QSORT(3) | Library Functions Manual | QSORT(3) |
qsort
, heapsort
,
mergesort
—
#include <stdlib.h>
void
qsort
(void
*base, size_t
nmemb, size_t size,
int (*compar)(const void *,
const void *));
int
heapsort
(void
*base, size_t
nmemb, size_t size,
int (*compar)(const void *,
const void *));
int
mergesort
(void
*base, size_t
nmemb, size_t size,
int (*compar)(const void *,
const void *));
qsort
() function is a modified partition-exchange
sort, or quicksort. The heapsort
() function is a
modified selection sort. The mergesort
() function is a
modified merge sort with exponential search intended for sorting data with
pre-existing order.
The qsort
() and
heapsort
() functions sort an array of
nmemb objects, the initial member of which is pointed
to by base. The size of each object is specified by
size. mergesort
() behaves
similarly, but requires that size be
greater than “sizeof(void *) / 2”.
The contents of the array base are sorted in ascending order according to a comparison function pointed to by compar, which requires two arguments pointing to the objects being compared.
The comparison function must return an integer less than, equal to, or greater than zero if the first argument is considered to be respectively less than, equal to, or greater than the second.
The functions qsort
() and
heapsort
() are not stable, that
is, if two members compare as equal, their order in the sorted array is
undefined. The function mergesort
() is stable.
The qsort
() function is an implementation
of C.A.R. Hoare's ``quicksort'' algorithm, a variant of partition-exchange
sorting; in particular, see D.E. Knuth's Algorithm Q.
qsort
() takes O N lg N average time. This
implementation uses median selection to avoid its O N**2 worst-case
behavior.
The heapsort
() function is an
implementation of J.W.J. William's ``heapsort'' algorithm, a variant of
selection sorting; in particular, see D.E. Knuth's Algorithm H.
heapsort
() takes O N lg N worst-case time. Its
only advantage over qsort
() is
that it uses almost no additional memory; while
qsort
() does not allocate memory, it is implemented
using recursion.
The function mergesort
() requires
additional memory of size nmemb *
size bytes; it should be used only when space is not
at a premium. mergesort
() is optimized for data with
pre-existing order; its worst case time is O N lg N; its best case is O
N.
Normally, qsort
() is faster than
mergesort
() is faster than
heapsort
(). Memory availability and pre-existing
order in the data can make this untrue.
qsort
() function returns no value.
Upon successful completion, heapsort
() and
mergesort
() return 0. Otherwise, they return -1 and
the global variable errno is set to indicate the
error.
qsort
() did not permit the
comparison routine itself to call qsort
(). This is no
longer true.
heapsort
() function succeeds unless:
Hoare, C.A.R., Quicksort, The Computer Journal, 5:1, pp. 10-15, 1962.
Williams, J.W.J, Heapsort, Communications of the ACM, 7:1, pp. 347-348, 1964.
Knuth, D.E., Sorting and Searching, The Art of Computer Programming, Vol. 3, pp. 114-123, 145-149, 1968.
McIlroy, P.M., Optimistic Sorting and Information Theoretic Complexity, Proceedings of the Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp. 467-474, 1993.
Bentley, J.L. and McIlroy, M.D., Engineering a Sort Function, Software-Practice and Experience, Vol. 23, pp. 1249-1265, 1993.
qsort
() function conforms to ANSI
X3.159-1989 (“ANSI C89”).
June 4, 1993 | NetBSD 9.2 |