TypeError: 'NoneType' - Selection Sort on Python

Question

I'm trying to make a selection sort using python, I just recently learned python following the book "Python Crash Course" then now I'm reading the book "grokking algorithms"

I pretty much understood the selection sort algorithm but I just can't figure out why I'm getting a TypeError: 'NoneType' object cannot be interpreted as an integer

So the way I test it is to populate my array using random.randint(1, 101) and range(0,11), what I can't figure out is why python can't interpret it as an integer? since both function returns an integer. The error seems to happen when I do the new_array.append(arr.pop(smallest)).

see my full code below. (I'm using Python 3)

import random

def find_smallest(arr):
    """Return the index of the smallest value in an array"""
    smallest = arr[0]
    smallest_index = 0

    for i in range(1, len(arr)):
        if arr[i] < smallest:
            smallest = arr[i]
            smallest_index = i
        return smallest_index

def selection_sort(arr):
    """sort an array by storing smallest value to new array 1 by 1"""
    new_array = []
    for i in range(len(arr)):
        smallest = find_smallest(arr)
        new_array.append(arr.pop(smallest))
    return new_array


my_array = []
for i in range(0, 11):
    my_array.append(random.randint(1, 101))
    print(my_array[i])

sorted_array = selection_sort(my_array)
for i in range(len(sorted_array)):
    print(sorted_array[i])

Dima Chubarov · Accepted Answer · 2018-03-17 06:01:07Z

1

Looks like an indentation problem in your find_smallest() function

Consider

def find_smallest(arr):
    """Return the index of the smallest value in an array"""
    smallest = arr[0]
    smallest_index = 0

    for i in range(1, len(arr)):
        if arr[i] < smallest:
            smallest = arr[i]
            smallest_index = i
    return smallest_index  # <--- this is the line

Note that the return statement is indented differently than in your code.

To debug such problems you should notice that arr.pop() requires an integer or a list of integers as an argument, so you could insert

 assert(smallest is not None)

above the failing line new_array.append(arr.pop(smallest)) and confirm that assertion is triggered, then check at which iteration this happens replacing assert statement with

 assert(smallest is not None), "Array length is %d" % len(arr)

This would trigger an exception

 AssertionError: Array length is 1

Then you would know that you need to check what does your find_smallest function return when its argument is an array of length 1.

edited Mar 17, 2018 at 6:01

answered Mar 17, 2018 at 5:21

Dima Chubarov

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1 Comment

Darren Over a year ago

thank you so much! I understand it now since the return statement is incorrectly indented the way the script works is that it will only return when the "if statement" has been met. thanks, I appreciate it how you include debugging steps for such problem. you're awesome!

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