Chapter 8 : Strings | class 11th | revision notes computer science

String Manipulation in Python

Topics:

• Strings – introduction,
• Indexing
• String operations – concatenation, repetition, membership & slicing,
• Traversing a string using loops,
• Built-in functions: len(), capitalize(), title(), lower(), upper(), count(), find(), index(), endswith(), startswith(), isalnum(), isalpha(), isdigit(), islower(), isupper(), isspace(), lstrip(), rstrip(), strip(), replace(), join(), partition(), split()

Strings in Python – Introduction

Python strings are characters enclosed in quotes of any type – single quotation marks, double quotation marks, triple quotation marks.

S = ‘Anjeev Singh Academy’ ,    

T =  “Singh@123”

Mline = ‘‘‘Anjeev Singh Academy

is the best free online academy on YouTube ’’’

Python strings is an immutable data type, i.e. not allowed to change in place. S[0] =‘R’ not allowed

String Indexing – Forward and Backward Indexing in Python

Strings are sequence of characters, where each character has a unique index number.

string = “mycstutorial.in”

>>> string[0]

‘m’

>>> string[-1]

‘n’

Two types of Indexing is supported in python.

(A) Forward Indexing and (B) Backward Indexing

The indexes of strings begin from 0 to length-1, in forward direction and  -1, -2, -3, …. , -length in backward direction.

String operations – concatenation, repetition, membership & slicing,

Strings can be manipulated by using string operators. String operators are

Operator	Name	Description
+	Concatenation	Add two strings i.e. concatenate two strings. e.g.      S = “Hello” + “Shyam”
*	Replication  / Repetition	Replicate the string upto n times.
In / not in	Membership	To check given character exists in the string or not.
[index]	Element Accessor	Extract the character from the given index position.
[ : ]	Slicing [start:end:step]	Extracts the characters from the given range i.e start to stop
==, !=, < , > , <=, >=	Relational / Comparison Operator	These relational operators can do comparison between strings, based on character by character  comparison rules for Unicode or Ordinal Value

Traversing a string using loops

Traversing a String or Text means accessing all characters of string, character by character, through its index position.

#Traversal of a string using loop
name = "mycstutorial.in"
for char in name:
    print (char, end=' ')

Output

m y c s t u t o r i a l . i n

Traversing a String using index position

#Traversal of a string using index - forward indexing
name = "mycstutorial.in"
for index in range(len(name)):
    print (name[index], end=' ')

Output

m y c s t u t o r i a l . i n

#Traversal of a string using index - backward indexing
name = "mycstutorial.in"
for index in range(-1, -len(name)-1, -1):
    print (name[index], end=' ')

Output

n i . l a i r o t u t s c y m 

String Slicing

Slice means ‘a part of’.  In Python, string slice refers to a part of string containing some contiguous characters from the string.

Syntax: string[start : end : step]

• where start and end are integers with legal indices.
• will return the characters falling between indices start and end. i.e. start, start+1, start+2,…, till end-1.
• By default value of step is 1. It is an optional argument.

Slice	Output, if str = “Hello Anjeev”
str[ 2 : ]	‘llo Anjeev”
str[ : 5]	“Hello”
str[4 : 8]	“o Anj”
str[ : : 2]	“HloAje”
str[ : : -1]	“veejnA olleH”
str[-6 : -2 ]	“Anje”

String Built-in functions

Python provides several built-in-methods to manipulate string. 

Syntax : stringvariable.methodName( )

Example: isalnum(), isalpha(), isdigit(), islower(), isupper(), isspace(), etc

Methods	Description	Example
isalpha( )	Returns True, if the string contains only letters, otherwise returns False.	>>>”abc”.isalpha() True >>>”1abc”.isalpha() False
isalnum()	Returns True, if the string contains alphanumeric (i.e. alphabets or number or both), otherwise returns False	>>>”abc”.isalnum() True >>>”123″.isalnum() True >>>”a123″.isalnum() True >>>”@#2″.isalnum() False >>>”@#12ab”.isalnum() False
isdigit( )	Return True, if the string contains only digits, otherwise returns False.	>>>”123″.isdigit() True >>>”abc123″.isdigit() False
isspace( )	Returns True, if the string contains only spaces, otherwise returns False.	>>>” “.isspace() True >>>”a “.isspace() False >>>” a”.isspace() False
islower( )	Returns True if the string contains all lowercase characters, otherwise returns False.	>>>”mycstutorial.in”.islower() True >>>”MYCSTUTORIAL.IN”.islower() False
isupper()	Returns True, if the string contains all uppercase characters, otherwise returns False	>>>”MYCSTUTORIAL.IN”.isupper() True >>>”mycstutorial.in”.isupper() False
istitle( )	Returns True, if string is properly “TitleCasedString”, otherwise returns False.	>>>”Anjeev Singh Academy”.istitle() True >>>”Anjeev singh academy”.istitle() False >>>”Anjeevsinghacademy”.istitle() True
endswith()	Returns Ture, if string is ends with the given substring.	>>> name=”tanmay singh” >>> name.endswith(‘singh’) True >>> name.endswith(‘nh’) False
startswith()	Returns True, if string is starts with the given substring.	>>> name=”tanmay singh” >>> name.startswith(‘tan’) True >>> name.startswith(‘tannu’) False
len(string)	Returns the length of string, i.e. number of characters.	>>> name=”tanmay singh” >>> len(name) 12
capitalize()	Returns a copy of string with its first character capitalized.	>>> name=”tanmay singh” >>> name.capitalize() ‘Tanmay singh’
lower()	Returns a copy of string converted to lowercase.	>>> name=”TANMAY SINGH” >>> name.lower() ‘tanmay singh’
upper()	Returns a copy of string converted to uppercase.	>>> name=”tanmay singh” >>> name.upper() ‘TANMAY SINGH’
title()	Returns a copy of string converted to title case.	>>> name=”tanmay singh” >>> name.title() ‘Tanmay Singh’
swapcase()	Converts and returns all uppercase characters to lowercase and vice-versa.	>>> name=”Tanmay Singh” >>> name.swapcase() ‘tANMAY sINGH’
count()	Returns the number of times a specified value occurs in a string	>>> name=”tanmay singh” >>> name.count(‘a’) 2
find()	Returns the first occurrence of given string/character in the string.	>>> name=”tanmay singh” >>> name.find(‘y’) 5
index(char)	Search and returns the first occurrence of given character or substring.	>>> name=”tanmay singh” >>> name.index(‘y’) 5
lstrip() / lstrip(char)	Returns a copy of the string with leading characters removed. By default removed space.	>>> data = ‘ hello ‘ >>> data.lstrip() ‘hello ‘ >>> mesg = ‘aaaRAMaaa’ >>> mesg.lstrip(‘a’) ‘RAMaaa’
rstrip()/ rstrip(char)	Returns a copy of the string with trailing characters removed. By default removed space.	>>> data = ‘ hello ‘ >>> data.rstrip() ‘ hello’ >>> mesg = ‘aaaRAMaaa’ >>> mesg.rstrip(‘a’) ‘aaaRAM’
strip()	Returns a copy of the string with leading and trailing characters removed.	>>> data = ‘ hello ‘ >>> data.strip() ‘hello’ >>> mesg = ‘aaaRAMaaa’ >>> mesg.strip(‘a’) ‘RAM’
replace()	Returns a copy of the string with replaced old substring with new substring.	>>> name=”tanmay singh” >>> name.replace(‘singh’, ‘verma’) ‘tanmay verma’
split()	Splits the string at the specified separator, and returns a list. Default separator is space ‘ ‘. Separator character is ignored, i.e not available in the list.	>>> name=”tanmay singh” >>> name.split(‘n’) [‘ta’, ‘may si’, ‘gh’] >>> message = “Hello dear welcome to mycstutorial.in” >>> message.split() [‘Hello’, ‘dear’, ‘welcome’, ‘to’, ‘mycstutorial.in’]
partition()	Splits the string at the specified separator, and returns a tuple. Tuple is having only three elements, characters before separator, separator and characters after separator.	>>> name=”tanmay singh” >>> name.partition(‘n’) (‘ta’, ‘n’, ‘may singh’) >>> message = “Hello dear welcome to mycstutorial.in” >>> message.partition(‘ ‘) (‘Hello’, ‘ ‘, ‘dear welcome to mycstutorial.in’)
join()	Returns a string in which the string elements have been joined by a string separator.  Eg. ‘-’.join(‘anjeev” => ‘a-n-j-e-e-v’	>>> name=”tanmay singh” >>> ‘#’.join(name) ‘t#a#n#m#a#y# #s#i#n#g#h’ >>> “–He–“.join(name) ‘t–He–a–He–n–He–m–He–a–He–y–He– –He–s–He–i–He–n–He–g–He–h’
ord(char)	Returns the ordinal value (ASCII) of given character.	>>> ord(‘a’) 97 >>> ord(‘A’) 65 >>> ord(‘#’) 35
chr(num)	Returns character represented by inputted ASCII number.	>>> chr(35) ‘#’ >>> chr(65) ‘A’ >>> chr(97) ‘a’