COMP284 Scripting Languages – Handouts (8 on 1)
COMP284 Scripting Languages
Lecture 3: Perl (Part 2)
Handouts (8 on 1)
Ullrich Hustadt
Department of Computer Science
School of Electrical Engineering, Electronics, and Computer Science
University of Liverpool
Contents
1 Control structures
Conditional statements
Switch statements
While- and Until-loops
For-loops
2 Lists and Arrays
Identifiers
List literals
Contexts
List and array functions
Foreach-loops
3 Hashes
Identifiers
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 =
@lines =
• 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 $#array will the array’s size shrink to the
position of the highest element that returns true for exists()
@array = (0, 11, 22, 33);
delete($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.
O’Reilly, 2011.
Harold Cohen Library: 518.579.86.S39 or e-book
COMP284 Scripting Languages Lecture 3 Slide L3 – 31
Lecture 3
Control structures
Conditional statements
Switch statements
While- and Until-loops
For-loops
Lists and Arrays
Identifiers
List literals
Contexts
List and array functions
Foreach-loops
Hashes
Identifiers
Basic hash operations
Foreach
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