Make quicksort work the other way?

This is a discussion on Make quicksort work the other way? within the C Programming forums, part of the General Programming Boards category; Hi, How can i modify the quicksort algorithm so that it sorts in descending order? thxn...

  1. #1
    Registered User Nutshell's Avatar
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    Make quicksort work the other way?

    Hi,

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

    thxn

  2. #2
    Registered User
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    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.
    Callou collei we'll code the way
    Of prime numbers and pings!

  3. #3
    Unregistered
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    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;
    }

  4. #4
    Registered User Nutshell's Avatar
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    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.

  5. #5
    and the hat of wrongness Salem's Avatar
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    > 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)

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