[SOLVED] python COMP1730/COMP6730 Programming for Scientists

COMP1730/COMP6730 Programming for Scientists
Strings

Announcements
* Homework 3 is due next Monday (26th Aug, 11:55pm).
* Week 6&7 conflict questionnaire is due this Friday (23rd Aug). But do not fill out if you have no conflict.
* Lab this week will be large, working time outside 2 hours lab is required.
* Drop-in consultation: every Monday, 4pm, room 1.17 Hanna Neumann blg.

Lecture outline
* Character encoding & strings
* Indexing, slicing & sequence operations * Iteration over sequences

Characters & strings

Strings
* Strings values of type str in python are used to store and process text.
* A string is a sequence of characters. str is a sequence type.
* String literals can be written with
single quotes, as in hello there
double quotes, as in hello there
triple quotes, as in hello there

* Beware of copypasting code from slides (and other PDF files or web pages).

* Quoting characters other than those enclosing a string can be used inside it:
>>> its true!
>>> To be, said he,
* Quoting characters of the same kind can be used inside a string if escaped by backslash ():
>>> its true
>>> its a quote
* Escapes are used also for some non-printing characters:
>>> print(t1mt38s
t12mt9s)

Character encoding
* Idea: Every character has a number.
* Baudot code
(1870).
* 5-bit code, but also sequential (letter and figure mode).

Unicode, encoding and font
* Unicode defines numbers (code points) for >120,000 characters (in a space for >1 million).
Encoding (UTF-8)
Font
Byte(s) Code point
0100 0101 1110 0010 1000 0010
1010 1100
Glyph
69
8364

* python 3 uses the unicode character representation for all strings.
* Functions ord and chr map between the character and integer representation:
>>> ord(A)
>>> chr(65 + 4)
>>> chr(32)
>>> chr(8364)
>>> chr(20986)+chr(21475)
>>> ord(3)
* See unicode.org/charts/.

More about sequences

Indexing & length (reminder)
Image from Punch & Enbody
* In python, all sequences are indexed from 0.
* or from end, starting with -1.
* The index must be an integer.
* The length of a sequence is the number of elements, not the index of the last element.

* len(sequence) returns sequence length.
* Sequence elements are accessed by placing the index in square brackets, [].
>>> s = Hello World
>>> s[1]
e
>>> s[-1]
d
>>> len(s)
11
>>> s[11]
IndexError:string index out of range

Slicing
* Slicing returns a subsequence:
s[start:end]
start is the index of the first element in the
subsequence.
end is the index of the first element after the
end of the subsequence.
* Slicing works on all built-in sequence types
(list, str, tuple) and returns the same type.
* If start or end are left out, they default to the beginning and end (i.e., after the last element).

* The slice range is half-open: start index is included, end index is one after last included element.
>>> s = Hello World
>>> s[6:10]
Worl
Image from Punch & Enbody

* The end index defaults to the end of the sequence.
>>> s = Hello World
>>> s[6:]
World
Image from Punch & Enbody

* The start index defaults to the beginning of the sequence.
>>> s = Hello World
>>> s[:5]
World
Image from Punch & Enbody

>>> s = Hello World
>>> s[9:1]

>>> s[-100:5]
Hello
* An empty slice (index range) returns an empty sequence
* Slice indices can go past the start/end of the sequence without raising an error.

Operations on sequences
* Reminder: value types determine the meaning of operators applied to them.
* Concatenation: seq + seq >>> comp + 1730
* Repetition: seq * int >>>Oi! *3
* Membership: value in seq
Note: str in str tests for substring.
* Equality: seq == seq, seq != seq.
* Comparison (same type): seq < seq, seq <= seq, seq > seq, seq >= seq.

Sequence comparisons
* Two sequences are equal if they have the same length and equal elements in every position.
* seq1 < seq2 if- seq1[i] < seq2[i] for some index i andthe elements in each position before i areequal; or- seq1 is a prefix of seq2.* Note: Comparison of NumPy arrays is element-wise and returns an array of bool.String comparisons* Each character corresponds to an integer. – ord(‘ ‘) == 32- ord(‘A’) == 65,…,ord(‘Z’) == 90 – ord(‘a’) == 97,…,ord(‘z’) == 122* Character comparisons are based on this. >>> the ANU < “The anu” >>> the ANU < “the anu” >>> nontrivial < “non trivial”Iteration over sequencesThe for .. in .. statement for name in expression :suite1. Evaluate the expression, to obtain an iterable collection.- If value is not iterable: TypeError. 2. For each element E in the collection:2.1 assign name the value E; 2.2 execute the loop suite.for char in “The quick brown fox”:print(char, “is”, ord(char))vs.s = “The quick brown fox” i=0while i < len(s):char = s[i]print(char, “is”, ord(char)) i=i+1Iteration over sequences* Sequences are an instance of the general concept of an iterable data type.- An iterable type is defined by supporting the iter() function.- python also has data types that are iterable but not indexable (for example, sets and files).* The for .. in .. statement works on any iterable data type.- On sequences, the for loop iterates through the elements in order.String methodsMethods* Methods are only functions with a slightly different call syntax:”Hello World”.find(“o”)instead ofstr.find(“Hello World”, “o”)* pythons built-in types, like str, have many useful methods.- help(str)- docs.python.orgProgramming problem* Find a longest repeated substring in a word: – ‘backpack’ ‘ack’- ‘singing’ ‘ing’- ‘independent’ ‘nde’- ‘philosophically ‘phi’- ‘monotone’ ‘on’- ‘wherever’ ‘er’- ‘repeated’ ‘e’- ‘programming’ ‘r’ (or ‘g’, ‘m’) – ‘problem’ ”