Is there any way to refer to another array in qsort's compare function?

[88888 Dihedral]

Ben Bacarisseæ–¼ 2011å¹´12月29日星期四UTC+8下åˆ7時58分14秒寫é“：

Here's the problem.

I wrote a function 'int my_compare(const void *, const void *);' and
passed it to qsort() of stdlib.h, in order to sort an array named
*ptr* which contains indices of another array *data*. By that I
intended to sort elements of *ptr* (i.e. ptr) using data[ptr] as
the key value.

However, the *data* array was defined as static in main(), and seemed
not easy to access in my_compare(). Is there any way to achieve this?

You can either move the array so that it is defined at file scope
(i.e. outside main) or you can arrange for there to be a file-scope
pointer to it:

#include <stdlib.h>
#include <stdio.h>

int *aux_sort_ptr;

int my_compare(const void *p1, const void *p2)
{
const int *ip1 = p1, *ip2 = p2;
return aux_sort_ptr[*ip1] - aux_sort_ptr[*ip2];
}

int main(void)
{
static int data[] = { 3, 1, 2 };
int idx[] = { 1, 2, 3 };
aux_sort_ptr = data;
qsort(idx, sizeof idx/sizeof *idx, sizeof *idx, my_compare);
for (int i = 0; i < sizeof idx/sizeof *idx; i++)
printf("idx[%d] = %d, data[%d] = %d\n",
i, idx, idx, data[idx]);
}

This is ugly (and problematic in threaded code), but there's no elegant
way round it using standard C. Many systems have other sort functions
whose comparison function can be handed another void * (see Dr Nick's
reply) but I don't think and have become common enough to though of as
"almost standard".

This will turn the quick sort algorithm to be of O(nlog(n)) in speed
and O(n) used for the heap space.

Thus the non-choking merge sort is a distributed system is better.

Of course sorting integers of 32 bits in a radix sort of billions of items
in an 64 bit OS is the obvious way to replace the quick sort in the standard
library of C.

An array of 32 bit integers can be decomposed as 12-12-8 in 3 passes or 16-16 in two passes in a descent radix sort implementation that needs the O(n) chunk
in the heap space.

 

[Ben Bacarisse]

88888 Dihedral wrote:

First of all,
whether or not qsort uses the quicksort algorithm,
is up to the implementation.

It's up to you, of course, but there doesn't seem to be much point in
correcting 88888 Dihedral's posts. They seem to consist of random
truths and falsehoods only tangentially related to the posting.

Second,
quicksort can be implemented with O(1) use of memory.

I think this is an abuse of notation. Since you post C code to back
this up, I presume that you are saying that quicksort's maximum memory
usage is reasonably small and can be predicted (indeed it can be) but
that's not what O(1) means. The memory usage of all implementations of
all sort algorithms is bounded above by a constant (maybe the number of
particles in the universe) so big O is not very helpful here.

struct {
size_t bytes;
void *base;
} stack[CHAR_BIT * sizeof nmemb], *stack_ptr;

<snip>

The abstract algorithm whose code you posted uses O(log n) stack space.

 

[88888 Dihedral]

Ben Bacarisseæ–¼ 2012å¹´1月8日星期日UTC+8下åˆ8時50分49秒寫é“：

pete
writes:


It's up to you, of course, but there doesn't seem to be much point in
correcting 88888 Dihedral's posts. They seem to consist of random
truths and falsehoods only tangentially related to the posting.

Are you joking for nonsense again?

Feeding a quick sort with a segment of 3,4,5,6 repeated for 2 to 10 thousand
times, then the quick sort will be choked to be so slow.

A non-choking quick sort is of O(nlog(n)) in time spend
for sorting arbitrary n items even in the worst case.

I think this is an abuse of notation. Since you post C code to back
this up, I presume that you are saying that quicksort's maximum memory
usage is reasonably small and can be predicted (indeed it can be) but
that's not what O(1) means. The memory usage of all implementations of
all sort algorithms is bounded above by a constant (maybe the number of
particles in the universe) so big O is not very helpful here.

struct {
size_t bytes;
void *base;
} stack[CHAR_BIT * sizeof nmemb], *stack_ptr;

<snip>

The abstract algorithm whose code you posted uses O(log n) stack space.

 

[Phil Carmody]

Second,
quicksort can be implemented with O(1) use of memory.

If you were going to make such a statement, why stop at quicksort?

Anything which only needs to accept a bounded amount of input can
be implemented with O(1) use of memory.

Phil