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

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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.

Implementation:

C

// 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]) { 
            cnt++; 
 
            // 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; 
        } 
        else
            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); 
 
    free(result); 
    return 0; 
} 

Output:

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.

Implementation:

C

// 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: ");
    gets(str);
 
    printf("Enter the word to be replaced: ");
    gets(oldWord);
 
    printf("Replace with: ");
    gets(newWord);
 
    replaceWord(str, oldWord, newWord);
 
    printf("\nModified string: %s", str);
 
    return 0;
}