Merge sort algorithm not working correctly

Question

As part of a homework assignment, I need to be able to implement a merge sort using a structure as the main argument. Having not been familiar with the merge sort until today, i have attempted to write my own implementation of it. For some reason I cannot get it to work.

Here is my code:

#include<iostream>
#include<stdlib.h>

using namespace std;

struct MergeArgument
{
    int *numArray;
    int *tempArray;
    int lowIndex, highIndex;
};

void merge(MergeArgument*);
void merge_sort(MergeArgument*);

int main(int argc, char** argv)
{   int SIZE = 25;
    MergeArgument arg;
    int arr[SIZE];
    int temp[SIZE];

    for(int k = 0; k < SIZE; k++)
    {
        arr[k] = rand() % 100;
        cout << arr[k] << " ";
    }

    arg.numArray = arr;
    arg.tempArray = temp;
    arg.lowIndex = 0;
    arg.highIndex = SIZE - 1;

    cout << endl;

    merge_sort(&arg);

    cout << "Sorted array: \n";

    for (int i = 0; i < SIZE; i++)
        cout << arr[i] << " ";
    cout << endl;

    return 0;
}

void merge_sort(MergeArgument *arg)
{   int tempHigh, tempLow;
    if(arg->lowIndex < arg->highIndex)
    {
        tempHigh = arg->highIndex;
        tempLow = arg->lowIndex;
        arg->highIndex = (tempHigh + tempLow) / 2;
        merge_sort(arg);
        arg->highIndex = tempHigh;
        arg->lowIndex = ((tempHigh + tempLow) / 2) + 1;
        merge_sort(arg);
        arg->lowIndex = tempLow;
        merge(arg);
    }

}

void merge(MergeArgument *arg)
{   int low = arg->lowIndex, mid = ((arg->lowIndex + arg->highIndex) / 2), high = arg->highIndex;
    int i = low, lowCounter = low, highCounter = mid + 1;

    while((lowCounter <= mid) && (highCounter <= high))
    {
        if(arg->numArray[lowCounter] < arg->numArray[highCounter])
        {
            arg->tempArray[i] = arg->numArray[lowCounter];
            lowCounter++;
        }
        else
        {
            arg->tempArray[i] = arg->numArray[highCounter];
            highCounter++;
        }
        i++;
    }

    if (lowCounter < mid)
    {
        for (int k = lowCounter; k < mid; k++)
        {
            arg->tempArray[i] = arg->numArray[k];
            i++;
        }
    }
    else
    {
        for (int k = highCounter; k <= arg->highIndex; k++)
        {
            arg->tempArray[i] = arg->numArray[k];
            i++;
        }

    }

    for(int k = arg->lowIndex; k <= arg->highIndex; k++)
    {
        arg->numArray[k] = arg->tempArray[k];
    }
}

Here is the output I am getting:

83 86 77 15 93 35 86 92 49 21 62 27 90 59 63 26 40 26 72 36 11 68 67 29 82    
Sorted array:  
11 -1216235240 15 0 21 26 -1079135248 26 27 -1079135396 29 -1216770650 35 -1216235240 49 -1216492084 59 0 68 72 82 83 0 86 82

Can anyone point out what exactly I am doing wrong?

This is very rough code, I simply want to get the merge sort working before I move on to the big part of the assignment. It will look like C++ by the time I am done with it. — Brian Zable
– Brian Zable, Commented Oct 30, 2011 at 2:00
I'm trying to figure out how you could make it work with only one instance of MergeArgument, and only one argument passed to merge(). — Hot Licks
– Hot Licks, Commented Oct 30, 2011 at 2:05
@michaelnett -- I'm not sure I'm seeing enough local variables. — Hot Licks
– Hot Licks, Commented Oct 30, 2011 at 3:07

clstrfsck · Accepted Answer · 2011-10-30 06:46:38Z

1

This is pretty close to working, although you might want to consider some of the comments folks have made on making this more C++ like. No doubt this is from hard-won experience that there is never enough time to go back and do what you really should do.

The problem I see is here, in merge:

if (lowCounter < mid)
{
    for (int k = lowCounter; k < mid; k++)
    {
        arg->tempArray[i] = arg->numArray[k];
        i++;
    }
}

You might want to compare and contrast the bounds here to the initial loop.

answered Oct 30, 2011 at 6:46

clstrfsck

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1 Comment

Brian Zable Over a year ago

Thank you! Although I got it working by redoing my merge function, I am glad to finally see what was wrong with my original implementation.

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