Replace a sub-string in a string in C

Understanding String Manipulation

Replacing a sub-string in a string means finding a specific sequence of characters and replacing it with another sequence.

We will explore three different methods to achieve this in C.

Method 1: Using Loop

This method iterates through the string and replaces occurrences of the sub-string.

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

void replaceSubstring(char *str, char *oldWord, char *newWord) {
    char buffer[1000];
    char *pos;
    int index = 0, oldLen = strlen(oldWord);
    
    buffer[0] = '\0';
    while ((pos = strstr(str, oldWord)) != NULL) {
        strncat(buffer, str, pos - str);
        strcat(buffer, newWord);
        str = pos + oldLen;
    }
    strcat(buffer, str);
    strcpy(str, buffer);
}

int main() {
    char str[1000] = "Hello World!";
    replaceSubstring(str, "World", "C");
    printf("Modified String: %s", str);
    return 0;
}
            

Method 2: Using Recursion

This method replaces the sub-string recursively.

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

void replaceRecursive(char *str, char *oldWord, char *newWord, char *buffer) {
    char *pos = strstr(str, oldWord);
    if (!pos) {
        strcat(buffer, str);
        return;
    }
    strncat(buffer, str, pos - str);
    strcat(buffer, newWord);
    replaceRecursive(pos + strlen(oldWord), oldWord, newWord, buffer);
}

int main() {
    char str[1000] = "I love programming";
    char buffer[1000] = "";
    replaceRecursive(str, "love", "enjoy", buffer);
    printf("Modified String: %s", buffer);
    return 0;
}
            

Method 3: Using String Functions

This method uses C's built-in strstr() and strcpy() functions.

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

void replaceUsingFunctions(char *str, char *oldWord, char *newWord) {
    char buffer[1000];
    char *pos = strstr(str, oldWord);
    if (!pos) return;
    
    strncpy(buffer, str, pos - str);
    buffer[pos - str] = '\0';
    strcat(buffer, newWord);
    strcat(buffer, pos + strlen(oldWord));
    strcpy(str, buffer);
}

int main() {
    char str[1000] = "Replace old text";
    replaceUsingFunctions(str, "old", "new");
    printf("Modified String: %s", str);
    return 0;
}