[SOLVED] Merge Sort IndexError


This Content is from Stack Overflow. Question asked by user366018

This is my first merge sort implementation with Python.

unsorted_list = [7, 4, 2, 1]

def merge(left_array, right_array):
    lst = []

    # compare the element in both array
    i = 0
    while len(left_array) > 0 and len(right_array) > 0:
        if left_array[i] <= right_array[i]:
            del left_array[i]
            i += 1
            del right_array[i]
            i += 1

    # append the rest if one of the array is empty
    if len(left_array) > 0:
        for j in left_array:

    if len(right_array) > 0:
        for h in right_array:

    # return the sorted sub-list
    return lst

def merge_sort(array):
    # return the array if there is only 1 element
    if len(array) <= 1:
        return array
    # divide the array in 2 halves
    mid = (len(array))//2
    left = array[:mid]
    right = array[mid:]

    # recursion
    return merge(left, right)


But the console showed this error instead of a sorted list:

 File "c:Users...merge_sort.py", line 64, in <module>
  File "c:Users...merge_sort.py", line 62, in merge_sort
    return merge(left, right)
  File "c:Users...merge_sort.py", line 28, in merge
    if left_array[i] <= right_array[i]:
IndexError: list index out of range

I expect the result should be this or somthing like that:

[1, 2, 4, 7]

As i goes from 0 and it stop adding if one array i.e. right_array & left_array goes empty, i should not be greater than the largest index of the array.

I dont know how to solve my error. how to correct it?


Remove the 2 lines where i+=1. and replace i with 0.

In your current code while you deleting num from list you increment i by 1. Then actualy miss some numbers in in both left and right arrays.

Ex: in a list(variable name as arr) have 2 number as follows[1,2]. If you delete 1 then new list is [2]. When you call arr[1].Now it return indexError because now there is no 2 nums in list.

This Question was asked in StackOverflow by user366018 and Answered by YJR It is licensed under the terms of CC BY-SA 2.5. - CC BY-SA 3.0. - CC BY-SA 4.0.

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