Python: Working with Strings

Working with Strings

String is basically the sequence of characters.

Any desired character can be accessed using the index. For example ‒

In [1]: country = "India"

country [1]

Out[1]: 'n'

In [2]: country[2]

Out[2]: 'd'

The index is an integer value if it is a decimal value, then it will raise an error.

For example ‒

In [3]: country [1.5]

‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒ ‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒ ‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒

‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒

TypeError                                                  Trace

back (most recent call last)

Cell In[3], line

‒‒‒‒> 1 country[1.5]

The string can be created using double quotes or single quotes. For example ‒

In[1]: msg="Hello"

Hello

print(msg)

In[2]: msg='Goodbye'

print(msg)

Goodbye

Finding length of a string

There is an in‒built function to find length of the string. This is len function.

For example ‒

In[3]: msg='Goodbye'

    print(msg)

Goodbye

In[4]: len(msg)

7

Traversing the string

We can traverse the string using the for loop or using while loop.

Example 1 ‒ Traversing a string using while string.

In [4]: msg = 'GoodBye'

index = 0

while index < len(msg):

    letter = msg[index]

    print(letter)

    index = index + 1

G

o

o

d

B

y

e

Example 2 ‒ The string can be traversed using for loop.

msg = 'GoodBye'

for index in range(0, len(msg)):

letter = msg[index]

print(letter)

index index + 1

Output

G

o

o

d

B

y

е

Example:1

Write a program to display a set of strings using range() function.

Solution :

handsets = ['Samsung', 'OPPO', 'OnePlus', 'Apple']

    print("The mobile handsets are...")

for i in range(len(handsets)):

print(handsets[i], end ="")

Output

The mobile handsets are...

Samsung OPPO OnePlus Apple

1. String Slices

String slice is an extracted chunk of characters from the original string. In Python we can obtain the string slice with the help of string indices. For example ‒ We can obtain

In[5]: msg="Good Morning"

msg[0:4]

'Good'

In[6]: msg[5:12]

'Morning'

Here the string from 0 to less than 4 indexes will be displayed. In the next command the string from 5^th index to 11^th index is displayed.

Fig. 2.9.1 String slice

 We can omit the beginning index. In that case, the beginning index is considered as 0.

For example ‒

In[7]: msg[:4]  ←Here the starting index will be 0

'Good'

Similarly we can omit ending index. In that case, the string will be displayed upto its

ending character. For example ‒

msg[5:] ←Here the last character of the string is the ending index

'Morning'

If we do not specify any starting index or ending index then the string from starting index 0 to ending index as last character position will be considered and the entire string will be displayed. For example ‒

msg[:]

'Good Morning'

 

2. Immutability

Strings are immutable i.e. we cannot change the existing strings. For example –

msg="Good Morning"

msg[0]='g'

Output

TypeError: 'str' object does not support item assignment

To make the desired changes we need to take a new string and manipulate it as per our requirement. Here is an illustration

msg = 'Good Morning'

new_msg = 'g'+ msg[1:]

print(new_msg)

Output

Good Morning

Program explanation:

In above example the new_msg string is created to display "good morning" instead of "Good Morning".

The string slice from character 1 to end of string is concatenated with the character 'g'. The concatenation is performed using the operator +.

3. String Functions and Methods

In this section we will discuss various string functions and methods.

1. String concatenation

Joining of two or more strings is called concatenation.

In python we use + operator for concatenation of two strings.

For example ‒

In [5]:

msg1 = 'Good'

msg2 = Morning'

print(msg1+msg2)

Good Morning

2. String comparison

The string comparison can be done using the relational operators like <,>== .

For example ‒

In [6]:

msg1 = 'aaa'

msg2 = 'aaa'

msg1 = = msg2

Out[6]: True

In [7]:

msg1 = 'aaa'

msg2 = 'bbb'

print(msg1<msg2)

True

Note that the string comparison is made based on alphabetical ordering. All the upper case letters appear before all the lower case letters.

3. String repetition

We can repeat the string using * operator. For example ‒

msg="Welcome!"

print(msg*3)

Welcome!Welcome!Welcome!

4. Membership test

The membership of a particular character is determined using the keyword in.

For example –

msg="Welcome"

'm' in msg

True

't'in msg

False

 

Methods in string manipulation

Some commonly used methods are enlisted in the following table.

Method  : Purpose

count() ‒ This method searches the substring and returns how many times the substring is

present in it.

capitalize() ‒ This function returns a string with first letter capitalized. It doesn't modify the old string.

find() ‒ The find() method returns the lowest index of the substring (if found). If not found, it returns ‒1.

Index ‒ This method returns the index of a substring inside the string (if found). If the substring is not found, it raises an exception.

isalnum() ‒ The isalnum() method returns true if all characters in the string are alphanumeric.

isdigit() ‒ The isdigit() method returns true if all characters in a string are digits. If not, it returns false.

islower() ‒ The islower() method returns true if all alphabets in a string are lowercase alphabets. if the string contains at least one uppercase alphabet, it returns false.

 

Python Programming examples based on string

 

Example:2

Write a Python program to find the length of a string.

Solution:

def str_length(str):

len=0

for ch in str:

    len+=1

    return len

print("\t\t Program to find the length of a string")

print("Enter some string: ")

str=input()

print("Length of a string is: ",str_length(str))

Output

Program to find the length of a string

Enter some string:

python

Length of a string is: 6

 

Example:3

Write a Python program to count occurrences of each word in given sentence.

Solution:

def count_occur(str):

    data=dict()

    words=str.split()

    for word in words:

        if word in data:

            data[word]+=1

        else:

            data[word]=1

        return data

print("Enter some string: ")

str=input()

print(count_occur(str))

Output

Enter some string :

A big black bear sat on a big black rug

{'A': 1, 'big': 2, 'black': 2, 'bear': 1, 'sat': 1, 'on': 1, 'a': 1, 'rug': 1}

 

Example:4

Write a Python program to copy one string to another.

Solution :

print("Enter some string: ")

str1=input()

str2="

for i in range (len(str1)):

    str2=str2+str1[i]

print("The copied string is: ",str2)

Output

Enter some string:

Technical

The copied string is: Technical

 

Example:5

Write a Python program to check if a substring is present in the given string or not.

Solution :

print("Enter some string: ")

str1=input()

print("Enter a word: ")

str2=input()

if(str1.find(str2)= = ‒1):

        print("The substring ",str2," is not present in ",str1)

else:

        print("The substring ",str2," is present in ",str1)

Output

Enter some string:

sky blue

Enter a word:

blue

The substring blue is present in sky blue

 

Example:6

Write a Python program to count number of digits and letters in a string.

Solution :

print("Enter some string: ")

str=input()

digit_count = 0

letter_count = 0

for i in str:

    if(i.isdigit()):

    digit_count+=1

else:

    letter count+=1

print("Total number of digits in ",str," are ",digit_count)

print("Total number of letters in ",str," are ",letter_count)

Output

Enter some string :

Python123Program

Total number of digits in Python123Program are 3

Total number of letters in Python123Program are 13

 

Example:7

Write a Python program to count number of vowels in a string.

Solution :

print("Enter some string: ")

str=input()

vowel_count=0

for i in str:

if((i= ='a' or i= ='e' or i= ='i' or i= ='o' or i= ='u')

or((i= ='A' or i= ='E' or i= ='I' or i= ='0' or i= ='U'))):

    vowel_count + =1

print("Total number of vowels in ",str," are ",vowel_count)

Output

Enter some string:

India

Total number of vowels in India are 3

 

Example:8

Write a Python program to check if the string is palindrome or not.

Solution :

print("Enter some string: ")

str=input()

rev_str=reversed(str)

if(list(str)==list(rev_str)):

    print("The string ",str," is palindrome")

else:

    print("The string ",str," is not palindrome")

Output

Enter some string :

madam

The string madam is palindrome

 

Example:9

Write a Python program to sort the word in a sentence in an alphabetic order.

Solution :

print("Enter some string: ")

str=input()

words_list=str.split()

words_list.sort()

print("The words in sorted order are...")

for word in words_list:

    print(word)

Output

Enter some string :

I like python program very much

The words in sorted order are...

I

like

much

program

python

very

 

Review Questions

1. Analyse the string slicing. Illustrate how it is done in Python with examples,

2. Write a Python code to search a string in the given list.

3. Python strings are immutable, Justify with an example.