我试图理解的字符串指针行为

Question

I have written a replacechar function which replaces an instance of a source char with a replacement char. The function works in that the string is changed as expected but when I attempt to use the return value of the function, puts just outputs a blank line.

Can someone please explain what is happening and what I would need to change in replacechar to fix.

#include <stdio.h>  /* puts */
#include <string.h> /* strcpy */
#include <stdlib.h> /* malloc, free */

char* replacechar(char* s, char ch1, char ch2) {
   while (*s) {
      if (*s == ch1)
         *s = ch2;

      *s++;
   }

   return s;
}

int main()
{
   char* s = malloc(8);

   strcpy(s, "aarvark");

   puts(replacechar(s, 'a', 'z')); /* prints blank line */

   puts(s);  /* prints zzrvzrk as expected */

   free(s);

   return 0;
}

Thanks for all the responses.

I have changed to this (which now works fine).

char* replacechar(char* s, char ch1, char ch2) {
   char* p = s;
   while (*p) {
      if (*p == ch1)
         *p = ch2;

      p++;
   }

   return s;
}

Answer 1

It returns the value of the s pointer once it's been incremented past the end of the string. Make a local variable in replacechar() , and increment it, and return the original value of s .

Answer 2

Thats because the while loop in replacechar increment s until it's \\0 . At the end of the function you're returning the pointer, which points to \\0 and printing \\0 is a blank line. You should manage it like this:

char *replacechar(char *s, char ch1, char ch2) {
    char *start = s;
    ...
    return start;
}

Answer 3

Your issue here, with otherwise pretty nice looking code, is one of variable scope.

Use a different local char* within replacechar, like

char* replacechar(char* s, char ch1, char ch2) {
   char* tmpstr;

   tmpstr=s;
   while (*tmpstr) {
      if (*tmpstr == ch1)
         *tmpstr = ch2;

      tmpstr++;  /* Note, no "*" here as in your code.  */
   }

   return s; /* s has remained unchanged */
}

Answer 4

#include <stdio.h>  /* puts */
#include <string.h> /* strcpy */
#include <stdlib.h> /* malloc, free */

char* replacechar(char* s, char ch1, char ch2) {
   char* t = s;
   while (*s) {
      if (*s == ch1)
         *s = ch2;

      *s++;
   }

  return t;
}

int main()
{
   char* s = malloc(8);

   strcpy(s, "aarvark");

   puts(replacechar(s, 'a', 'z')); /* prints blank line */

   puts(s);  /* prints zzrvzrk as expected */

   free(s);

   return 0;
}