Xref: utzoo comp.lang.c:25888 comp.unix.xenix:10062
Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uunet!bii!gegu
From: gegu@bii.UUCP (Gerhard Gucher)
Newsgroups: comp.lang.c,comp.unix.xenix
Subject: Re: qsort() - HELP
Summary: qsort - HELP
Message-ID: <275@bii.UUCP>
Date: 13 Feb 90 15:10:12 GMT
References: <5916@ozdaltx.UUCP>
Followup-To: comp.lang.c comp.unix.xenix
Organization: Bruker Instruments Inc., Billerica MA.
Lines: 69

In article <5916@ozdaltx.UUCP>, root@ozdaltx.UUCP (root) writes:
> After going over the manuals more times than I'd like, I still can't
> figure out how to get qsort() (S) to operate.  The example shows
> something like:
> 
> 	void qsort(base, nel, width, compar)
> 	char *base
> 	unsigned nel, width
> 	int (*compar)()
> 
> Is base supposed to be an array?
> nel & width are self-explanitory
> What is compar() looking for or how is it assigned?
> 
> 
> The objective is to sort an array of strings in alpha order and then
> be able to read them.  So far I'm getting is core dumps.
> 

qsort is a little bit tricky for arrays of strings. compar() will
get the address of two items to be sorted as arguments. Since your
arguments are strings (char*'s) you must supply a compare function
which accepts (char**)'s as args.
The Example has been tested on a SUN OS 4.03 cc compiler.

Example:

#define NUM_ELEM(a)	( sizeof(a)/sizeof(a[0]) )

/* array to be sorted */
char * array[] = { "beta", "gamma", "alpha", "delta", "epsilon" };

extern int qsort();

/****************************************************************/
int ind_strcmp(s,t)	/* indirect string compare for qsort */
char** s;
char** t;
{
  return( strcmp(*s,*t) );
}


/****************************************************************/
/* ARGSUSED */
int
main(argc,argv)
int argc;
char *argv[];
{
  int i;

  (void) qsort( (char*)(&array[0]), 
         NUM_ELEM(array), 
         sizeof(array[0]),
         ind_strcmp		/* gets the address of the char ptrs */
  );

  /* print the sorted array */
  for( i=0; i< NUM_ELEM(array); i++ )
    (void)printf( "%s\n", array[i] );

} /* main() */

--
+------------------------------------------------------------------+
| Gerry Gucher    uunet!bii!gegu       gegu@bii.bruker.com	   |
| Bruker Instruments Inc. Manning Park Billerica MA 01821          | 
+------------------------------------------------------------------+