Find and Replace Text Function

**Hammer** · 03-02-2003

>>OK, OK, damn and blast,
>>Rereading your post, you are technically correct
Damn, I'd just written a nice reply too.

Oh well, I've deleted it and I'll make my summary by quoting myself:
>>strncpy() ... isn't guaranteed to write a nul

We're done

**mart_man00** · 03-02-2003

i was about to fix up the replace function up alittle more and i attempt to but the process in a loop in case string search is in src a couple times

Code:

int replace(char *src, char search[], char replace[]) {
    char *value = malloc( strlen(src) - strlen(search) + strlen(replace) +1 );
    
    char *found;
    
    while(found != NULL) { /* never ends, why? */
        found = strstr(src, search);
    }

    printf("%s", value);
    free(value);
    return 0;
}

my while loop never ends, why? i thought the fact that a pointer is never NULL would make this work. unfortunally im doing this in windows again, but if its wrong why wouldnt i get a seg fault for checking out memory thats not mine?

**Hammer** · 03-03-2003

>>found = strstr(src, search);
strchr() is being asked to look for the same thing each time the loop goes round, so it will return the same answer.

Also, the value of found is undefined the first time into the loop.

**mart_man00** · 03-03-2003

strchr() is being asked to look for the same thing each time the loop goes round, so it will return the same answer.

doesnt it increment itself so it can look for a string more than once? can i make it?

**quzah** · 03-03-2003

Originally posted by mart_man00
doesnt it increment itself so it can look for a string more than once? can i make it?

No. It doesn't. No, you can't. That is to say, you can't change the function itself. You can change what you pass to it:

Code:

for( something = strchr( somestring, somechar ); sometest; something = strchr( something +1, somechar ) );

That should work. Initialize it by having the return value of the initialization be stored in 'something'. Use subsequent calls to check 'something', updating it on each call.

Naturally you'll want better error checking than what I use.

Quzah.

**Hammer** · 03-03-2003

>>doesnt it increment itself so it can look for a string more than once?
No, you need to update the pointers you pass to strstr() each time.

Code:

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

int main(void)
{
  char  buf[] = "this is something is blah is OK";
  char  *p;
  
  puts(buf);
  for (p = buf; p = strstr(p, "is"); p++)
    puts(p);

  return(0);
}

/*
 * Output:
this is something is blah is OK
is is something is blah is OK
is something is blah is OK
is blah is OK
is OK
 
*/

[edit]Doh, beaten

**mart_man00** · 03-04-2003

i tried to rewrite my function so it will look more than once, but with no luck. any one see why?

Code:

int replace(char *src, char search[], char replace[]) {
	char *value = malloc(strlen(src) - strlen(search) + strlen(replace) + 1 );
    	char check[ strlen(src) + 1 ];

	char *found = src;
	char *spot = src;
    
    	while(*spot != '\0' && found != NULL) {
		found = strstr(spot, search);
		
		if( strcmp(found, check) != 0 ) {
			printf("%s\n", found);
			
			strncat(value, spot, found - src);
			strcat(value, replace);
			strcat(value, found + strlen(search));

			printf("\t%s\n", value);
		}

		strcpy(check, found);
		++*spot;				
	}
    	
	free(value);
    	
	return 0;
}

**quzah** · 03-04-2003

Code:

char *value = malloc(strlen(src) - strlen(search) + strlen(replace) + 1 );

Never assume passed argument pointers are non-NULL. Always check before using them.

Code:

char check[ strlen(src) + 1 ];

Variable length arrays are not part of the pre-C99 standard. It's doubtful your compiler actually is a C99 compiler. Avoid this.

Code:

strcpy(check, found);
++*spot;

Why are you dereferencing this pointer? You shouldn't be.

Finally, what is the purpose of value? Debugging, or what?

Quzah.

**mart_man00** · 03-04-2003

value is a just a temp varible to work with, it will eventually get copied to src.

im using gcc for this, and it works, just only once. char check[ strlen(src) + 1 ]; doesnt seem right, atleast with my malloc call i get exactly how much i need.

im really not sure what to do here now, im alittle confused. i dont see why this wouldnt work. how would you guys do this?

**quzah** · 03-05-2003

Originally posted by mart_man00
char check[ strlen(src) + 1 ]; doesnt seem right, atleast with my malloc call i get exactly how much i need.

Um... that line of code is from your example. I was pointing out that variable sized arrays, such as the one listed above, is not part of the pre-C99 standard. (IE: It's bad form.)

Quzah.

**mart_man00** · 03-06-2003

can any one show me how they would write this function, im getting now where. i just want it to scan a string for a string and replace ever occurence.

heres my most recent failure

Code:

int replace(char *src, char search[], char replace[]) {
    char *value = malloc( strlen(src) + 1);
        value[0] = '\0';
    char *spot = src;
    char *found = src;
    
    char check[ strlen(src) ];
    
    while(spot != NULL && (found = strstr(spot, search)) != NULL ) {
        if( strcmp(found, check) != 0 ) {
            realloc(value, strlen(found) + 1);  /* Problem here, how do i find the length of everything before found? */
            strncat(value, spot, found - src);

            realloc(value, strlen(replace) + 1);
            strcat(value, replace);
            
            realloc(value, strlen(found) + 1);
            strcat(value, found + strlen(search));
        }
        
        printf("\n%s\n", value);
        strcpy(check, found);
        ++spot;
    }

    free(value);
    strcpy(src, value);
    
    return 0;
}

i think its time to learn by example.............................

im also getting garbage data somewhere.

**Scarlet7** · 03-07-2003

This will search and replace with larger or smaller text. First count the number of replacements then allocate a new buffer from the count, then copy the old buffer+replacement text to the new buffer in the while loop.

Code:

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

void replace(char **newbuffer, char *source, char *search, char *replace) 
{
	int count=0,slen = strlen(search),rlen = strlen(replace);
	char *s1 = source;

	while(s1 = strstr(s1, search))
	{
		count++;
		s1 += slen;
	}
//	char *n1 = *newbuffer = new char[ strlen(source)+((strlen(replace)-slen)* count)+1 ];
         char *n1 = *newbuffer = (char *) malloc( strlen(source)+((strlen(replace)-slen)* count)+1 );
	s1 = source;

	while(s1 = strstr(s1, search))
	{
		strncpy(n1,source, s1-source);
	   	n1 += s1-source;
		strcpy(n1,replace);
		n1 += rlen;
		s1 += slen;
		source = s1;
	}
	strcpy(n1, source );
}

int main() 
{
        char temp2[] = "testing a new idea, testing a new idea, testing a new idea";
	char *newbuffer;
	
        //replacing with larger
	replace(&newbuffer, temp2, "new", "WORKING");
	printf("%s\n", newbuffer);
//	delete [] newbuffer;
         free(newbuffer);

       //replacing with smaller
	replace(&newbuffer, temp2, "new", "X");
	printf("%s\n", newbuffer);
//	delete [] newbuffer;
         free(newbuffer);

         return 0;
}

**quzah** · 03-07-2003

For the record, delete and new are C++ operators, and are not available in C.

Quzah.

**Scarlet7** · 03-07-2003

I was just showing the program structure, but just replace 'new' and 'delete' with the following code 'malloc()' and 'free()':

Code:

char *n1 = *newbuffer = (char *) malloc( strlen(source)+((strlen(replace)-slen)* count)+1 );

free(newbuffer);

I have corrected the post,
Cheers....

troublemaker · 03-08-2003

Thanks Scarlet7,
I have been wrestling with a way to implement replacing of multiple instances of multiple different place holders in a string.
Your way will work very nicely and about ten times more efficiently than what I had been thinking of.

I think it will be something like this:

Code:

struct found {
lpstr placeholder
lpstr replace
lpstr position
}

for (loop through placeholders)
  while _tcsstr(src, placeholders[i]) {
    add replace length to length needed
    add found pointer to array of found structs
  }
}

sort array of found structs  by the position
member so that those
first in the string are first in the array.

allocate required mem for dest

for (loop through array of found structs.) {
  memcpy(dest,src, foundstruct[i].position - src)
  dest+= (.position - src)
  src = .position + strlen(.placeholder)
  memcpy(dest, .replace, strlen(replace) )
  dest += strlen(replace);
}