I'm trying to implement Heap Sort in Python, but I can't seem to get it right. I've tried to implement this pseudo code, but my code does not sort! It just sifts to ridiculous effect. I'm inclined to think that the problem is in this line:
swap the root(maximum value) of the heap with the last element of the heap
How do I get the maximum value?
That is what I have:
def my_heap_sort(sqc):
def heapify(count):
start = (count-2)/2
while start >= 0:
sift_down(start, count-1)
start -= 1
def swap(i, j):
sqc[i], sqc[j] = sqc[j], sqc[i]
def sift_down(start, end):
root = start
while (root * 2 + 1) <= end:
child = root * 2 + 1
temp = root
if sqc[temp] < sqc[child]:
temp = child+1
if temp != root:
swap(root, temp)
root = temp
else:
return
count = len(sqc)
heapify(count)
end = count-1
while end > 0:
swap(end, 0)
end -= 1
sift_down(0, end)
And I found an example with almost the same problem:
def heap_sort_example(a):
def heapify(a):
start = (len(a) - 2) / 2
start -= 1
def sift_down(a, start, end):
root = start
while root * 2 + 1 <= end:
child = root * 2 + 1
if child + 1 <= end and a[child] < a[child+1]:
child += 1
if child <= end and a[root] < a[child]:
a[root], a[child] = a[child], a[root]
root = child
else:
return
heapify(a)
end = len(a) - 1
while end > 0:
a[end], a[0] = a[0], a[end]
sift_down(a, 0, end-1)
end -= 1
The results are different, but both are ridiculous:
>>> my_heap_sort(sqc)
[2, 7, 1, -2, 56, 5, 3]
>>> heap_sort_example(sqc)
[-2, 1, 7, 2, 56, 5, 3]
How do I get the maximum value? You don't need to "get" it. The root is exactly the maximum, that's a defined property of a heap.
If you feel tough to understand heap sort, this chapter will be extremely helpful.
I rewrote your code:
def swap(i, j):
sqc[i], sqc[j] = sqc[j], sqc[i]
def heapify(end,i):
l=2 * i + 1
r=2 * (i + 1)
max=i
if l < end and sqc[i] < sqc[l]:
max = l
if r < end and sqc[max] < sqc[r]:
max = r
if max != i:
swap(i, max)
heapify(end, max)
def heap_sort():
end = len(sqc)
start = end // 2 - 1 # use // instead of /
for i in range(start, -1, -1):
heapify(end, i)
for i in range(end-1, 0, -1):
swap(i, 0)
heapify(i, 0)
sqc = [2, 7, 1, -2, 56, 5, 3]
heap_sort()
print(sqc)
It gives:
[-2, 1, 2, 3, 5, 7, 56]