diff --git a/searches/quick_select.py b/searches/quick_select.py new file mode 100644 index 000000000..e5e2ce99c --- /dev/null +++ b/searches/quick_select.py @@ -0,0 +1,47 @@ +import collections +import sys +import random +import time +import math +""" +A python implementation of the quick select algorithm, which is efficient for calculating the value that would appear in the index of a list if it would be sorted, even if it is not already sorted +https://en.wikipedia.org/wiki/Quickselect +""" +def _partition(data, pivot): + """ + Three way partition the data into smaller, equal and greater lists, + in relationship to the pivot + :param data: The data to be sorted (a list) + :param pivot: The value to partition the data on + :return: Three list: smaller, equal and greater + """ + less, equal, greater = [], [], [] + for element in data: + if element.address < pivot.address: + less.append(element) + elif element.address > pivot.address: + greater.append(element) + else: + equal.append(element) + return less, equal, greater + + def quickSelect(list, k): + #k = len(list) // 2 when trying to find the median (index that value would be when list is sorted) + smaller = [] + larger = [] + pivot = random.randint(0, len(list) - 1) + pivot = list[pivot] + count = 0 + smaller, equal, larger =_partition(list, pivot) + count = len(equal) + m = len(smaller) + + #k is the pivot + if m <= k < m + count: + return pivot + # must be in smaller + elif m > k: + return quickSelect(smaller, k) + #must be in larger + else: + return quickSelect(larger, k - (m + count))