I created this program for an assignment in which we were required to create an implementation of Quichesort. This is a hybrid sorting algorithm that uses Quicksort until it reaches a certain recursion depth (log2(N), where N is the length of the list), then switches to Heapsort, to avoid exceeding the maximum recursion depth.
While testing my implementation, I discovered that although it generally performed better than regular Quicksort, Heapsort consistently outperformed both. Can anyone explain why Heapsort performs better, and under what circumstances Quichesort would be better than both Quicksort and Heapsort?
Note that for some reason, the assignment referred to the algorithm as "Quipsort".
Edit: Apparently, "Quichesort" is actually identical to Introsort.
I also noticed that a logic error in my medianOf3() function was
causing it to return the wrong value for certain inputs. Here is an improved
version of the function:
def medianOf3(lst):
"""
From a lst of unordered data, find and return the the median value from
the first, middle and last values.
"""
first, last = lst[0], lst[-1]
if len(lst) <= 2:
return min(first, last)
middle = lst[(len(lst) - 1) // 2]
return sorted((first, middle, last))[1]
Would this explain the algorithm's relatively poor performance?
Code for Quichesort:
import heapSort # heapSort
import math # log2 (for quicksort depth limit)
def medianOf3(lst):
"""
From a lst of unordered data, find and return the the median value from
the first, middle and last values.
"""
first, last = lst[0], lst[-1]
if len(lst) <= 2:
return min(first, last)
median = lst[len(lst) // 2]
return max(min(first, median), min(median, last))
def partition(pivot, lst):
"""
partition: pivot (element in lst) * List(lst) ->
tuple(List(less), List(same, List(more))).
Where:
List(Less) has values less than the pivot
List(same) has pivot value/s, and
List(more) has values greater than the pivot
e.g. partition(5, [11,4,7,2,5,9,3]) == [4,2,3], [5], [11,7,9]
"""
less, same, more = [], [], []
for val in lst:
if val < pivot:
less.append(val)
elif val > pivot:
more.append(val)
else:
same.append(val)
return less, same, more
def quipSortRec(lst, limit):
"""
A non in-place, depth limited quickSort, using median-of-3 pivot.
Once the limit drops to 0, it uses heapSort instead.
"""
if lst == []:
return []
if limit == 0:
return heapSort.heapSort(lst)
limit -= 1
pivot = medianOf3(lst)
less, same, more = partition(pivot, lst)
return quipSortRec(less, limit) + same + quipSortRec(more, limit)
def quipSort(lst):
"""
The main routine called to do the sort. It should call the
recursive routine with the correct values in order to perform
the sort
"""
depthLim = int(math.log2(len(lst)))
return quipSortRec(lst, depthLim)
Code for Heapsort:
import heapq # mkHeap (for adding/removing from heap)
def heapSort(lst):
"""
heapSort(List(Orderable)) -> List(Ordered)
performs a heapsort on 'lst' returning a new sorted list
Postcondition: the argument lst is not modified
"""
heap = list(lst)
heapq.heapify(heap)
result = []
while len(heap) > 0:
result.append(heapq.heappop(heap))
return result
