diff --git a/sorts/counting_sort.py b/sorts/counting_sort.py new file mode 100644 index 000000000..13e4554ae --- /dev/null +++ b/sorts/counting_sort.py @@ -0,0 +1,72 @@ +""" +This is pure python implementation of counting sort algorithm +For doctests run following command: +python -m doctest -v counting_sort.py +or +python3 -m doctest -v counting_sort.py +For manual testing run: +python counting_sort.py +""" + +from __future__ import print_function + + +def counting_sort(collection): + """Pure implementation of counting sort algorithm in Python + :param collection: some mutable ordered collection with heterogeneous + comparable items inside + :return: the same collection ordered by ascending + Examples: + >>> counting_sort([0, 5, 3, 2, 2]) + [0, 2, 2, 3, 5] + >>> counting_sort([]) + [] + >>> counting_sort([-2, -5, -45]) + [-45, -5, -2] + """ + # if the collection is empty, returns empty + if collection == []: + return [] + + # get some information about the collection + coll_len = len(collection) + coll_max = max(collection) + coll_min = min(collection) + + # create the counting array + counting_arr_length = coll_max + 1 - coll_min + counting_arr = [0] * counting_arr_length + + # count how much a number appears in the collection + for number in collection: + counting_arr[number - coll_min] += 1 + + # sum each position with it's predecessors. now, counting_arr[i] tells + # us how many elements <= i has in the collection + for i in range(1, counting_arr_length): + counting_arr[i] = counting_arr[i] + counting_arr[i-1] + + # create the output collection + ordered = [0] * coll_len + + # place the elements in the output, respecting the original order (stable + # sort) from end to begin, updating counting_arr + for i in reversed(range(0, coll_len)): + ordered[counting_arr[collection[i] - coll_min]-1] = collection[i] + counting_arr[collection[i] - coll_min] -= 1 + + return ordered + + +if __name__ == '__main__': + import sys + # For python 2.x and 3.x compatibility: 3.x has not raw_input builtin + # otherwise 2.x's input builtin function is too "smart" + if sys.version_info.major < 3: + input_function = raw_input + else: + input_function = input + + user_input = input_function('Enter numbers separated by a comma:\n') + unsorted = [int(item) for item in user_input.split(',')] + print(counting_sort(unsorted))