Objective - Remove even numbers from a given list
Example Input = [1,2,3,4,5,6]
Output = [1,3,5]

My initial solution is as follows:-

def remove_even_error(lst):
    for number in lst:
        if number % 2 == 0:
            lst.remove(number)                           
    return lst

I used the below list to test my function my_list = [-147,17,-73,80,-188,198,-4,-10,-12,-20,-40]

The result was not as expected.

remove_even_error(my_list)

[-147, 17, -73, -4, -40]

My initial thought was modulus operator is not working as expected. I checked the modulus of a few numbers and it was working fine. \

-40 % 2

Output was 0

-73 % 2

Output was 1

Even though the modulus is working fine, I was not able to remove the even numbers from the list.

I googled what might the reason, and found something interesting about modulus operator.

Guess what is the result of -5 % 4 without reading further? Astonishingly it is 3.

The modulus operator works as follows in python \

modulus_git \

(Image Credit: https://stackoverflow.com/questions/3883004/the-modulo-operation-on-negative-numbers-in-python)

But this was not happening in my case. I used a bunch of print statements to check what was going wrong. After spending some time, I found the reason. \

function_error

I was modifying the list itself which I was iterating over. Once created a new list, I was able to remove the even numbers from the list.

def remove_even(lst):
    new_list = lst.copy()
    for i in range(len(lst)):
        if (lst[i] % 2) == 0:
            new_list.remove(lst[i])                           
    return new_list

remove_even(my_list)

[-81, 187, -199, 3, 9, 11, 19, 39]

Further using list comprehension, the code is as follows

def remove_even(lst):
    return [~number for number in lst if number % 2 == 0]

Two important things I learnt from this challenge:-

  • Behaviour of modulus operator in python
  • To change perspective when you are struck for a long time. I was very sure that issue was with the modulus operator. At last the issue turned out to be with modifying the list itself.

In my next challenge, I will be tackling a _challenge due to which I lost a job oppurtunity in the past._ More about it in my next challenge.