replace a substring with another string in C

kaz2100 · 04-24-2007, 04:42 PM

Ciao,

Code:

char *stringa_sost(char *buffer3, char *buffer1, char *buffer2)

is a definition. And when you call it

Code:

stringa_sost(char buffer3[1000][1000], char buffer1[256], char buffer2[]);

INCONSISTANT!! You do not need type identifier when you call function.

I do not know anything about your compiler, dose it work?

ozmosis · 04-25-2007, 06:21 AM

delete!!!!

reverse · 04-25-2007, 08:31 AM

There are a couple of problems with your code, so I'll try to say some things about some of them.

#1.

Code:

  strncpy(buffer5, buffer3, p-buffer3); /*???*/
  buffer5[p-buffer3] = '\0';

Looking at that comment it would seem you have no idea what the two instructions do. Do you understand what strncpy() does? I'll paste an excerpt from the manual page of strcpy, on my Gentoo Linux system:

Quote:

char *strcpy(char *dest, const char *src);

char *strncpy(char *dest, const char *src, size_t n);

DESCRIPTION
The strcpy() function copies the string pointed to by src (including the terminat-
ing `\0' character) to the array pointed to by dest. The strings may not overlap,
and the destination string dest must be large enough to receive the copy.

The strncpy() function is similar, except that not more than n bytes of src are
copied. Thus, if there is no null byte among the first n bytes of src, the result
will not be null-terminated.

#2. Another thing:

Code:

  char buffer1 [256];	/*prima stringa da acquisire da cercare*/
  char buffer2 [256];	/*seconda stringa da sostituire alla prima stringa trovata*/
  char buffer3 [1000];	/*per contenere il testo nel file1*/

Yet:

Code:

  char buffer5[4096];

buffer5 seems to be too big. Why not allocate it dynamically? If you don't want to do this but decide to fine tune the size to a lower value .. make sure it's not so low as to allow buffer overflows to occur.

#3. I think this is what kaz2100 meant.. Your function returns a pointer to char. However you do nothing with this return value:

Code:

  stringa_sost(buffer3, buffer1, buffer2);

^^ That is the actual call to the function. As you can see, stringa_sost() could be of type void for all main() cares. I'm thinking: you probably think that by calling:

Code:

stringa_sost(string, substring-to-replace, string-to-replace-with);

"string" will be modified. In fact it isn't.

Code:

(kind of pseudo-code:)

string = "abcXXXdef";
substring-to-replace = "XXX";
string-to-replace-with = "YYY";

stringa_sost(string, substring-to-replace, string-to-replace-with);

-- At this point, you think: string = "abcYYYdef". In fact, it isn't. string = "abcXXXdef".

However: newstring = stringa_sost(string, substring-to-replace, string-to-replace-with);
Then: newstring = "abcYYYdef".

#4. You should develop the habit of checking the return values of functions. I.e. fopen(), fgets() etc.

ozmosis · 04-25-2007, 04:16 PM

Thanks a lot for your interest.

paulhope123 · 07-07-2009, 02:33 PM

Hi All,

The problem that I found with the original code was that whenever you attempt to replace a substring that's shorter in lenght that the new string, the buffer overflows and the logic crashes. I have added additional logic to fix this problem should you ever encounter such a problem.

char *replace(char *st, char *orig, char *repl)
{
static char buffer[4096];
static char buffer2[4096];

char *ch;

if (!(ch = (char*)strstr(st, orig)))
return (char*)st;

strncpy(buffer, st, ch-st);
strncpy(buffer2, st, ch-st+strlen(orig));

buffer[ch-st] = '\0';
buffer2[ch-st+strlen(orig)] = '\0';

if (strlen(orig)>=strlen(repl))
{
sprintf(buffer+(ch-st), "%s%s", repl, ch+strlen(orig));
}
else
{
sprintf(buffer2+(ch-st), "%s%s", repl, ch+strlen(orig));
strcpy(buffer, buffer2, strlen(buffer));
}

return buffer;
}

dhondi · 11-09-2009, 08:16 AM

Thanks, well done

Quote:

Originally Posted by itsme86

Code:

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

char *replace_str(char *str, char *orig, char *rep)
{
  static char buffer[4096];
  char *p;

  if(!(p = strstr(str, orig)))  // Is 'orig' even in 'str'?
    return str;

  strncpy(buffer, str, p-str); // Copy characters from 'str' start to 'orig' st$
  buffer[p-str] = '\0';

  sprintf(buffer+(p-str), "%s%s", rep, p+strlen(orig));

  return buffer;
}

int main(void)
{
  puts(replace_str("Hello, world!", "world", "Miami"));

  return 0;
}

itsme@dreams:~/C$ ./repstr
Hello, Miami!
itsme@dreams:~/C$

eric.r.turner · 11-09-2009, 09:59 PM

Quote:

Originally Posted by ozmosis

delete!!!!

Dude, you deleted 6 posts on this forum! What's going on???