Why empty list to function?

August 18, 2023

Why is an empty list passed to the binary_search function? After all, it’s not really empty.

from typing import List


def binary_search(nums: List[int], item: int):
    print("binary_search nums", nums)
    low = 0
    high = len(nums) - 1

    while low <= high:
        middle = (low + high) // 2

        if nums[middle] == item:
            return middle
        if nums[middle] > item:
            high = middle - 1
        if nums[middle] < item:
            low = middle + 1
    return None


def smallest_find(nums: List[int]):
    smallest = nums[0]
    smallest_index = 0

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


def selection_sort(nums: List[int]):
    sorted_nums = []

    for _ in range(len(nums)):
        smallest_index = smallest_find(nums)
        sorted_nums.append(nums.pop(smallest_index))
    return sorted_nums

import random

while True:
    nums = [random.randint(-100, 100) for _ in range(20)]
    item = 24
    if item in nums:
        print(nums)
        print(selection_sort(nums))
        print(binary_search(selection_sort(nums), item))
        break

Result:

[20, 84, -65, -76, -46, -7, 19, 48, -55, 31, 62, -86, 16, 24, 67, -87, -65, -57, 12, 61]
[-87, -86, -76, -65, -65, -57, -55, -46, -7, 12, 16, 19, 20, 24, 31, 48, 61, 62, 67, 84]
binary_search nums []
None

Why is an empty list passed to the binary_search function? After all, it’s not really empty.

Python 3.8.10

>Solution :

Your second call to selection_sort actually returns an empty list, that is why binary_search is getting an empty list.

The reason is this line:

sorted_nums.append(nums.pop(smallest_index))

You are pop-ing all values from nums in your selection_sort routine. This is emptying out (and overwriting) input list nums

You can avoid this by making a shallow copy of nums:

from typing import List


def binary_search(nums: List[int], item: int):
    print("binary_search nums", nums)
    low = 0
    high = len(nums) - 1

    while low <= high:
        middle = (low + high) // 2

        if nums[middle] == item:
            return middle
        if nums[middle] > item:
            high = middle - 1
        if nums[middle] < item:
            low = middle + 1
    return None


def smallest_find(nums: List[int]):
    smallest = nums[0]
    smallest_index = 0

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


def selection_sort(nums: List[int]):
    sorted_nums = []
    nums_ = nums[:] # shallow copy
    for _ in range(len(nums_)):
        smallest_index = smallest_find(nums_)
        sorted_nums.append(nums_.pop(smallest_index))
    return sorted_nums

import random

while True:
    nums = [random.randint(-100, 100) for _ in range(20)]
    item = 24
    if item in nums:
        print(nums)
        print(selection_sort(nums))
        print(binary_search(selection_sort(nums), item))
        break

Output:

[41, -59, 38, -50, 25, -62, 58, -81, 7, 24, -93, 41, 92, 3, 65, -61, 47, -54, 72, 65]
[-93, -81, -62, -61, -59, -54, -50, 3, 7, 24, 25, 38, 41, 41, 47, 58, 65, 65, 72, 92]
binary_search nums [-93, -81, -62, -61, -59, -54, -50, 3, 7, 24, 25, 38, 41, 41, 47, 58, 65, 65, 72, 92]
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