C program to Replace a word in a text by another given word

In this program, we have given three strings: text, oldword, and newword. Our task is to create a C program to replace a word in a text by another given word.

The program will search for all the occurrences of the oldword in the text and replace it with newword.

Syntax

void replaceWordInText(const char *text, const char *oldWord, const char *newWord);

Example: Input and Expected Output

Let's take an example to understand the problem −

Input:

text = "I am learning programming"
oldword = "learning"
newword = "practicing"

Output:

"I am practicing programming"

Algorithm

To solve this problem, we will first count the occurrences of the oldword in the string, then create a new string with sufficient memory to store the text with replaced words.

Example: Complete Program

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

void replaceWordInText(const char *text, const char *oldWord, const char *newWord) {
    int i = 0, cnt = 0;
    int len1 = strlen(newWord);
    int len2 = strlen(oldWord);
    
    // Count occurrences of oldWord
    for (i = 0; text[i] != '\0'; i++) {
        if (strstr(&text[i], oldWord) == &text[i]) {
            cnt++;
            i += len2 - 1;
        }
    }
    
    // Allocate memory for new string
    char *newString = (char *)malloc(i + cnt * (len1 - len2) + 1);
    if (newString == NULL) {
        printf("Memory allocation failed
");
        return;
    }
    
    i = 0;
    while (*text) {
        if (strstr(text, oldWord) == text) {
            strcpy(&newString[i], newWord);
            i += len1;
            text += len2;
        }
        else {
            newString[i++] = *text++;
        }
    }
    
    newString[i] = '\0';
    printf("New String: %s
", newString);
    free(newString);
}

int main() {
    char str[] = "I am learning programming";
    char oldWord[] = "learning";
    char newWord[] = "practicing";
    
    printf("Original string: %s
", str);
    replaceWordInText(str, oldWord, newWord);
    
    return 0;
}

Output

Original string: I am learning programming
New String: I am practicing programming

How It Works

• First, the program counts how many times the oldWord appears in the text using strstr().
• It calculates the required memory for the new string based on the difference in lengths between old and new words.
• Memory is allocated dynamically using malloc() with proper NULL checking.
• The program iterates through the original text, replacing occurrences of oldWord with newWord.
• Finally, it prints the modified string and frees the allocated memory.

Key Points

• The function uses strstr() to find word occurrences in the text.
• Dynamic memory allocation ensures the new string has sufficient space.
• Always check for NULL after malloc() and use free() to prevent memory leaks.
• The algorithm handles multiple occurrences of the same word correctly.

Conclusion

This C program efficiently replaces all occurrences of a word in a text string using dynamic memory allocation and string manipulation functions. The approach ensures memory safety and handles variable-length replacements correctly.