I was originally using only a single random pivot given by
pivots = random.randrange(l,r)
Here l and r will be integers that define my range
I wanted to improve the run time by greatly increasing the likely hood that my pivot would be a good pivot by selecting the median of three random pivots. Below is the code I used and it caused my run time to increase by 20%-30%.
rr = random.randrange
pivots = [ rr(l,r) for i in range(3) ]
pivots.sort()
How do I implement the above to be much faster?
Edit: Entire code added below
import random
def quicksort(array, l=0, r=-1):
# array is list to sort, array is going to be passed by reference, this is new to me, try not to suck
# l is the left bound of the array to be acte on
# r is the right bound of the array to act on
if r == -1:
r = len(array)
# base case
if r-l <= 1:
return
# pick the median of 3 possible pivots
#pivots = [ random.randrange(l,r) for i in range(3) ]
rr = random.randrange
pivots = [ rr(l,r) for i in range(3) ]
pivots.sort()
i = l+1 # Barrier between below and above piviot, first higher element
array[l], array[pivots[1]] = array[pivots[1]], array[l]
for j in range(l+1,r):
if array[j] < array[l]:
array[i], array[j] = array[j], array[i]
i = i+1
array[l], array[i-1] = array[i-1], array[l]
quicksort(array, l, i-1)
quicksort(array, i, r)
return array
Edit 2: This is the corrected code due. There was an error in the algorithm for picking the 3 pivots
import random
def quicksort(array, l=0, r=-1):
# array is list to sort, array is going to be passed by reference, this is new to me, try not to suck
# l is the left bound of the array to be acte on
# r is the right bound of the array to act on
if r == -1:
r = len(array)
# base case
if r-l <= 1:
return
# pick the median of 3 possible pivots
mid = int((l+r)*0.5)
pivot = 0
#pivots = [ l, mid, r-1]
if array[l] > array[mid]:
if array[r-1]> array[l]:
pivot = l
elif array[mid] > array[r-1]:
pivot = mid
else:
if array[r-1] > array[mid]:
pivot = mid
else:
pivot = r-1
i = l+1 # Barrier between below and above piviot, first higher element
array[l], array[pivot] = array[pivot], array[l]
for j in range(l+1,r):
if array[j] < array[l]:
array[i], array[j] = array[j], array[i]
i = i+1
array[l], array[i-1] = array[i-1], array[l]
quicksort(array, l, i-1)
quicksort(array, i, r)
return array
Though it can be outperformed by random choice on occasion, it's still worth looking into the median-of-medians algorithm for pivot selection (and rank selection in general), which runs in O(n) time. It's not too far off of what you are currently doing, but there is a stronger assurance behind it that it picks a "good" pivot as opposed to just taking the median of three random numbers.
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