C＃while循环未退出（快速排序）

Question

for some reason, my while loop for (i<=j) does not end when j goes lower than i.

I watched the debug values, and have repeatedly seen values of (4,3), (6,5) etc for i and j (respectively).

public static List<Item> QuickSort(List<Item> a, int left, int right)
{
    int i = left;
    int j = right;
    double pivotValue = ((left + right) / 2);
    Item x = a[Convert.ToInt32(pivotValue)];
    Item w;
    while (i <= j)
    {
        //these while loops continue looping after i<=j is false
        while (a[i] < x)
        {
            i++;
        }
        while (x < a[j])
        {
            j--;
        }
        if (i <= j)
        {
            w = a[i];
            a[i++] = a[j];
            a[j--] = w;
        }
    }
    if (left < j)
    {
        QuickSort(a, left, j);
    }
    if (i < right)
    {
        QuickSort(a, i, right);
    }
    return a;
}

Answer 1

I'll bet it's either the recursion or one of the internal while loops that's still going on. I don't have the quicksort algorithm memorized, but I'd wager that if you broke in the debugger you'd find yourself in one of the two inner while loops.

Answer 2

Coding quicksort is a very good exercice, it's not as easy as it may look to get everything right! ;)

Here's one problem with your code. Consider what happens if j bumps into the pivot, while i does not. You exchange the pivot (at j ) with the value at i and increment i past the pivot. This cannot continue correctly (if you understand Quicksort you should understand why).

After choosing the pivot, I like to exchange it with one bound (either left or right ), go exchange values until i and j meet and then put the pivot back at this place. There are other ways, of course.

Answer 3

It is not a infinite loop.

while (a[i] < x)
    {
        i++;
    }

If x is the largest number in the list, it will raise 'i' as long as the array is out of Range, and the programm will throw a OutOfRangeException.

Answer 4

What happens if i==j and a[i]==x ...? No increment, no decrement, just swap and continue...

Consider changing while(i<=j) to while(i<j) . Also swap is not necessary if i==j ...