merge sort dividing in three ways instead of two

[zenakent]

Hello, I am modifying a merge sort program that instead of dividing a list into two, it divides it into 3 instead and do its thing. so far, I have this.

#include <iostream>
using namespace std;

void merge(int *, int, int, int);

void merge(int *a, int low, int high, int mid, int mid2)
{
	int i, j, k, l, c[50];
	i = low;
	k = low;
	j = mid + 1;
	l = mid2 + 1;

	while (i <= mid && j <= mid2 && l <= high)
	{
		if (a[i] < a[j])
		{
			c[k] = a[i];
			k++;
			i++;
		}
		else if (a[j] < a[l])
		{
			c[k] = a[j];
			k++;
			j++;
		}
		else
		{
			c[k] = a[l];
			k++;
			l++;
		}
	}

	while (i <= mid)
	{
		c[k] = a[i];
		k++;
		i++;
	}

	while (j <= mid2)
	{
		c[k] = a[j];
		k++;
		j++;
	}

	while (l <= high)
	{
		c[k] = a[l];
		k++;
		l++;
	}
	for (i = low; i < k; i++)
	{
		a[i] = c[i];
	}
}

void mergesort(int *a, int low, int high)
{
	int mid;
	int mid2;
	if (low < high)
	{
		mid = (low + high) / 3;
		mid2 = 2 * ((low + high) / 3);
		mergesort(a, low, mid);
		mergesort(a, mid + 1, mid2);
		mergesort(a, mid2 + 1, high);
		merge(a, low, mid, mid2, high);
	}
	return;
}

int main()
{
	int a[20], i;
	cout << "Enter 5 elements to sort" << endl;
	for (i = 0; i < 5; i++)
	{
		cin >> a[i];
	}

	mergesort(a, 0, 4);

	cout << "Sorted list: " << endl;

	for (i = 0; i < 5; i++)
	{
		cout << a[i] << endl;
	}
	system("pause");
	return 0;
}

I seem to have a problem at the mergesort function but I can't figure out what it is.

was hoping someone could comment on what I should do.

[helios]

You're determining the pivots incorrectly. For example, if sorting the range [5; 8), (5+8)/3 = 4. Times 2 is 8. Neither pivot is within the range.

[zenakent]

sorry... pivots?

[Zhuge]

You should look over descriptions of the sorting algorithms again, as "pivot" is a technical term you need to know to understand them.