Python Program to Replace all Occurrences of ‘a’ with $ in a String

Given a string, the task is to write a Python program to replace all occurrence of 'a' with $.

Examples:

Input: Ali has all aces
Output: $li h$s $ll $ces

Input: All Exams are over
Output: $ll Ex$ms $re Over

Method 1:  uses splitting of the given specified string into a set of characters. An empty string variable is used to store modified string . We loop over the character array and check if the character at this index is equivalent to 'a' , and then append '$' sign, in case the condition is satisfied. Otherwise, the original character is copied into the new string.  

# declaring a string variable
str = "Amdani athani kharcha rupaiya."

# declaring an empty string variable for storing modified string
modified_str = ''

# iterating over the string
for char in range(0, len(str)):
    # checking if the character at char index is equivalent to 'a'
    if(str[char] == 'a' or str[char] == 'a'.upper()):
        # append $ to modified string
        modified_str += '$'
    else:
        # append original string character
        modified_str += str[char]

print("Modified string : ")
print(modified_str)

Output:

Modified string :
$md$ni $th$ni kh$rch$ rup$iy$.

Time Complexity: O(n), where n is length of str string.

Auxiliary Space: O(n), where n is length of modified_str string to store result.

Method 2: uses the inbuilt method replace() to replace all the occurrences of a particular character in the string with the new specified character. The method has the following syntax : 

replace( oldchar , newchar)

This method doesn't change the original string, and the result has to be explicitly stored in the String variable. 

# declaring a string variable
str = "An apple A day keeps doctor Away."

# replacing character a with $ sign
str = str.replace('a', '$')
str = str.replace('a'.upper(),'$')
print("Modified string : ")
print(str)

Modified string : 
$n $pple $ d$y keeps doctor $w$y.

Time Complexity: O(n)
Auxiliary Space: O(n)

Method 3: Using Python re module

The re.sub() function from the re module is used to replace all occurrences of a particular pattern in a string with a specified replacement. In this case, we are using the pattern 'a' and the replacement '$' to replace all occurrences of 'a' in the string str. The result of the re.sub() function is stored in the variable modified_str.
 

import re

#declaring a string variable
str = "Amdani athani kharcha rupaiya."

#using re.sub() function to replace all occurrences of 'a' with '$'
modified_str = re.sub("a", "$", str.lower())

#print("Modified string : ")
print(modified_str)

$md$ni $th$ni kh$rch$ rup$iy$.

Time Complexity: O(n)
Auxiliary Space: O(n)

Method 4: Using list comprehension:

This approach uses a list comprehension to iterate over each character in the lowercase version of the input string str. If the character is equal to 'a', it is replaced with a '$' symbol. Otherwise, the original character is used. The resulting list of characters is then joined back into a string using the join() method.

# declaring a string variable
str = "Amdani athani kharcha rupaiya."

# using list comprehension to replace all occurrences of 'a' with '$'
modified_str = ''.join(['$' if c == 'a' else c for c in str.lower()])

# print modified string
print(modified_str)

$md$ni $th$ni kh$rch$ rup$iy$.

Time complexity: O(n), where n is the length of the input string.
Auxiliary space: O(n).