@node Searching and Sorting, Pattern Matching, Locales, Top
@chapter Searching and Sorting

This chapter describes functions for searching and sorting arrays of
arbitrary objects.  You pass the appropriate comparison function to be
applied as an argument, along with the size of the objects in the array
and the total number of elements.

@menu
* Comparison Functions::        Defining how to compare two objects.
				 Since the sort and search facilities
                                 are general, you have to specify the
                                 ordering.
* Array Search Function::       The @code{bsearch} function.
* Array Sort Function::         The @code{qsort} function.
* Search/Sort Example::         An example program.
@end menu

@node Comparison Functions, Array Search Function,  , Searching and Sorting
@section Defining the Comparison Function
@cindex Comparison Function

In order to use the sorted array library functions, you have to describe
how to compare the elements of the array.

To do this, you supply a comparison function to compare two elements of
the array.  The library will call this function, passing as arguments
pointers to two array elements to be compared.  Your comparison function
should return a value the way @code{strcmp} (@pxref{String/Array
Comparison}) does: negative if the first argument is ``less'' than the
second, zero if they are ``equal'', and positive if the first argument
is ``greater''.

Here is an example of a comparison function which works with an array of
numbers of type @code{double}:

@smallexample
int
compare_doubles (const double *a, const double *b)
@{
  return (int) (*a - *b);
@}
@end smallexample

The header file @file{stdlib.h} defines a name for the data type of
comparison functions.  This type is a GNU extension.

@comment stdlib.h
@comment GNU
@tindex comparison_fn_t
@smallexample
int comparison_fn_t (const void *, const void *);
@end smallexample

@node Array Search Function, Array Sort Function, Comparison Functions, Searching and Sorting
@section Array Search Function
@cindex search function (for arrays)
@cindex binary search function (for arrays)
@cindex array search function

To search a sorted array for an element matching the key, use the
@code{bsearch} function.  The prototype for this function is in
the header file @file{stdlib.h}.
@pindex stdlib.h

@comment stdlib.h
@comment ISO
@deftypefun {void *} bsearch (const void *@var{key}, const void *@var{array}, size_t @var{count}, size_t @var{size}, comparison_fn_t @var{compare})
The @code{bsearch} function searches the sorted array @var{array} for an object
that is equivalent to @var{key}.  The array contains @var{count} elements,
each of which is of size @var{size} bytes.

The @var{compare} function is used to perform the comparison.  This
function is called with two pointer arguments and should return an
integer less than, equal to, or greater than zero corresponding to
whether its first argument is considered less than, equal to, or greater
than its second argument.  The elements of the @var{array} must already
be sorted in ascending order according to this comparison function.

The return value is a pointer to the matching array element, or a null
pointer if no match is found.  If the array contains more than one element
that matches, the one that is returned is unspecified.

This function derives its name from the fact that it is implemented
using the binary search algorithm.
@end deftypefun

@node Array Sort Function, Search/Sort Example, Array Search Function, Searching and Sorting
@section Array Sort Function
@cindex sort function (for arrays)
@cindex quick sort function (for arrays)
@cindex array sort function

To sort an array using an arbitrary comparison function, use the
@code{qsort} function.  The prototype for this function is in
@file{stdlib.h}.
@pindex stdlib.h

@comment stdlib.h
@comment ISO
@deftypefun void qsort (void *@var{array}, size_t @var{count}, size_t @var{size}, comparison_fn_t @var{compare})
The @var{qsort} function sorts the array @var{array}.  The array contains
@var{count} elements, each of which is of size @var{size}.

The @var{compare} function is used to perform the comparison on the
array elements.  This function is called with two pointer arguments and
should return an integer less than, equal to, or greater than zero
corresponding to whether its first argument is considered less than,
equal to, or greater than its second argument.

@cindex stable sorting
@strong{Warning:} If two objects compare as equal, their order after
sorting is unpredictable.  That is to say, the sorting is not stable.
This can make a difference when the comparison considers only part of
the elements.  Two elements with the same sort key may differ in other
respects.

If you want the effect of a stable sort, you can get this result by
writing the comparison function so that, lacking other reason
distinguish between two elements, it compares them by their addresses.
Note that doing this may make the sorting algorithm less efficient, so
do it only if necessary.

Here is a simple example of sorting an array of doubles in numerical
order, using the comparison function defined above (@pxref{Comparison
Functions}):

@smallexample
@{
  double *array;
  int size;
  @dots{}
  qsort (array, size, sizeof (double), compare_doubles);
@}
@end smallexample

The @code{qsort} function derives its name from the fact that it was
originally implemented using the ``quick sort'' algorithm.
@end deftypefun

@node Search/Sort Example,  , Array Sort Function, Searching and Sorting
@section Searching and Sorting Example

Here is an example showing the use of @code{qsort} and @code{bsearch}
with an array of structures.  The objects in the array are sorted
by comparing their @code{name} fields with the @code{strcmp} function.
Then, we can look up individual objects based on their names.

@comment This example is dedicated to the memory of Jim Henson.  RIP.
@smallexample
@include search.c.texi
@end smallexample

@cindex Kermit the frog
The output from this program looks like:

@smallexample
Kermit, the frog
Piggy, the pig
Gonzo, the whatever
Fozzie, the bear
Sam, the eagle
Robin, the frog
Animal, the animal
Camilla, the chicken
Sweetums, the monster
Dr. Strangepork, the pig
Link Hogthrob, the pig
Zoot, the human
Dr. Bunsen Honeydew, the human
Beaker, the human
Swedish Chef, the human

Animal, the animal
Beaker, the human
Camilla, the chicken
Dr. Bunsen Honeydew, the human
Dr. Strangepork, the pig
Fozzie, the bear
Gonzo, the whatever
Kermit, the frog
Link Hogthrob, the pig
Piggy, the pig
Robin, the frog
Sam, the eagle
Swedish Chef, the human
Sweetums, the monster
Zoot, the human

Kermit, the frog
Gonzo, the whatever
Couldn't find Janice.
@end smallexample
