C program to replace all occurrence of a character in a string

In C programming, replacing characters in a string is a common string manipulation task. This involves searching for a specific character and replacing it with another character either for all occurrences or just the first occurrence.

Syntax

for(i = 0; string[i] != '\0'; i++) {
    if(string[i] == old_char) {
        string[i] = new_char;
    }
}

Method 1: Replace All Occurrences

This method replaces every occurrence of a character in the string −

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

int main() {
    char string[100], ch1, ch2;
    int i;
    
    printf("Enter a string: ");
    fgets(string, sizeof(string), stdin);
    string[strcspn(string, "<br>")] = '\0'; /* Remove newline */
    
    printf("Enter character to search: ");
    scanf("%c", &ch1);
    getchar(); /* Clear buffer */
    
    printf("Enter replacement character: ");
    scanf("%c", &ch2);
    
    for(i = 0; string[i] != '\0'; i++) {
        if(string[i] == ch1) {
            string[i] = ch2;
        }
    }
    
    printf("String after replacing '%c' with '%c': %s<br>", ch1, ch2, string);
    return 0;
}

Enter a string: Tutorials Point
Enter character to search: i
Enter replacement character: %
String after replacing 'i' with '%': Tutor%als Po%nt

Method 2: Replace First Occurrence Only

This method replaces only the first occurrence of a character and stops −

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

int main() {
    char string[100], ch1, ch2;
    int i;
    
    printf("Enter a string: ");
    fgets(string, sizeof(string), stdin);
    string[strcspn(string, "<br>")] = '\0'; /* Remove newline */
    
    printf("Enter character to search: ");
    scanf("%c", &ch1);
    getchar(); /* Clear buffer */
    
    printf("Enter replacement character: ");
    scanf("%c", &ch2);
    
    for(i = 0; string[i] != '\0'; i++) {
        if(string[i] == ch1) {
            string[i] = ch2;
            break; /* Stop after first replacement */
        }
    }
    
    printf("String after replacing first '%c' with '%c': %s<br>", ch1, ch2, string);
    return 0;
}

Enter a string: Tutorial Point
Enter character to search: o
Enter replacement character: #
String after replacing first 'o' with '#': Tut#rial Point

Key Points

• Use fgets() instead of gets() for safer string input
• Remove newline character from fgets() input using strcspn()
• Use getchar() to clear input buffer after scanf()
• The break statement in Method 2 stops after the first replacement

Conclusion

Character replacement in strings can be done efficiently using simple loops. Choose Method 1 for replacing all occurrences or Method 2 for replacing only the first occurrence based on your requirements.