Question

I have written the following heap sort code and I get the wrong output (not sorted) at times and I can’t seem to find why…any help will be much appreciated!

def heap_sort(self, a):

    heapsize = self.build_max_heap(a)

    n = len(a)-1
    i = len(a)-1

    for i in range(i, 0, -1):
        temp = a[0]
        a[0] = a[i]
        a[i] = temp
        heapsize = heapsize - 1
        self.max_heapify(heapsize, a, 0)       #rebuild max heap at with new root

    return a

def max_heapify(self, heapsize, a, i):

    left = (2*(i+1))-1      #left child of i
    right = 2*(i+1)             #right child of i
    largest = i

    if left < heapsize and a[left] > a[i]:
        largest = left

    if right < heapsize and a[right] > a[largest]:
        largest = right

    if largest != i:
        temp = a[largest]
        a[largest] = a[i]
        a[i] = temp
        self.max_heapify(heapsize, a, largest)

def build_max_heap(self, a):

    heapsize = len(a)
    i = int(heapsize/2)-1

    for i in range(i, 0):
        self.max_heapify(heapsize, a, i)

    return heapsize

My tests:

#--Test for 0 in array--#
def zero_array(self):
    a = [12,0,232]
    print self.sort.heap_sort(a)
    return

#--Test for duplicate in array--#
def duplicate_array(self):
    a = [12, 12, 7]
    print self.sort.heap_sort(a)
    return

#--Test for all same values in array--#
def allsame_array(self):
    a = [1,1,1]
    print self.sort.heap_sort(a)
    return

#--Test for negative values in array--#
def negative_array(self):
    a = [-23, -2, 123]
    print self.sort.heap_sort(a)
    return

Output (which is supposed to all be sorted):

    [0, 232, 12]
    [7, 12, 12]
    [1, 1, 1]
    [-2, 123, -23]

Answer 1

It could be because your second range() is incrementing forwards instead of backwards.

for i in range(i, 0):

Should be:

for i in range(i, 0, -1):

You also need to keep in mind that range() will stop just before the second argument, so range(5, 0, -1) will return [5, 4, 3, 2, 1].