Handling Strings

This section brings the whole chapter together: user-defined functions that perform practical operations on strings. Each program below defines a function, reads input from the user, calls the function, and prints the result. Together they exercise traversal, membership testing, negative indexing, concatenation and comparison of characters.

Program 8-1 — Count occurrences of a character

Task: a user-defined function that counts how many times a character (passed as an argument) occurs in a given string.

The function traverses the string with a for loop and increments a counter each time the current character equals the one being searched for.

# Function to count how many times a character occurs in a string
def charCount(ch, st):
    count = 0
    for character in st:
        if character == ch:
            count += 1
    return count
# end of function

st = input("Enter a string: ")
ch = input("Enter the character to be searched: ")
count = charCount(ch, st)
print("Number of times character", ch, "occurs in the string is:", count)

Enter a string: Today is a Holiday
Enter the character to be searched: a
Number of times character a occurs in the string is: 3

Program 8-2 — Replace all vowels with *

Task: a user-defined function taking a string as parameter that replaces every vowel in it with the character '*'.

Because strings are immutable (Section 8.2.2), the function cannot overwrite characters in place. Instead it builds a new string: starting from an empty string, it appends either '*' (if the current character is a vowel) or the character itself. The vowel test uses the membership operator in against the string 'aeiouAEIOU', which covers both cases at once.

# Function to replace all vowels in the string with '*'
def replaceVowel(st):
    newstr = ''                        # create an empty string
    for character in st:
        if character in 'aeiouAEIOU':  # is it a vowel?
            newstr += '*'              # replace vowel with *
        else:
            newstr += character
    return newstr
# end of function

st = input("Enter a String: ")
st1 = replaceVowel(st)
print("The original String is:", st)
print("The modified String is:", st1)

Enter a String: Hello World
The original String is: Hello World
The modified String is: H*ll* W*rld

Note that the original string survives unchanged — the function returned a fresh string.

Program 8-3 — Print a string in reverse (no new string)

Task: input a string and print it in reverse order without creating a new string.

The trick is negative indexing: range(-1, -len(st)-1, -1) generates the indices -1, -2, ..., -len(st) — i.e., the characters from the rightmost to the leftmost. Printing each with end='' displays the reversed text without ever building a second string.

# Program to display a string in reverse order
st = input("Enter a string: ")
for i in range(-1, -len(st) - 1, -1):
    print(st[i], end='')

Enter a string: Hello World
dlroW olleH

Program 8-4 — Reverse a string into a new string

Task: a user-defined function that reverses the string passed as parameter and stores the reversed string in a new string.

The traversal logic is the same negative-index range as Program 8-3, but instead of printing each character the function concatenates it onto a new string, which it then returns.

# Function to reverse a string
def reverseString(st):
    newstr = ''              # create a new string
    length = len(st)
    for i in range(-1, -length - 1, -1):
        newstr += st[i]
    return newstr
# end of function

st = input("Enter a String: ")
st1 = reverseString(st)
print("The original String is:", st)
print("The reversed String is:", st1)

Enter a String: Hello World
The original String is: Hello World
The reversed String is: dlroW olleH

Program 8-5 — Check whether a string is a palindrome

Task: a user-defined function to check if a string is a palindrome — a string that reads the same backwards as forwards (for example, kanak).

The function uses two indices moving towards each other: i starts at the first character and j at the last. At each step it compares st[i] with st[j]; the moment any pair differs, the string cannot be a palindrome and the function returns False immediately. If the two indices cross without finding a mismatch, every pair matched and the function returns True.