Python function and list not working as expected

Question

I created a fairly simple function; however, it isn’t performing as desired. I think i’m missing something with my if line but not sure.

I have two lists. I want to take the first number of list 1 and see if it is the same number as the first number in list 2. Then I want to do that all the way through.

I should end with 17 ‘Yes’ responses in my list (l)

preds = [1, 0, 0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 0, 1, 1, 0, 0, 1]
labels = [1, 0, 0, 1,  0, 1, 0, 0, 1, 1, 0, 0, 0, 0, 1,  0, 1]



def fix(x,y):
    l = []
    for i in x:
        if i == y:
            l.append('yes')
        else:
            l.append('no')
    return(l)


fix(labels, preds)

Asked By: Evan Day

||

Source

Answer 1

You are comparing the elements of x to the variable holding list y, which will always be false.
To accomplish this easiest in a pythonic way use zip, as below:

preds = [1, 0, 0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 0, 1, 1, 0, 0, 1]
labels = [1, 0, 0, 1,  0, 1, 0, 0, 1, 1, 0, 0, 0, 0, 1,  0, 1]



def fix(x,y):
    l = []
    for i,j in zip(x,y):
        if i == j:
            l.append('yes')
        else:
            l.append('no')
    return l


fix(labels, preds)

Zip returns a tuple with the elements of each list at a given index: https://www.programiz.com/python-programming/methods/built-in/zip

Here is a list comprehension that does what you want in one line:

['yes' if i == j else 'no'  for i, j in zip(preds, labels)]

Answered By: nklisch

Answer 2

If you wanted to see if all element of one list appear in another list in that order (with possible extra elements in the second list), this would be an approach:

preds = [1, 0, 0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 0, 1, 1, 0, 0, 1]
labels = [1, 0, 0, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 0, 1, 0, 1]


def contained_in_order(xs: list, ys: list) -> bool:
    i = 0
    try:
        for x in xs:
            i = ys.index(x, i) + 1
        return True
    except ValueError:
        return False


print(contained_in_order(labels, preds))

This prints True for the lists you provided as labels is indeed contained in pred in order.

Howevever, a list of 'yes' and 'no' values wouldn’t make sense in this context, since there may be many ways to match. For example, consider:

preds = [1, 0, 1]
labels = [0, 0]

Clearly, labels is not contained in preds in order. But both [1, 0] and [0, 1] would be. So should the result be ['yes', 'no'] or ['no', 'yes']?

You could write a function that generates all possible mismatches, but I doubt that’s what you’re after. Perhaps you can update the question to clarify what exactly you’re trying to achieve, or comment on the answer.

Edit: If you’re just after a list of matches, which you say you’d like to see as strings, this is all you need:

preds = [1, 0, 0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 0, 1, 1, 0, 0, 1]
labels = [1, 0, 0, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 0, 1, 0, 1]

yes_no = ['yes' if p == l else 'no' for p, l in zip(preds, labels)]
print(yes_no)

Output:

['yes', 'yes', 'yes', 'yes', 'no', 'no', 'no', 'yes', 'no', 'yes', 'no', 'yes', 'yes', 'yes', 'no', 'no', 'yes']

Answered By: Grismar

Python function and list not working as expected

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