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C program to Replace a word in a text by another given word
  • Difficulty Level : Hard
  • Last Updated : 25 Jul, 2020

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]) {
            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

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






#include <bits/stdc++.h>
using namespace std;
int main()
{
    // code
    int t;
    cin >> t;
    cin.ignore();
  
    while (t--) {
        string s;
        getline(cin, s);
  
        string x, y;
        getline(cin, x);
        getline(cin, y);
  
        reverse(s.begin(), s.end());
        reverse(x.begin(), x.end());
        reverse(y.begin(), y.end());
  
        int ls = s.length(), lx = x.length(), ly = y.length();
        int d = ly - lx;
        int ct = 0;
        int i = 0, j = 0;
  
        while (i < ls) {
            string temp = "";
            for (int k = 0; k < lx; k++) {
                temp += s[i + k];
            }
  
            if (temp == x) {
                ct++;
                i = i + lx;
            }
            else {
                i = i + 1;
            }
        }
  
        for (int i = 0; i < ct * d; i++)
            s += ' ';
  
        i = ls - 1;
        j = ls + ct * d - 1;
  
        while (i >= 0 && j >= 0) {
            string temp = "";
            for (int k = 0; k < lx; k++) {
                if (i < (lx - 1 - k))
                    break;
  
                temp += s[i - (lx - 1 - k)];
            }
  
            if (temp == x) {
                int k = ly - 1;
                while (k >= 0)
                    s[j--] = y[k--];
  
                i = i - lx;
            }
            else {
                s[j--] = s[i--];
            }
        }
        reverse(s.begin(), s.end());
        cout << s << endl;
    }
    return 0;
}
Input:
1
xxforxx xx for xx
xx
geeks
Output:
geeksforgeeks geeks for geeks

This article is contributed by Anuj Chauhan. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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