Make quicksort work the other way?

Hi,

How can i modify the quicksort algorithm so that it sorts in descending order?

thxn

Sure, just change the comparison function to return negative values of what it normally returns...

Code:

---

int cmp(int a, int b)
{
 return (a - b);
}

int othercmp (int a, int b)
{
 return (b - a);
}

---

qsort (base, n, sizeof(int), cmp);
sorts the array in ascending order....

qsort(base, n, sizeof(int), othercmp);
sorts the array in descending order.

See where it says "control order of alphebetizing?

Code:

---

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


int IsLess(char * x, char * mid)
{
        //control the order of alphebetizing here with < or >
        if(strcmp( x,mid) < 0)
                return -1;
        else
                return 1;
}

char * GetRecord(char * a[], int x)
{
        char * temp = (char*) malloc (sizeof(char) * 20);
        temp = a[x];
        return temp;
}

void QSort(char * a[], int left, int right)
{
        int i = left;
        int j = right;
        char * temp = NULL;

        //frees placeholder during recursive call
        free(temp);

        //get middle record
        temp = GetRecord(a, (i+j) / 2);

        do
        {
                while (IsLess(GetRecord(a,i), temp) < 0 && i < right)
                        i++;

                while (IsLess(temp,GetRecord(a,j)) < 0 && j > left)
                        j--;

                //swap
                if( i <= j)
                {
                        temp =  a[i];
                        a[i] =  a[j];
                        a[j] = temp;
                        i++;
                        j--;
                }
        }while( i <= j);
        //recursive calls
        if(left < j)
                QSort(a,left,j);
        if(i < right)
                QSort(a,i,right);
}

int main()
{
        int i;
        const int size = 5;

        //jagged array rather than a smooth array[x][size]
        char * array[size] = {"namev","namee","namex","namea","namen" };

        QSort(array, 0, size - 1);

        printf("Sorted array of strings.\n");
        for(i = 0; i < size; i++)
        {
                        printf("%s\n",array[i]);
        }
        return 0;
}

---

mmmm i think it really depends on the version of quicksort algorithm you have right? COz mine has only three parameters.
Ok then, nevermind,i 'll sort out myself.

> mmmm i think it really depends on the version of quicksort algorithm you have right
There is only one, it's called qsort and it's found in stdlib.h

And you use it as QuestionC posted

Any other qsort is some non-standard implementation which you would need to provide more details on (if you're still stuck)