Sorting function revisited

Posted on 1998-08-30
Last Modified: 2010-04-02
Question by:wtc108
  • 2

Expert Comment

ID: 1171624
It is possible there is a bug in your compiler, but I think it's more like that it's you calling qsort incorrectly.

qsort is defined:

qsort is defined byvoid qsort( void *base, size_t num, ...etc...

where base is the start of the array, num is number of elements in the array.  

My guess is you are out by 1 for num.  Remember a 2 element array starts from 0 (the last element of such an array is '1').  However num should be set to the number of elements, not the highest allowable index in the array (in this case should be 2).

Try setting num 1 higher than you currently are.  Otherwise post enough sample code for us to see.

Incidentally a good way to debug if this is happening would be to set up the following
1. Output the last item in the array (to screen or debugger)
2. Do qsort
3. Output the last item in the array

if #1 and #3 give the same answer, it pretty much proves by guess.


Accepted Solution

Answers2000 earned 50 total points
ID: 1171625
Well I figured I may as well blow the points to look up the original question...

(begin quote snoeggler)

// This functions tests for less/equal/greater(see above)
int _cdecl qsortHelper(const void *elem1,const void *elem2)
const float &val1=*((float *)elem1); // elem1, elem2 point to 'float' var's which
const float &val2=*((float *)elem2); // are elements of the array you pass(see below)

if(val1<val2) return -1;
if(val1==val2) return 0;
if(val1>val2) return 1;
// this will never be called
return 0;

// array is a pointer to the float array you wish to be sorted, size contains the
// number of elements your array contains
void SortFloats(float* array,int size)

(end quote)

Make sure you pass the number of elements (not the highest element of the array) into Snoeggler's SortFloats function.

A good way to do this is to use sizeof

float x[5] ;
SortFloats( &x, sizeof(x) / sizeof(float) ) ;

Expert Comment

ID: 1171626
This is a generic selection sort algorithm it can work with any data type. You can change it to quick sort and it will work faster that qsort which exist in C, because templates mechenism is smarter than qsort. qsort requeres more overhead in the recursive routine.

// A generic selection sort.

#include <iostream.h>
#include <string.h>

template <class Stype> void select(Stype *item, int count);

 int i;

 // sort some characters
 char str[]="DJABGRC";;
 select(str,(int) strlen(str));
 cout << "The sorted string is         : " << str << endl;

 // sort some integers
 int nums[]={5,7,3,9,5,1,8};
 cout << "The sorted numbers are       : ";
 for (i=0;i<7;i++) cout << nums[i] << " ";
 cout << endl;

 // sort some floats
 float fnums[]={5.25,7.35,3.15,9.45,5.25,1.05,8.4};
 cout << "The sorted float numbers are : ";
 for (i=0;i<7;i++) cout << fnums[i] << " ";
 cout << endl;

 return 0;

// The generic selection sort.
template <class Stype> void select(Stype *item, int count)
 register int a,b,c;
 int exchange;
 Stype t;

 for (a=0; a<count-1;++a) {
  for(b=a+1;b<count;++b) {
    if (item[b]<t){
  if (exchange) {

Regard, Motaz from Sudan.


Author Comment

ID: 1171627
Thanks a lot, the sizeof() function worked like a champ. . .

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