Number of occurrences of a number in a list

January 19, 2023

This program below is supposed to count the number of occurrences of x in a list. Can not identify the error in the code

def count_x( items, x ):
    if items==[]:
        return 0
    first = items.pop(0)
    if first == x:
        return 1 + count_x(items, x)

>Solution :

There are better ways to do this, but it’s worth addressing why your code is throwing an error.

You don’t have a case for when the item popped is not equal to your search item. This causes the function to return a None. Since it is working recursively, it tries to compute int + None, which leads to an error.

The other issue is that you are modifying the list with the function, which you may not want. For example:

def count_x( items, x ):
    print(items)
    if items==[]:
        return 0
    first = items.pop(0)
    if first == x:
        return 1 + count_x(items, x)
    
    else:
        return count_x(items, x)

items = [1, 2, 1, 1]

print(count_x(items, 1))

print(items)

Your items will become an empty list after you run the function.