C Program to swap first occurrence of a character in a String

In this article, we will write a C program to swap first occurrence of a character in a String with the given character.

C Program to swap first occurrence of character with another character

A brief explanation of the statements is provided in the program itself using comments. You can find the detailed explanation at the end of this article.

#include <stdio.h>
#include <string.h>

// It swaps the first occurrence of char1 with char2 in the string str
void swap_first_occurrence(char *str, char char1, char char2) {
    // Get the length of the string
    int len = strlen(str); 

    // Iterate though the string str to find the first occurrence of char1
    for (int i = 0; i < len; i++) {
        if (str[i] == char1) {
            // Replace char1 with char2
            str[i] = char2; 
            break; // Exit the loop after the first occurrence is replaced
        }
    }
}

int main() {
    // It stores the string entered by user
    char str[100]; 

    //char1: character that needs to be replaced.
    //char2: character that will replace char1.
    char char1, char2;

    // Read the input string
    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);
    // Remove the newline character at the end of the string if present
    str[strcspn(str, "\n")] = '\0';

    // Get the character from user that needs to be swapped out
    printf("Enter the character to be swapped out: ");
    scanf(" %c", &char1);

    // Get the character from user that needs to be swapped in
    printf("Enter the character to be swapped in: ");
    scanf(" %c", &char2);

    // Call the function to swap the first occurrence of char1 with char2
    swap_first_occurrence(str, char1, char2);

    // Print the modified string
    printf("Modified string: %s\n", str);

    return 0;
}

Output:

Enter a string: hello world
Enter the character to be swapped out: o
Enter the character to be swapped in: a
Modified string: hella world

Explanation of the program

1. Header files:
   • #include <stdio.h>: Includes the standard input/output library for printf and scanf functions.
   • #include <string.h>: Includes the string handling library.
2. swap_first_occurrence() Function:
   • int len = strlen(str); It calculates the length of the input string.
   • It runs a for loop to iterate through the string to find the first occurrence of char1:
     • if (str[i] == char1): Checks if the current character is char1.
     • If char1 is found, replace it with char2.
     • User break statement to terminate the loop after replace is done.
3. main Function:
   • char str[100];: This char array is used to hold the input string.
   • char char1, char2;: Both of these variables are entered by user, the first occurrence of char1 is need to be replaced with char2.
   • fgets(str, sizeof(str), stdin);: Reads the input string from the user.
   • str[strcspn(str, "\n")] = '\0';: Removes the newline character from the input string if it is present.
   • scanf(" %c", &char1); and scanf(" %c", &char2);: Get the characters value from the user. The space before %c ensures that any leading whitespace characters are ignored.
   • swap_first_occurrence(str, char1, char2);: Calls the function to swap the first occurrence of char1 with char2.
   • printf("Modified string: %s\n", str);: Prints the modified string.