qs

Algorithms for Image Processing and Computer Vision Source Code

@c ----------------------------------------------------------------------
@node qsort, misc
@heading @code{qsort}
@subheading Syntax

@example
#include <stdlib.h>

void qsort(void *base, size_t numelem, size_t size,
           int (*cmp)(const void *e1, const void *e2));
@end example

@subheading Description

This function sorts the given array in place.  @var{base} is the address
of the first of @var{numelem} array entries, each of size @var{size}
bytes.  @code{qsort} uses the supplied function @var{cmp} to determine
the sort order for any two elements by passing the address of the two
elements and using the function's return address. 

The return address of the function indicates the sort order:

@table @asis

@item Negative

Element @var{e1} should come before element @var{e2} in the resulting
array. 

@item Positive

Element @var{e1} should come after element @var{e2} in the resulting
array. 

@item Zero

It doesn't matter which element comes first in the resulting array. 

@end table

@subheading Return Value

None.

@subheading Example

@example
typedef struct @{
  int size;
  int sequence;
@} Item;

int qsort_helper_by_size(void *e1, void *e2)
@{
  return ((Item *)e2)->size - ((Item *)e1)->size;
@}

Item list[100];

qsort(list, 100, sizeof(Item), qsort_helper_by_size);

int qsort_stringlist(void *e1, void *e2)
@{
  return strcmp(*(char **)e1, *(char **)e2);
@}

char *slist[10];

/* alphabetical order */
qsort(slist, 10, sizeof(char *), qsort_stringlist);

@end example