Question

Pretty confused about this because I’ve verified correct logical output for small enough testcases (N = 20). I try doing N = 10,000 numbers and the program just hangs and I don’t understand why… I’ve implemented the algorithm as simply as I can.

Also, calling sorted(data) on my N = 10k list seems to work almost instantly. So I’m convinced my algorithm just gets stuck somewhere.

Here is the code:

def QuickSort(array):
    qsort(array, 0, len(array))


def qsort(arr, left, right):
    if ((right - left) < 2):
        return

    pivotIndex = choosePivot0(arr, left, right)

    newPivotIndex = partition(arr, pivotIndex, left, right)

    qsort(arr, 0, newPivotIndex)
    qsort(arr, newPivotIndex + 1, right)

def partition(arr, pivotIndex, left, right):
    swap(arr, pivotIndex, left)
    pivot = arr[left]
    i = left + 1
    for j in range(left+1, right):
        if (arr[j] < pivot):
            swap(arr, i, j)
            i = i + 1

    swap(arr, left, i-1) #put pivot back where it belongs
    #cobj.count = cobj.count + (right - left - 1) #add m-1 the #of comparisons
    return (i-1) #give back where the pivot resides



def swap(array, i, j):
    temp = array[i]
    array[i] = array[j]
    array[j] = temp

def choosePivot0(array, left, right):
    return randint(left, right-1) #random

So I’m pretty lost as to why this could be happening. Thanks for any help.

Answer 1

There seems to be a typo in the following line:

qsort(arr, 0, newPivotIndex)

I think it should be like this.

qsort(arr, left, newPivotIndex)

Otherwise the function will work somehow only for some of the input data sets. That is why the algorithm gets stuck.