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C Program to Replace a Word in a Text By Another Given Word

Last Updated : 11 Mar, 2023
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Given three strings ‘str’, ‘oldW’ and ‘newW’. The task is find all occurrences of the word ‘oldW’ and replace then with word ‘newW’. Examples:

Input : str[] = "xxforxx xx for xx", 
        oldW[] = "xx", 
        newW[] = "geeks"
Output : geeksforgeeks geeks for geeks

The idea is to traverse the original string and count the number of times old word occurs in the string. Now make a new string of sufficient size so that new word can be replaced. Now copy original string to new string with replacement of word. 



// C program to search and replace
// all occurrences of a word with
// other word.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
// Function to replace a string with another
// string
char* replaceWord(const char* s, const char* oldW,
                const char* newW)
    char* result;
    int i, cnt = 0;
    int newWlen = strlen(newW);
    int oldWlen = strlen(oldW);
    // Counting the number of times old word
    // occur in the string
    for (i = 0; s[i] != '\0'; i++) {
        if (strstr(&s[i], oldW) == &s[i]) {
            // Jumping to index after the old word.
            i += oldWlen - 1;
    // Making new string of enough length
    result = (char*)malloc(i + cnt * (newWlen - oldWlen) + 1);
    i = 0;
    while (*s) {
        // compare the substring with the result
        if (strstr(s, oldW) == s) {
            strcpy(&result[i], newW);
            i += newWlen;
            s += oldWlen;
            result[i++] = *s++;
    result[i] = '\0';
    return result;
// Driver Program
int main()
    char str[] = "xxforxx xx for xx";
    char c[] = "xx";
    char d[] = "Geeks";
    char* result = NULL;
    // oldW string
    printf("Old string: %s\n", str);
    result = replaceWord(str, c, d);
    printf("New String: %s\n", result);
    return 0;


Old string: xxforxx xx for xx
New String: GeeksforGeeks Geeks for Geeks

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

Method 2: This method involves the in-place update of the string. It is more efficient as it uses only extra space for the new characters to be inserted. 



// C Program to replace a word in a text by another given
// word by inplace updation
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void replaceWord(char* str, char* oldWord, char* newWord)
    char *pos, temp[1000];
    int index = 0;
    int owlen;
    owlen = strlen(oldWord);
    // Repeat This loop until all occurrences are replaced.
    while ((pos = strstr(str, oldWord)) != NULL) {
        // Bakup current line
        strcpy(temp, str);
        // Index of current found word
        index = pos - str;
        // Terminate str after word found index
        str[index] = '\0';
        // Concatenate str with new word
        strcat(str, newWord);
        // Concatenate str with remaining words after
        // oldword found index.
        strcat(str, temp + index + owlen);
int main()
    char str[1000], oldWord[100], newWord[100];
    printf("Enter the string: ");
    printf("Enter the word to be replaced: ");
    printf("Replace with: ");
    replaceWord(str, oldWord, newWord);
    printf("\nModified string: %s", str);
    return 0;


xxforxx xx for xx

geeksforgeeks geeks for geeks

Time Complexity: O(n)
Auxiliary Space: O(1)

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