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The following function provides a simple alternative to the standard
library function `qsort`

. It is intended for systems lacking
`qsort`

, not as a replacement for it. The function `qsort`

should be used whenever possible, as it will be faster and can provide
stable ordering of equal elements. Documentation for `qsort`

is
available in the GNU C Library Reference Manual.

The functions described in this section are defined in the header file
`gsl_heapsort.h`.

- Function:
*void***gsl_heapsort***(void **`array`, size_t`count`, size_t`size`, gsl_comparison_fn_t`compare`) -
This function sorts the

`count`elements of the array`array`, each of size`size`, into ascending order using the comparison function`compare`. The type of the comparison function is defined by,int (*gsl_comparison_fn_t) (const void * a, const void * b)

A comparison function should return a negative integer if the first argument is less than the second argument,

`0`

if the two arguments are equal and a positive integer if the first argument is greater than the second argument.For example, the following function can be used to sort doubles into ascending numerical order.

int compare_doubles (const double * a, const double * b) { if (*a > *b) return 1; else if (*a < *b) return -1; else return 0; }

The appropriate function call to perform the sort is,

gsl_heapsort (array, count, sizeof(double), compare_doubles);

Note that unlike

`qsort`

the heapsort algorithm cannot be made into a stable sort by pointer arithmetic. The trick of comparing pointers for equal elements in the comparison function does not work for the heapsort algorithm. The heapsort algorithm performs an internal rearrangement of the data which destroys its initial ordering.

- Function:
*int***gsl_heapsort_index***(size_t **`p`, const void *`array`, size_t`count`, size_t`size`, gsl_comparison_fn_t`compare`) -
This function indirectly sorts the

`count`elements of the array`array`, each of size`size`, into ascending order using the comparison function`compare`. The resulting permutation is stored in`p`, an array of length`n`. The elements of`p`give the index of the array element which would have been stored in that position if the array had been sorted in place. The first element of`p`gives the index of the least element in`array`, and the last element of`p`gives the index of the greatest element in`array`. The array itself is not changed.

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