Python: Unexpected print values from within a simple function

Question

I have created an simple, arbritrary function that prints out a value (1) based on what the first element from a randomly shuffled list is.

Looking at the code below: I am creating a variable and setting it to 1, then I'm a creating a small list (1,2,3) and then shuffling the list. The idea is, if the first element of that list (after shuffle) is 1 or 2, then print the value of 'correct', BUT if the first element of the list happens to be 3, set 'correct' to 0, and then run the function again. As you can see from the if statement, when 'value' is set to 0, I don't expect this to be printed at all because the function is being run again (chair()) before any printing takes place.

Having run this, I expected to find a print out of 5 values of 1. The problem is, I am seeing 0 values printed too. This seems like a very simple function, and I feel a little but stupid for not seeing why this is happenig. Any ideas?

def chair():

    correct = 1

    a = [1,2,3]
    random.shuffle(a)

    if a[0] == 3:
        correct = 0
        chair()               #run function again without printing

    print correct


for i in range(5):
    chair()

zondo · Accepted Answer · 2016-04-02 23:43:18Z

1

Just calling chair() does not modify correct. You need to do correct = chair(). The problem with that is chair() does not return anything; it prints it. You should move the printing to outside of the function and return instead:

def chair():

    correct = 1

    a = [1,2,3]
    random.shuffle(a)

    if a[0] == 3:
        correct = chair()

    return correct


for i in range(5):
    print chair()

answered Apr 2, 2016 at 23:43

zondo

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6 Comments

danidee Over a year ago

why can't you just return chair() instead of assigning it to correct

zondo Over a year ago

@danidee: No reason. I don't see a benefit either way.

jDo Over a year ago

@danidee Wouldn't that basically make correct superfluous? Would the value still be returned to the for loop?

danidee Over a year ago

yes it will...it could simply be removed instead of assigning chair() to it again for no reason

jDo Over a year ago

@danidee ah, you mean having two return statements? Replacing correct = chair() with return chair() and leaving return correct as it is? I thought you meant deleting return correct and only returning chair() from the function (and only if a[0] == 3)

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Have_Mercy · Accepted Answer · 2016-04-02 23:51:19Z

0

Your function is recursive, so when you recall the function again to avoid printing, the function will still finish. If you added an else statement, you'd see those zeros disappear. Ex:

def chair():

    correct = 1

    a = [1,2,3]
    random.shuffle(a)

    if a[0] == 3:
        correct = 0
        chair()               #run function again without printing
    else:
        print correct


for i in range(5):
    chair()

edit: grammar edit: example

edited Apr 2, 2016 at 23:51

answered Apr 2, 2016 at 23:44

Have_Mercy

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1 Comment

Steve Over a year ago

Perfect. That small change was exactly what I needed. Thank you.

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