problem with pointers and strings (replace pattern program)

Hi guys i need your help for this replace_pattern program.

This is a program that takes a string
i.e.:"The boy was sitting in the room"
then it takes a string to compare
i.e.:"the"
Finally it takes a string to replace the second string
i.e:"a"

The final result is :"a boy was sitting in a room"

I must use this function:
char *replace_pattern(char *s, char *p ,char *np,int firstOnly);

Here is my program ...(i cant find what is wrong)
The bug is in replace_pattern function

Code:

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

char *replace_pattern(char *s, char *p ,char *np,int firstOnly);

//this funct allocates dynamicly memory for the three strings
//s,p and np
char *mem_alloc(char *buffer);

char buffer_str[81];
char *cur_str,*same_str,*repl_str,*new_str;

int firstOnly,c;

int main(void){
   
   char *str_ptr;
  
   printf("Enter a string and then press enter...\n");
 
   while( *(str_ptr=gets(buffer_str)) == NULL ) 
   printf("try again...\n");

   cur_str=mem_alloc(buffer_str);
  	cur_str=buffer_str;

	printf("Enter a string to compare,then press enter...\n"); 
	while( *(str_ptr=gets(buffer_str)) == NULL )
   
   same_str=mem_alloc(buffer_str);
   same_str=buffer_str;
   
 	printf("Enter a string to replace,then press enter...\n");  
	while( *(str_ptr=gets(buffer_str)) == NULL )
	printf("try again...\n");
	
   repl_str=mem_alloc(buffer_str);
   repl_str=buffer_str;
   
   printf("\nEnter 0 if you want to check the whole string...");
   printf("\nor enter a different number if you want to find");
   printf("\nand replace the 1st similarity:");
   c=scanf("%d",&firstOnly);
   
   while(c!=1){
	   while((c=getchar())!='\n');
	   c=scanf("%d",&firstOnly);
	   }
	  
	  new_str=replace_pattern(cur_str, same_str ,repl_str, firstOnly);
 return 0;
}

/*memory allocation for the strings*/

Code:

char *mem_alloc(char *buffer)
{
	char *str;
	
	   str=(char *)malloc((strlen(buffer)+1)); 
	   if(str==NULL){
	   printf("\n memory allocation error! ");
		printf("\n exiting now... ");
		exit(1);
		}
		else return(str);  
}

Replace Pattern function *buggy one*
/* *s is the given string, *p is a string to search in *s,
if we find *p string in *s string we replace with *np string

if firstOnly !=0 i must replace only the first substring with np
else i must replace substrings with np string in the whole string*/

Code:

char *replace_pattern(char *s, char *p ,char *np)
{
char *start_ptr;
int counter=0,i=0,j=0;
	
	counter=strlen(p);
	while((start_ptr=strstr(s,p))!=NULL){
	  
	  while(s<start_ptr)
	  
	  buffer_str[i++]=*s++;
 	  while(j<counter)
	  buffer_str[i++]=*np++;
	  s+=counter;
   }
   buffer_str[i++]='\0';	
	return buffer_str;
}

Why are you using a global variable in your function? It makes it annoyingly hard to follow when you have to look all over to see what a given variable actually is. Rethink the process:

Code:

count the occurances of the word to replace
newstring = strlen oldstring - word to replace * occurances + strlen newword * occurances + 1
allocate space for newstring
while not at the end of the big string
    while not at the first letter of the word to replace
        copy characters into new string
    copy in one occurance of new word
    increment past word to replace in old string

Something along those lines.

Quzah.

but you dont help a lot

i tried but no results

some source code will help a lot

>>some source code will help a lot
Then try writing some, and post it here when it doesn't do what you want.

Here's a couple of pointers
http://faq.cprogramming.com/cgi-bin/...&id=1043284385
http://faq.cprogramming.com/cgi-bin/...&id=1043284385

> same_str=mem_alloc(buffer_str);
> same_str=buffer_str;

You do realize you just created a memory leak, right? You allocated some memory for same_str, then pointed it back to buffer_str, thereby losing the original memory.

> repl_str=mem_alloc(buffer_str);
> repl_str=buffer_str;

Oops, you just did it again. Try strcpy() instead:
same_str=mem_alloc(buffer_str);
strcpy( same_str, buffer_str );

You should also free same_str and repl_str once you're finished using them:
free(same_str);
free(repl_str);

Code:

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

void replace ( char *string, char *find ) {
    char *p = strstr( string, find );
    if ( p != NULL ) {
        printf( "Left  Substring='%.*s'\n", p-string, string );
        printf( "Match Substring='%s'\n", find );
        printf( "Right Substring='%s'\n", p+strlen(find) );
    }
}

int
main(void)
{
    replace( "can you find the floogle", "find" );
    return 0;
}

I suggest you write a function which makes ONE substitution in a string, and returns a boolean indicating whether a substitution was made.

Then write another function to call it either
- once (firstOnly = true)
- until it returns false (firstOnly = false)