Fundamentals of Programming(Python)

Strings

Sina SajadmaneshSharif University of Technology

Fall 2017Some slides have been adapted from “Python Programming: An Introduction to

Computer Science”

Outline1. String Data Type

2. String Representation

3. String Indexing

4. Operations on Strings

5. String Methods

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String Data TypeText is represented in programs by the string data type

A string is a immutable sequence of charactersenclosed within quotation marks (") or apostrophes (').◦ Example:

◦ ‘This is a sample sentence.’

◦ “2x2=4”

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String RepresentationCharacter Encoding◦ A string is stored as a sequence of binary numbers,

one number per character

◦ The mapping used to convert characters to numbers and vice-versa is called character encoding

◦ Python 3 uses Unicode

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>>> ord('a') # Return the Unicode code point for a one-character string.

97

>>> chr(97) # Return a Unicode string of one character with ordinal i

'a'

String Indexing

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>>> greet = "Hello Bob"

>>> greet[0]

'H'

>>> print(greet[0], greet[2], greet[4])

H l o

>>> x = 8

>>> print(greet[x - 2])

B

>>> greet[-1]

'b'

>>> greet[-3]

'B'

H e l l o B o b

0 1 2 3 4 5 6 7 8

String Indexing

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H e l l o B o b

0 1 2 3 4 5 6 7 8

>>> greet[0:3]

'Hel'

>>> greet[5:9]

' Bob'

>>> greet[:5]

'Hello'

>>> greet[5:]

' Bob'

>>> greet[:]

'Hello Bob'

Operations on StringsTraversal:

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fruit = 'banana'

n = len(fruit)

for i in range(n):

c = fruit[i]

print(c)

Output:

b

a

n

a

n

a

fruit = 'banana'

for c in fruit:

print(c)

Output:

b

a

n

a

n

a

Operations on StringsComparison:

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Operations on StringsStrings are immutable

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Operations on StringsThe in Operator

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String Methods◦ s.capitalize() – Copy of s with only the first

character capitalized

◦ s.title() – Copy of s; first character of each word capitalized

◦ s.center(width) – Center s in a field of given width

◦ s.count(sub) – Count the number of occurrences of sub in s

◦ s.find(sub) – Find the first position where sub occurs in s

◦ s.join(list) – Concatenate list of strings into one large string using s as separator.

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String Methods◦ s.ljust(width) – Like center, but s is left-justified

◦ s.lower() – Copy of s in all lowercase letters

◦ s.lstrip() – Copy of s with leading whitespace removed

◦ s.replace(oldsub, newsub) – Replace occurrences of oldsub in s with newsub

◦ s.rfind(sub) – Like find, but returns the right-most position

◦ s.rjust(width) – Like center, but s is right-justified

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String Methods◦ s.rstrip() – Copy of s with trailing whitespace

removed

◦ s.split() – Split s into a list of substrings

◦ s.upper() – Copy of s; all characters converted to uppercase

For more, refer to:https://docs.python.org/3/library/stdtypes.html#string-methods

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