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)

CodePudding user response：

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.

CodePudding user response：

I think it should work as following:

def count_x(items:list, x ):
    count = 0
    for item in items:
        if item == x:
            count = count   1
    return count

count = count_x(items, x)

You could also simply use count = items.count(x) tho.

CodePudding user response：

There are three cases in this recursion:

1. The list is empty
2. The popped item is the value you are counting
3. The popped item is not the value you are counting

You have treated the first two cases, but not the third.

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

    return count_x(items, x) # Fix