C Program: Replacing a substring from a string

This is a nice piece of program I wrote a few months ago. One of my students requested me to write this. So, here it is! Hope this might be useful for you…

#1: What the program does:

This program finds all the occurrences of a sub-string within a given string and replaces that with our provided string. Basically there are many similar programs but, most of them lack the ability to find multiple occurrences. So I wrote this keeping in mind that the search string may appear many times in the original string. Here is the algo:

• Get the original, search and replace string from the user.
• Write a user defined function to replace the first occurrence of the search string with the replace string.
• Recursively call the function until there is no occurrence of the search string.

Yes, there are many flaws in this program, like if the search string is a part of the replace string, the function will fail. I will try to remove these bugs when I get enough time. But this program will do the job for you in your college semesters 😉

#2: Source Code:

/**
 ****************************************************|
 * String replace Program                            |
 ****************************************************|
 * Takes three string input from the user
 * Replaces all the occurances of the second string
 * with the third string from the first string
 * @author Swashata
 */

/** Include Libraries */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

/** Define the max char length */
#define MAX_L 4096

/** Prototypes */
void replace (char *, char *, char *);

int main(void) {
    char o_string[MAX_L], s_string[MAX_L], r_string[MAX_L]; //String storing variables

	printf("Please enter the original string (max length %d characters): ", MAX_L);
	fflush(stdin);
	gets(o_string);

	printf("\nPlease enter the string to search (max length %d characters): ", MAX_L);
	fflush(stdin);
	gets(s_string);

	printf("\nPlease enter the replace string (max length %d characters): ", MAX_L);
	fflush(stdin);
	gets(r_string);

	printf("\n\nThe Original string\n*************************************\n");
	puts(o_string);

	replace(o_string, s_string, r_string);

	printf("\n\nThe replaced string\n*************************************\n");
	puts(o_string);

    return 0;
}

/**
 * The replace function
 *
 * Searches all of the occurrences using recursion
 * and replaces with the given string
 * @param char * o_string The original string
 * @param char * s_string The string to search for
 * @param char * r_string The replace string
 * @return void The o_string passed is modified
 */
void replace(char * o_string, char * s_string, char * r_string) {
      //a buffer variable to do all replace things
      char buffer[MAX_L];
      //to store the pointer returned from strstr
      char * ch;

      //first exit condition
      if(!(ch = strstr(o_string, s_string)))
              return;

      //copy all the content to buffer before the first occurrence of the search string
      strncpy(buffer, o_string, ch-o_string);

      //prepare the buffer for appending by adding a null to the end of it
      buffer[ch-o_string] = 0;

      //append using sprintf function
      sprintf(buffer+(ch - o_string), "%s%s", r_string, ch + strlen(s_string));

      //empty o_string for copying
      o_string[0] = 0;
      strcpy(o_string, buffer);
      //pass recursively to replace other occurrences
      return replace(o_string, s_string, r_string);
 }

#3: Output:

So that was it. If you have any better programming approach, then do share with us!