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.?

Answer 1

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