Thread: qsort

Hi all,

i was reading in a book how to use the function qsort. the function needs a pointer to a compare function comp.
sth like this was done :

Code:

char *data[MAX]; // memory was allocated and so on
qsort(data, MAX, sizeof(data[0]), comp); // the call of qsort
.
.
.
int comp(const void *s1, const void *s2)
{
    return (strcmp(*(char **)s1, *(char **)s2)); // what i don't understand
}

why in our exaplme we were using so many "*"
i understand that (char **) is a typecasting, and the other one for dereferencing the pointer...

so far when i wanted to compare two strings pointed to by ptr1 and ptr2 i used
strcmp(ptr1, ptr2).

i mean the comp function will be given pointers to strings, why not only :

Code:

 return (strcmp((char *)s1, (char *)s2)); // typecasting from void* to char*

hmm, my comp function wil be given 2 void pointers.
1. i need to typecast to int --> (int *)
2. i need the value in the adress pointed to by the pointer (*)
so :

Code:

int comp(const void *a, const void *b)
{
    retrun(*(int *)a - *(int *)b)
}

Originally Posted by Scatman

hmm, my comp function wil be given 2 void pointers.
1. i need to typecast to int --> (int *)
2. i need the value in the adress pointed to by the pointer (*)
so :

Code:

int comp(const void *a, const void *b)
{
    retrun(*(int *)a - *(int *)b)
}

Notice: You get the pointers to two elements from the original array. If you are dealing with an array containing pointers to arrays of chars (strings), these void pointers need to be cast to char **.

Code:

int comp( const void *a, const void *b )
{
  const char **pa, **pb;

  pa = ( const char **)a;
  pb = ( const char **)b;

  return strcmp( *pa, *pb );
}

Originally Posted by Salem

It's easier if you just declare some intermediate variables.

Code:

int compare ( const void *a, const void *b ) {
  const int *pa = a;
  const int *pb = b;
  // Note *pa - *pb may fail with numeric underflow
  if ( *pa < *pb ) return -1;
  if ( *pa > *pb ) return -1;
  return 0;
}

Both return -1 ?