filter continuous duplicate chars in a string

Question

Is there any problems in my code ,or to be improved ?

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

char * filterdupchar(char* s,int n){
    char *tmp2 = s ;
    char *tmp =(char*)malloc(n*sizeof(char));
    for(int j=0;j<n/2;j++){
        int i=0,p=0;
        while(i<n){
            *(tmp+p) = s[i];
            if(s[i]==s[i+1])
                i = i+2;
            else
                i++;
            p++;
        }
    s = tmp;
   }
    s = tmp2;
    return tmp;
}
int main(){
     char * str= "bbaaan";
     printf("%s",str);
     printf("\n");
     char * strnew;
     strnew = filterdupchar(str,6);
     printf("%s",strnew);
     return 1;
}

The result should be "ban", but it is appended to something disordered character. In my func , is it necessary to give tmp2 to s, and do i need to free sth.?

@Purag output:ban+(something confused code), i thought i may not process the end — Roger, Aug 31 '15 at 04:28
It outputted `ban` for me as well. What compiler are you using, what OS, etc? — Purag, Aug 31 '15 at 04:28
windows 64bit, visual studio , and where should i free tmp , or i free(strnew) instead ? — Roger, Aug 31 '15 at 04:30
At last there r only 3 chars , but the tmp have more space , the remained space is filled with sth. else. — Roger, Aug 31 '15 at 04:34
You use the return value in `main`, so you should free it in main. — Purag, Aug 31 '15 at 04:36
@Lundin I ran the code and it worked; if the close vote was misplaced it'll expire soon enough anyway. :) — Purag, Aug 31 '15 at 11:17
@Purag You can always withdraw your close vote. The bugs are standard beginner ones, the accepted answer points all of them out, plus some style issues. One should be able to spot those bugs without running the code... Likely the program works on your computer because you are using a debug build which zero-out all variables and so by luck the string happened to appear null terminated on your system. — Lundin, Aug 31 '15 at 11:23
@Lundlin I thought I saw an explicit assignment for the null terminator, but I think I was confusing this question with one I had just seen. I probably just got lucky with a zero somewhere in the malloc'd space. Only ran it once. — Purag, Aug 31 '15 at 11:32

amdixon · Accepted Answer · 2015-08-31T04:54:55.560

changes

only need single loop and lag character to do the filter ( above was unnecessarily complicated )
need to allocate number of characters + 1 ( for nullchar ) for tmp..
malloc cast not required
added nullchar to end of tmp
main should return 0 on success. otherwise processes consuming this will get confused
free strnew is required..
use valgrind if you have any memory doubts
consecutive calls to print are redundant..

code

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

char * filterdupchar(char* s,int n)
{
  int from,to; char last;
  char *tmp = malloc((n+1)*sizeof(char));
  from = to = 0;
  last = 0;
  for(from = 0; from < n; from++)
  {
    if(s[from] != last)
    {
      tmp[to] = s[from];
      to++;
    }
    last = s[from];
  }
  tmp[to] = '\0';
  return tmp;
}
int main()
{
  char * str= "bbaaan";
  printf("%s\n",str);
  char * strnew;
  strnew = filterdupchar(str,6);
  printf("%s\n",strnew);
  free(strnew);
  return 0;
}

output

$ valgrind ./test
==11346== Memcheck, a memory error detector
==11346== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==11346== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info
==11346== Command: ./test
==11346== 
bbaaan
ban
==11346== 
==11346== HEAP SUMMARY:
==11346==     in use at exit: 0 bytes in 0 blocks
==11346==   total heap usage: 1 allocs, 1 frees, 7 bytes allocated
==11346== 
==11346== All heap blocks were freed -- no leaks are possible
==11346== 
==11346== For counts of detected and suppressed errors, rerun with: -v

notes

always use a memory checker ( like valgrind ) to test for memory leaks

filter continuous duplicate chars in a string

1 Answers1