3 Slide L3 30

Hashes Foreach

Revision

Read

Chapter 3: Lists and Arrays

Chapter 6: Hashes

of

R. L. Schwartz, brian d foy, T. Phoenix:
Learning Perl.
OReilly, 2011.
Harold Cohen Library: 518.579.86.S39 or e-book

COMP284 Scripting Languages Lecture 3 Slide L3 31

COMP284 Scripting Languages
Lecture 4: Perl (Part 3)

Handouts

Ullrich Hustadt

Department of Computer Science
School of Electrical Engineering, Electronics, and Computer Science

University of Liverpool

Contents

9 Regular expressions (1)
Introduction
Characters
Character classes
Quantifiers

COMP284 Scripting Languages Lecture 4 Slide L4 1

Regular expressions (1) Introduction

Regular expressions: Motivation

Suppose you are testing the performance of a new sorting algorithm by
measuring its runtime on randomly generated arrays of numbers
of a given length:

Generating an unsorted array with 10000 elements took 1.250 seconds

Sorting took 7.220 seconds

Generating an unsorted array with 10000 elements took 1.243 seconds

Sorting took 10.486 seconds

Generating an unsorted array with 10000 elements took 1.216 seconds

Sorting took 8.951 seconds

Your task is to write a program that determines the average runtime of
the sorting algorithm:

Average runtime for 10000 elements is 8.886 seconds

Solution: The regular expression /^Sorting took (d+.d+) seconds/
allows us to get the required information

; Regular expressions are useful for information extraction
COMP284 Scripting Languages Lecture 4 Slide L4 2

Regular expressions (1) Introduction

Regular expressions: Motivation

Suppose you have recently taken over responsibility for a companys
website. You note that their HTML files contain a large number of
URLs containing superfluous occurrences of .., e.g.

http://www.myorg.co.uk/info/refund/../vat.html

Your task is to write a program that replaces URLs like these with
equivalent ones without occurrences of ..:

http://www.myorg.co.uk/info/vat.html

while making sure that relative URLs like

../video/disk.html

are preserved

Solution: s!/[^/]+/..!!; removes a superfluous dot-segment

; Substitution of regular expressions is useful for text manipulation

COMP284 Scripting Languages Lecture 4 Slide L4 3

Regular expressions (1) Introduction

Regular expressions: Introductory example

Ahttps ?://[^/]+/.w./( cat|dog )/1

A is an assertion or anchor
h, t, p, s, :, /, c, a, t, d, o, g are characters
? and + are quantifiers
[^/] is a character class
. is a metacharacter and w is a special escape
(cat|dog) is alternation within a capture group
1 is a backreference to a capture group

COMP284 Scripting Languages Lecture 4 Slide L4 4

Regular expressions (1) Introduction

Pattern match operation

To match a regular expession regexpr against the special variable $_
simply use one of the expressions /regexpr/ or m/regexpr/

This is called a pattern match
$_ is the target string of the pattern match

In a scalar context a pattern match returns true (1) or false ()
depending on whether regexpr matches the target string

if (/ Ahttps ?://[^/]+/.w./( cat|dog )/1/) {

}

if (m/ Ahttps ?://[^/]+/.w./( cat|dog )/1/) {

}

COMP284 Scripting Languages Lecture 4 Slide L4 5

Regular expressions (1) Characters

Regular expressions: Characters

The simplest regular expression just consists of a sequence of

alphanumberic characters and
non-alphanumeric characters escaped by a backslash:
that matches exactly this sequence of characters occurring as a substring
in the target string

$_ = ababcbcdcde;

if (/cbc/) { print Match
} else { print No match
}

Output:

Match

$_ = ababcbcdcde;

if (/dbd/) { print Match
} else { print No match
}

Output:

No match

COMP284 Scripting Languages Lecture 4 Slide L4 6

Regular expressions (1) Characters

Regular expressions: Special variables

Often we do not just want to know whether a regular expession matches
a target string, but retrieve additional information

The special variable $-[0] can be used to retrieve the start position of
the match

Note that positions in strings are counted starting with 0

The special variable $+[0] can be used to retrieve the first position
after the match

The special variable $& returns the match itself

$_ = ababcbcdcde;

if (/cbc/) { print Match found at position $-[0]: $&
}

Output:

Match found at position 4: cbc

COMP284 Scripting Languages Lecture 4 Slide L4 7

Regular expressions (1) Characters

Regular expressions: Special escapes

There are various special escapes and metacharacters that match more
then one character:

. Matches any character except

w Matches a word character (alphanumeric
plus _, plus other connector punctuation
characters plus Unicode characters

W Matches a non-word character

s Match a whitespace character

S Match a non-whitespace character

d Match a decimal digit character

D Match a non-digit character

p{UnicodeProperty} Match UnicodeProperty characters

P{UnicodeProperty} Match non-UnicodeProperty characters

COMP284 Scripting Languages Lecture 4 Slide L4 8

Regular expressions (1) Characters

Regular expressions: Unicode properties

Each unicode character has one or more properties,
for example, which script it belongs it

p{UnicodeProperty} matches all characters that have a particular
property

P{UnicodeProperty} matches those that do not
Examples of unicode properties are

Arabic Arabic characters

ASCII ASCII characters

Currency_Symbol Currency symbols

Digit Digits in all scripts

Greek Greek characters

Han Chinese kanxi or Japanese kanji characters

Space Whitespace characters

See http://perldoc.perl.org/perluniprops.html for a complete list

COMP284 Scripting Languages Lecture 4 Slide L4 9

http://perldoc.perl.org/perluniprops.html

Regular expressions (1) Character classes

Regular expressions: Character class

A character class, a list of characters, special escapes, metacharacters
and unicode properties enclosed in square brackets, matches any single
character from within the class,
for example, [adt
-\09]

One may specify a range of characters with a hyphen -,
for example, [b-u]

A caret ^ at the start of a character class negates/complements it,
that is, it matches any single character that is not from within the class,
for example, [^01a-z]

$_ = ababcbcdcde;

if (/[bc][b-e][^ bcd ]/) {

print Match at positions $-[0] to ,$+[0]-1,: $&
};

Output:

Match at positions 8 to 10: cde

COMP284 Scripting Languages Lecture 4 Slide L4 10

Regular expressions (1) Quantifiers

Quantifiers

The constructs for regular expressions that we have so far are not
sufficient to match, for example, natural numbers of arbitrary size

Also, writing a regular expressions for, say, a nine digit number
would be tedious

This is made possible with the use of quantifiers

regexpr* Match regexpr 0 or more times

regexpr+ Match regexpr 1 or more times

regexpr? Match regexpr 1 or 0 times

regexpr{n} Match regexpr exactly n times

regexpr{n,} Match regexpr at least n times

regexpr{n,m} Match regexpr at least n but not more than m times

Quantifiers are greedy by default and match the longest leftmost sequence
of characters possible

COMP284 Scripting Languages Lecture 4 Slide L4 11

Regular expressions (1) Quantifiers

Quantifiers

regexpr* Match regexpr 0 or more times

regexpr+ Match regexpr 1 or more times

regexpr? Match regexpr 1 or 0 times

regexpr{n} Match regexpr exactly n times

regexpr{n,} Match regexpr at least n times

regexpr{n,m} Match regexpr at least n but not more than m times

Example:

$_ = Sorting took 10.486 seconds;

if (/d+.d+/) {

print Match at positions $-[0] to ,$+[0]-1,: $&
};

$_ = E00481370;

if (/[A-Z]0{2}(d+)/) {

print Match at positions $-[1] to ,$+[1]-1,: $1
};

Output:

Match at positions 13 to 18: 10.486

Match at positions 3 to 8: 481370

COMP284 Scripting Languages Lecture 4 Slide L4 12

Regular expressions (1) Quantifiers

Quantifiers

Example:

$_ = E00481370;

if (/d+/) {

print Match at positions $-[0] to ,$+[0]-1,: $&
};

Output:

Match at positions 1 to 8: 00481370

The regular expression d+ matches 1 or more digits
As the example illustrates, the regular expression d+
matches as early as possible
matches as many digits as possible

; quantifiers are greedy by default

COMP284 Scripting Languages Lecture 4 Slide L4 13

Regular expressions (1) Quantifiers

Revision

Read

Chapter 7: In the World of Regular Expressions

Chapter 8: Matching with Regular Expressions

of

R. L. Schwartz, brian d foy, T. Phoenix:
Learning Perl.
OReilly, 2011.

http://perldoc.perl.org/perlre.html
http://perldoc.perl.org/perlretut.html
http://www.perlfect.com/articles/regextutor.shtml

COMP284 Scripting Languages Lecture 4 Slide L4 14

http://perldoc.perl.org/perlre.html
http://perldoc.perl.org/perlretut.html
http://www.perlfect.com/articles/regextutor.shtml

COMP284 Scripting Languages
Lecture 5: Perl (Part 4)

Handouts

Ullrich Hustadt

Department of Computer Science
School of Electrical Engineering, Electronics, and Computer Science

University of Liverpool

Contents

10 Regular expressions (2)
Capture groups
Alternations
Anchors
Modifiers
Binding operator

COMP284 Scripting Languages Lecture 5 Slide L5 1

Regular expressions (2) Capture groups

Regular expressions: Capture groups and backreferences

We often encounter situations where we want to identify the repetition
of the same or similar text, for example, in HTML markup:

We might also not just be interested in the repeating text itself,
but the text between or outside the repetition

We can characterise each individual example above
using regular expressions:

.* </ strong >

.* </li >

but we cannot characterise both without losing fidelity, for example:

<w+>.*</w+>

does not capture the pairing of HTML tags

COMP284 Scripting Languages Lecture 5 Slide L5 2

Regular expressions (2) Capture groups

Regular expressions: Capture groups

The solution are capture groups and backreferences

(regexpr) creates a capture group

(? regexpr) creates a named capture group

(?:regexpr) creates a non-capturing group

N, gN, g{N} backreference to capture group N
(where N is a natural number)

g{name} backreference to a named capture group

Examples:

1 /Sorting took (d+.d+) seconds/
2 /<(w+)>.*</1 >/
3 /([A-Z])0{2}(d+)/
4 /(? w)(? w)g{c2}g{c1}/
5 /((? w)(? w)g{c2}g{c1})/

COMP284 Scripting Languages Lecture 5 Slide L5 3

Regular expressions (2) Capture groups

Regular expressions: Capture groups

Via capture variables the strings matched by a capture group are also
available outside the pattern in which they are contained

$N string matched by capture group N
(where N is a natural number)

$+{name} string matched by a named capture group

The matched strings are available until the end of the enclosing
code block or until the next successful match

Example:

$_ = Yabba dabba doo;

if (/((? w)(? w)g{c2}g{c1})/) {

print Match found: $1
}

Output:

Match found: abba

COMP284 Scripting Languages Lecture 5 Slide L5 4

Regular expressions (2) Alternations

Regular expressions: Alternations

The regular expression regexpr1|regexpr2 matches
if either regexpr1 or regexpr2 matches
This type of regular expression is called an alternation

Within a larger regular expression we need to enclose alternations
in a capture group or non-capturing group:
(regexpr1|regexpr2) or (?:regexpr1|regexpr2)

Examples:

1 /Mr|Ms|Mrs|Dr/
2 /cat|dog|bird/
3 /(?: Bill|Hillary) Clinton/

COMP284 Scripting Languages Lecture 5 Slide L5 5

Regular expressions (2) Alternations

Regular expressions: Alternations

The order of expressions in an alternation only matters
if one expression matches a sub-expression of another

Example:

1 $_ = cats and dogs;
2 if (/( cat|dog|bird )/) { print Match 1: $1
}
3 if (/( dog|cat|bird )/) { print Match 2: $1
}
4 if (/( dog|dogs )/) { print Match 3: $1
}
5 if (/( dogs|dog )/) { print Match 4: $1
}

Output:

Match 1: cat

Match 2: cat

Match 3: dog

Match 4: dogs

COMP284 Scripting Languages Lecture 5 Slide L5 6

Regular expressions (2) Anchors

Regular expressions: Anchors

Anchors allow us to fix where a match has to start or end

A Match only at string start

^ Match only at string start (default)
Match only at a line start (in //m)

Z Match only at string end modulo a preceding

z Match only at string end

$ Match only at string end modulo a preceding

Match only at a line end (in //m)

b Match word boundary (between w and W)

B Match except at word boundary

Example:

$_ = The girl who
played with fire
;

if (/firez/) { print fire at string end
}

if (/fireZ/) { print fire at string end modulo \n
}

fire at string end modulo

COMP284 Scripting Languages Lecture 5 Slide L5 7

Regular expressions (2) Modifiers

Regular expressions: Modifiers

Modifiers change the interpretation of certain characters in a regular
expression or the way in which Perl finds a match for a regular expression

/ / Default
. matches any character except

^ matches only at string start
$ matches only at string end modulo preceding

/ /s Treat string as a single long line
. matches any character including

^ matches only at string start
$ matches only at string end modulo preceding

/ /m Treat string as a set of multiple lines
. matches any character except

^ matches at a line start
$ matches at a line end

COMP284 Scripting Languages Lecture 5 Slide L5 8

Regular expressions (2) Modifiers

Regular expressions: Modifiers

Modifiers change the interpretation of certain characters in a regular
expression or the way in which Perl finds a match for a regular expression

/ /sm Treat string as a single long line, but detect multiple lines
. matches any character including

^ matches at a line start
$ matches at a line end

/ /i perform a case-insensitive match

Example:

$_ = bill
Clinton;

if (/( Bill|Hillary ). Clinton )/smi) { print Match: $1
}

Output:

Match: bill

Clinton

COMP284 Scripting Languages Lecture 5 Slide L5 9

Regular expressions (2) Modifiers

Regular expressions: Modifiers (/ /g and / /c)

Often we want to process all matches for a regular expression,
but the following code has not the desired effect

$_ = 11 22 33;

while (/d+/) { print Match starts at $ -[0]: $&
}

The code above does not terminate and endlessly prints out the same text:

Match starts at 0: 11

To obtain the desired behaviour of the while-loop we have to use
the / /g modifier:

/ /g In scalar context, successive invocations against a string will
move from match to match, keeping track of the position in the
string
In list context, returns a list of matched capture groups, or
if there are no capture groups, a list of matches to
the whole regular expression

COMP284 Scripting Languages Lecture 5 Slide L5 10

Regular expressions (2) Modifiers

Regular expressions: Modifiers (/ /g and / /c)

With the / /g modifier our code works as desired:

$_ = 11 22 33;

while (/d+/g) { print Match starts at $ -[0]: $&
}

Output:

Match starts at 0: 11

Match starts at 3: 22

Match starts at 6: 33

An example in a list context is the following:

$_ = ab 11 cd 22 ef 33;

@numbers = (/d+/g);

print Numbers: ,join( | ,@numbers),
;

Output:

Numbers: 11 | 22 | 33

Read / /g as: Start to look for a match from the position where the last
match using / /g ended

COMP284 Scripting Languages Lecture 5 Slide L5 11

Regular expressions (2) Modifiers

Regular expressions: Modifiers (/ /g and / /c)

The current position in a string for a regular expression regexpr
is associated with the string, not regexpr
; different regular expressions for the same strings will move forward the

same position when used with / /g

; different strings have different positions and their respective positions
move forward independently

Example:

$_ = ab 11 cd 22 ef 33;

if (/d+/g) { print Match starts at $-[0]: $&
}

if (/[a-z]+/g) { print Match starts at $-[0]: $&
}

if (/d+/g) { print Match starts at $-[0]: $&
}

Output:

Match starts at 3: 11

Match starts at 6: cd

Match starts at 9: 22

COMP284 Scripting Languages Lecture 5 Slide L5 12

Regular expressions (2) Modifiers

Regular expressions: Modifiers (/ /g and / /c)

A failed match or changing the target string resets the position

1 $_ = ab 11 cd 22 ef 33;
2 if (/d+/g) { print 2: Match starts at $-[0]: $&
}
3 if (/ab/g) { print 3: Match starts at $-[0]: $&
}
4 if (/d+/g) { print 4: Match starts at $-[0]: $&
}

Output:

2: Match starts at 3: 11

4: Match starts at 3: 11

To prevent the reset, an additional modifier / /c can be used

1 $_ = ab 11 cd 22 ef 33;
2 if (/d+/g) { print 2: Match starts at $-[0]: $&
}
3 if (/ab/gc) { print 3: Match starts at $-[0]: $&
}
4 if (/d+/g) { print 4: Match starts at $-[0]: $&
}

Output:

2: Match starts at 3: 11

4: Match starts at 9: 22

COMP284 Scripting Languages Lecture 5 Slide L5 13

Regular expressions (2) Binding operator

Generating regular expressions on-the-fly

The Perl parser will expand occurrences of $variable and @variable
in regular expressions
; regular expessions can be constructed at runtime

Example:

$_ = Bart teases Lisa;

@keywords = (bart,lisa,marge,Lw+,t\w+);

while ($keyword = shift(@keywords )) {

print Match found for $keyword: $&
if /$keyword/i;

}

Output:

Match found for bart: Bart

Match found for lisa: Lisa

Match found for Lw+: Lisa

Match found for tw+: teases

COMP284 Scripting Languages Lecture 5 Slide L5 14

Regular expressions (2) Binding operator

Binding operator

Perl offers two binding operators for regular expressions

string = /regexpr/ true iff regexpr matches string

string ! /regexpr/ true iff regexpr does not match string

Note that these are similar to comparison operators not assignments
Most of the time we are not just interested whether these expressions

return true or false, but in the side effect they have on the special
variables $N that store the strings matched by capture groups

Example:

$name = Dr Ullrich Hustadt;

if ($name = /(Mr|Ms|Mrs|Dr)?s*(w+)/) {print Hello $2
}

$name = Dave Shield;

if ($name = /(Mr|Ms|Mrs|Dr)?s*(w+)/) {print Hello $2
}

Hello Ullrich

Hello Dave

COMP284 Scripting Languages Lecture 5 Slide L5 15

Regular expressions (2) Binding operator

Pattern matching in a list context

When a pattern match /regexpr/ is used in a list context,
then the return value is

a list of the strings matched by the capture groups in regexpr
if the match succeeds and regexpr contains capture groups, or

(a list containing) the value 1
if the match succeeds and regexpr contains no capture groups, or

an empty list if the match fails

$name = Dr Ullrich Hustadt;

($t ,$f ,$l) = ($name = /(Mr|Ms|Mrs|Dr)?s*(w+)s+(w+)/);

print Name: $t, $f , $l
;

$name = Dave Shield;

($t ,$f ,$l) = ($name = /(Mr|Ms|Mrs|Dr)?s*(w+)s+(w+)/);

print Name: $t, $f , $l
;

Output:

Name: Dr, Ullrich , Hustadt

Name: , Dave , Shield

COMP284 Scripting Languages Lecture 5 Slide L5 16

Regular expressions (2) Binding operator

Pattern matching in a list context

When a pattern match /regexpr/g is used in a list context,
then the return value is

a list of the strings matched by the capture groups in regexpr
each time regex matches
provided that regexpr contains capture groups, or

a list containing the string matched by regexpr each time regexpr
matches provided that regexpr contains no capture groups, or

an empty list if the match fails

$string = firefox: 10.3 seconds; chrome: 9.5 seconds;

%performance = ($string = /(w+):s+(d+.d+)/g);

foreach $system (keys %performance) {

print $system -> $performance{$system }
}

Output:

firefox -> 10.3

chrome -> 9.5

COMP284 Scripting Languages Lecture 5 Slide L5 17

Regular expressions (2) Binding operator

Revision

Read

Chapter 7: In the World of Regular Expressions

Chapter 8: Matching with Regular Expressions

of

R. L. Schwartz, brian d foy, T. Phoenix:
Learning Perl.
OReilly, 2011.

http://perldoc.perl.org/perlre.html
http://perldoc.perl.org/perlretut.html
http://www.perlfect.com/articles/regextutor.shtml

COMP284 Scripting Languages Lecture 5 Slide L5 18

http://perldoc.perl.org/perlre.html
http://perldoc.perl.org/perlretut.html
http://www.perlfect.com/articles/regextutor.shtml

COMP284 Scripting Languages
Lecture 6: Perl (Part 5)

Handouts

Ullrich Hustadt

Department of Computer Science
School of Electrical Engineering, Electronics, and Computer Science

University of Liverpool

Contents

11 Substitution
Binding operators
Capture variables
Modifiers

12 Subroutines
Introduction
Defining a subroutine
Parameters and Arguments
Calling a subroutine
Persistent variables
Nested subroutine definitions

COMP284 Scripting Languages Lecture 6 Slide L6 1

Substitution Binding operators

Substitutions

s/regexpr/replacement/

Searches a variable for a match for regexpr, and if found,
replaces that match with a string specified by replacement

In both scalar context and list context returns the number of
substitutions made (that is, 0 if no substitutions occur)

If no variable is specified via one of the binding operators = or !,
the special variable $_ is searched and modified

The binding operator ! only negates the return value but does not
affect the manipulation of the text

The delimiter / can be replaced by some other paired or non-paired
character, for example:
s!regexpr!replacement! or s [replacement]

COMP284 Scripting Languages Lecture 6 Slide L6 2

Substitution Binding operators

Substitutions

Example:

$text = http ://www.myorg.co.uk/info/refund /../ vat.html;

$text = s!/[^/]+/..!!;

print $text
;

Output:

http :// www.myorg.co.uk/info/vat.html

Example:

$_ = Yabba dabba doo;

s/bb/dd/;

print $_,
;

Output:

Yadda dabba doo

Note: Only the first match is replaced

COMP284 Scripting Languages Lecture 6 Slide L6 3

Substitution Capture variables

Substitutions: Capture variables

s/regexpr/replacement/

Perl treats replacement like a double-quoted string

; backslash escapes work as in a double-quoted string

Newline
t Tab

l Lower case next letter
L Lower case all following letters until E
u Upper case next letter
U Upper case all following letters until E

; variable interpolation is applied, including capture variables

$N string matched by capture group N
(where N is a natural number)

$+{name} string matched by a named capture group

COMP284 Scripting Languages Lecture 6 Slide L6 4

Substitution Capture variables

Substitutions: Capture variables

Example:

$name = Dr Ullrich Hustadt;

$name = s/(Mr|Ms|Mrs|Dr)?s*(w+)s+(w+)/ U$3E, $2/;

print $name
;

$name = Dave Shield;

$name = s/(Mr|Ms|Mrs|Dr)?s*(w+)s+(w+)/ U$3E, $2/;

print $name
;

Output:

HUSTADT , Ullrich

SHIELD , Dave

COMP284 Scripting Languages Lecture 6 Slide L6 5

Substitution Modifiers

Substitutions: Modifiers

Modifiers for substitutions include the following:

s/ / /g Match and replace globally, that is, all occurrences

s/ / /i Case-insensitive pattern matching

s/ / /m Treat string as multiple lines

s/ / /s Treat string as single line

s/ / /e Evaluate the right side as an expression

Combinations of these modifiers are also allowed

Example:

$_ = Yabba dabba doo;

s/bb/dd/g;

print $_,
;

Output:

Yadda dadda doo

COMP284 Scripting Languages Lecture 6 Slide L6 6

Substitution Modifiers

Substitutions: Modifiers

Modifiers for substitutions include the following:

s/ / /e Evaluate the right side as an expression

Example:

1 $text = The temperature is 105 degrees Fahrenheit;
2 $text = s!(d+) degrees Fahrenheit!
3 (($1 -32)*5/9). degrees Celsius!e;
4 print $text
;
5 $text = s!(d+.d+)! sprintf(%d,$1 +0.5)!e;
6 print $text
;

The temperature is 40.5555555555556 degrees Celsius

The temperature is 41 degrees Celsius

Better:

1 $text = The temperature is 105 degrees Fahrenheit;
2 $text = s!(d+) degrees Fahrenheit!
3 sprintf(%d ,(($1 -32)*5/9)+0.5).
4 degrees Celsius!e;

COMP284 Scripting Languages Lecture 6 Slide L6 7

Substitution Modifiers

Regular Expressions and the Chomsky Hierarchy

In Computer Science, formal
languages are categorised
according to the type of grammar
needed to generate them (or the
type of automaton needed to
recognise them)

Perl regular expressions can
at least recognise all context-free
languages Chomsky Hiearchy of Formal Languages

Howerver, this does not mean regular expression should be used for
parsing context-free languages

Instead there are packages specifically for parsing context-free languages
or dealing with specific languages, e.g. HTML, CSV

COMP284 Scripting Languages Lecture 6 Slide L6 8

Subroutines Introduction

Java methods versus Perl subroutines

Java uses methods as a means to encapsulate sequences of instructions
In Java you are expected
to declare the type of the return value of a method
to provide a list of parameters, each with a distinct name,

and to declare the type of each parameter

public static int sum2( int f, int s) {

f = f+s;

return f;

}

public static void main(String [] args) {

System.out.println(Sum of 3 and 4 is + sum2(3, 4));

}

Instead of methods, Perl uses subroutines

COMP284 Scripting Languages Lecture 6 Slide L6 9

Subroutines Defining a subroutine

Subroutines

Subroutines are defined as follows in Perl:

sub identifier {

statements

}

Subroutines can be placed anywhere in a Perl script but preferably they
should all be placed at start of the script (or at the end of the script)

All subroutines have a return value (but no declaration of its type)
The statement
return value

can be used to terminate the execution of a subroutine and
to make value the return value of the subroutine

If the execution of a subroutine terminates without encountering
a return statement, then the value of the last evaluation of an expression
in the subroutine is returned

The return value does not have to be scalar value, but can be a list

COMP284 Scripting Languages Lecture 6 Slide L6 10

Subroutines Parameters and Arguments

Parameters and Arguments

Subroutines are defined as follows in Perl:

sub identifier {

statements

}

In Perl there is no need to declare the parameters of a subroutine
(or their types)
; there is no pre-defined fixed number of parameters

Arguments are passed to a subroutine via a special array @_
Individual arguments are accessed using $_[0], $_[1] etc
Is is up to the subroutine to process arguments as is appropriate
The array @_ is private to the subroutine

; each nested subroutine call gets its own @_ array

COMP284 Scripting Languages Lecture 6 Slide L6 11

Subroutines Parameters and Arguments

Parameters and Arguments: Examples

The Java method
public static int sum2( int f, int s) {

f = f+s;

return f;

}

could be defined as follows in Perl:

sub sum2 {

return $_[0] + $_[1];

}

A more general solution, taking into account that a subroutine can be
given arbitrarily many arguments, is the following:

1 sub sum {
2 return undef if (@_ < 1);3 $sum = shift(@_);4 foreach (@_) { $sum += $_ }5 return $sum;6 }COMP284 Scripting Languages Lecture 6 Slide L6 12Subroutines Parameters and ArgumentsPrivate variablessub sum {return undef if (@_ < 1);$sum = shift(@_);foreach (@_) { $sum += $_ }return $sum;}The variable $sum in the example above is global:$sum = 5;print “Value of $sum before call of sum: “,$sum ,”
“;print “Return value of sum: “,&sum(5,4,3,2,1),”
“;print “Value of $sum aftercall of sum: “,$sum ,”
“;produces the outputValue of $sum before call of sum: 5Return value of sum: 15Value of $sum after call of sum: 15This use of global variables in subroutines is often undesirable; we want $sum to be private/local to the subroutineCOMP284 Scripting Languages Lecture 6 Slide L6 13Subroutines Parameters and ArgumentsPrivate variables The operator my declares a variable or list of variables to be private:my $variable;my ($variable1 ,$variable2 );my @array; Such a declaration can be combined with a (list) assignment:my $variable = $_[0];my ($variable1 ,$variable2) = @_;my @array = @_; Each call of a subroutine will get its own copy of its private variablesExample:sub sum {return undef if (@_ < 1);my $sum = shift(@_);foreach (@_) { $sum += $_ }return $sum;}COMP284 Scripting Languages Lecture 6 Slide L6 14Subroutines Calling a subroutineCalling a subroutineA subroutine is called by using the subroutine name with an ampersand &in front possibly followed by a list of argumentsThe ampersand is optional if a list of arguments is presentsub identifier {statements}… &identifier …… &identifier(arguments) …… identifier(arguments) …Examples:print “sum0: ” ,&sum ,”
“;print “sum0: “,sum(),”
“;print “sum1: “,sum(5),”
“;print “sum2: “,sum(5,4),”
“;print “sum5: ” ,&sum(5,4,3,2,1),”
“;$total = sum(9,8,7,6)+sum(5,4,3,2,1);sum(1,2,3,4);COMP284 Scripting Languages Lecture 6 Slide L6 15Subroutines Persistent variablesPersistent variables Private variables within a subroutine are forgotten once a call of thesubroutine is completed In Perl 5.10 and later versions, we can make a variableboth private and persistent using the state operator; the value of a persistent variable will be retained betweenindependent calls of a subroutineExample:use 5.010;sub running_sum {state $sum;foreach (@_) { $sum += $_ }return $sum;}COMP284 Scripting Languages Lecture 6 Slide L6 16Subroutines Persistent variablesPersistent variablesExample:1 use 5.010;23 sub running_sum {4 state $sum;5 foreach (@_) { $sum += $_ }6 return $sum;7 }89 print “running_sum ():tt”, running_sum (), ”
“;10 print “running_sum (5):t”, running_sum (5), ”
“;11 print “running_sum (5 ,4):t”, running_sum (5,4), ”
“;12 print “running_sum (3,2 ,1):t”,running_sum (3,2,1),”
“;Output:running_sum ():running_sum (5): 5running_sum (5 ,4): 14running_sum (3,2,1): 20COMP284 Scripting Languages Lecture 6 Slide L6 17Subroutines Nested subroutine definitionsNested subroutine definitions Perl allows nested subroutine definitions (unlike C or Java)sub outer_sub {sub inner_sub { … }} Normally, nested subroutines are a means for information hiding; the inner subroutine should only be visible and executable frominside the outer subroutine However, Perl allows inner subroutines to be called from anywhere(within the package in which they are defined)sub outer_sub {sub inner_sub { … }}inner_sub ();COMP284 Scripting Languages Lecture 6 Slide L6 18Subroutines Nested subroutine definitionsNested subroutine definitionsIf an inner subroutine uses a local variable of an outer subroutine,then it refers to the instance of that local variable created the first timethe outer subroutine was calledsub outer {my $x = $_[0];sub inner { return $x }return inner (); # returns $_[0]?}print “1: “,outer (10),”
“;print “2: “,outer (20),”
“;1: 102: 10 # not 20!; Do not refer to local variables of an outer subroutine,pass information via arguments insteadCOMP284 Scripting Languages Lecture 6 Slide L6 19Subroutines Nested subroutine definitionsNested subroutine definitions: Examplesub sqrt2 {my $x = shift(@_);my $precision = 0.001;sub sqrtIter {my ($guess ,$x) = @_;if (isGoodEnough($guess ,$x)) {return int($guess/$precision +0.5)* $precision;} else { sqrtIter(improveGuess($guess , $x), $x) } }sub improveGuess {my ($guess ,$x) = @_;return ($guess + $x / $guess) / 2; }sub isGoodEnough {my ($guess ,$x) = @_;return (abs($guess * $guess – $x) < $precision ); }return sqrtIter (1.0,$x);}COMP284 Scripting Languages Lecture 6 Slide L6 20Subroutines Nested subroutine definitionsRevisionRead Chapter 9: Processing Text with Regular Expressions Chapter 4: SubroutinesofR. L. Schwartz, brian d foy, T. Phoenix:Learning Perl.OReilly, 2011. http://perldoc.perl.org/perlsub.htmlCOMP284 Scripting Languages Lecture 6 Slide L6 21http://perldoc.perl.org/perlsub.htmlCOMP284 Scripting LanguagesLecture 7: Perl (Part 6)HandoutsUllrich HustadtDepartment of Computer ScienceSchool of Electrical Engineering, Electronics, and Computer ScienceUniversity of LiverpoolContents13 Input/OutputFilehandlesOpenCloseReadSelectPrintHere documents14 Arguments and OptionsInvocation ArgumentsOptionsCOMP284 Scripting Languages Lecture 7 Slide L7 1Input/Output FilehandlesI/O Connections Perl programs interact with their environment via I/O connections A filehandle is the name in a Perl program for such an I/O connection,given by a Perl identifierBeware: Despite the terminology, no files might be involved There are six pre-defined filehandlesSTDIN Standard Input, for user input, typically the keyboardSTDOUT Standard Output, for user output, typically the terminalSTDERR Standard Error, for error output,typically defaults to the terminalDATA Input from data stored after __END__ at the end of aPerl programARGV Iterates over command-line filenames in @ARGVARGVOUT Points to the currently open output file when doing edit-in-place processing with -iperl -pi -e s/cat/dog/ fileCOMP284 Scripting Languages Lecture 7 Slide L7 2Input/Output FilehandlesI/O ConnectionsExcept for the six predefined I/O connections, all other I/O connections need to be opened before they can be usedopen filehandle, mode, expr should be closed once no longer neededclose filehandle can be used to read from

can be used to write to
print filehandle list

printf filehandle list

can be selected as default output
select filehandle

COMP284 Scripting Languages Lecture 7 Slide L7 3

Input/Output Filehandles

I/O Connections

Example:

open INPUT , <“, “oldtext.txt” or die “Cannot open file”;open OUTPUT , “>, newtext.txt;

while ( ) {

s!(d+) degrees Fahrenheit!

sprintf(%d ,(($1 -32)*5/9)+0.5). degrees Celsius!e;

print OUTPUT;

}

close(INPUT );

close(OUTPUT );

oldtext.txt:

105 degrees Fahrenheit is quite warm

newtext.txt:

41 degrees Celcius is quite warm

COMP284 Scripting Languages Lecture 7 Slide L7 4

Input/Output Open

Opening a filehandle

open filehandle, expr

open filehandle, mode, expr

Opens an I/O connection specified by mode and expr and associates it
with filehandle

expr specifies a file or command
mode is one of the following

Mode Operation Create Truncate

< read file> write file yes yes
>> append file yes

+< read/write file+> read/write file yes yes
+>> read/append file yes

|- write to command yes
-! read from command yes

COMP284 Scripting Languages Lecture 7 Slide L7 5

Input/Output Close

Closing a filehandle

close

close filehandle

Flushes the I/O buffer and closes the I/O connection associated with
filehandle

Returns true if those operations succeed
Closes the currently selected filehandle if the argument is omitted

COMP284 Scripting Languages Lecture 7 Slide L7 6

Input/Output Read

Reading

In a scalar context, returns a string consisting of all characters from
filehandle up to the next occurrence of $/
(the input record separator)

In a list context, returns a list of strings representing the whole content
of filehandle separated into string using $/ as a separator
(Default value of $/: newline
)

1 open INPUT , <“, “oldtext.txt” or die “Cannot open file”;2 $first_line = ;
3 while ($other_line = ) { }
4 close INPUT;
5
6 open LS, -|, ls -1;
7 @files = ;
8 close LS;
9 foreach $file (@files) { }

COMP284 Scripting Languages Lecture 7 Slide L7 7

Input/Output Select

Selecting a filehandle as default output

select

select filehandle

If filehandle is supplied, sets the new current default filehandle for
output
; write or print without a filehandle default to filehandle
; References to variables related to output will refer to filehandle

Returns the currently selected filehandle

COMP284 Scripting Languages Lecture 7 Slide L7 8

Input/Output Print

Printing

print filehandle list

print filehandle

print list

print

Print a string or a list of strings to filehandle
If filehandle is omitted, prints to the last selected filehandle
If list is omitted, prints $_
The current value of $, (if any) is printed between each list item

(Default: undef)

The current value of $ (if any) is printed after the entire list has
been printed
(Default: undef)

COMP284 Scripting Languages Lecture 7 Slide L7 9

Input/Output Print

Printing: Formatting

sprintf(format, list)

Returns a string formatted by the usual printf conventions of the C
library function sprintf (but does not by itself print anything)

sprintf (%10.3f) 1234.5678

format a floating-point number with minimum width 10 and precision 3
and put the result in parentheses:

( 1234.568)

See http://perldoc.perl.org/functions/sprintf.html for
further details

COMP284 Scripting Languages Lecture 7 Slide L7 10

http://perldoc.perl.org/functions/sprintf.html

Input/Output Print

Printing: Formatting

printf filehandle format, list

printf format, list

Equivalent to
print filehandle sprintf(format, list)

except that $ (the output record separator) is not appended

COMP284 Scripting Languages Lecture 7 Slide L7 11

Input/Output Print

Printing: Formatting

Format strings can be stored in variables and can be constructed
on-the-fly:

@list = qw(wilma dino pebbles );

$format = The items are:
. (%10s
x @list );

printf $format , @list;

Output:

The items are:

wilma

dino

pebbles

(The code above uses the quote word function qw()
to generate a list of words.
See http://perlmeme.org/howtos/perlfunc/qw_function.html
for details)

COMP284 Scripting Languages Lecture 7 Slide L7 12

http://perlmeme.org/howtos/perlfunc/qw_function.html

Input/Output Here documents

Here documents

A here document is a way of specifying multi-line strings in a scripting
or programming language

The basic syntax is
<

$title

Lots of HTML markup here