The use of "or" in evaluating integers

Question

The problem "Given 2 ints, a and b, return True if one if them is 10 or if their sum is 10."

In the first test why does entering (9,9) and (8,3) return True?

I have since solved the problem using a different solution (further below) but I am struggling to understand why the first solution does not work since neither integer is 10 nor do they equal 10 when summed together!

Thanks for your help.

def makes10(a, b):                                        

    if a or b == 10:
        return True

    if a + b == 10:
        return True

    else:
        return False

def makes10(a, b):

    if a == 10:
        return True

    if b == 10:
        return True

    if a + b == 10:
        return True

    else:
        return False

`a or b == 10` means `(a != 0) or (b == 10)`. The construct you wanted is `a == 10 or b == 10`. Is that enough for you to understand your mistake? — zwol, Mar 22 '16 at 14:16
Duplicate of [this](http://stackoverflow.com/questions/15112125/how-do-i-test-one-variable-against-multiple-values) — timgeb, Mar 22 '16 at 14:18
Ah, yes. @timgeb: that is a much better duplicate than the one I picked. Variations of this question have been asked so many times. Thanks! — Bryan Oakley, Mar 22 '16 at 14:18
Aside: `return (a == 10) or (b == 10) or (a+b == 10)` may be more readable. — P.P, Mar 22 '16 at 14:19
This is the proposed solution: def makes10(a, b): return (a == 10 or b == 10 or a+b == 10) — Gav Mac, Mar 22 '16 at 14:21

score 3 · Answer 1 · answered Mar 22 '16 at 14:15

3

It says a or b == 10, since a is true, i.e not zero it returns true.

answered Mar 22 '16 at 14:15

uSeemSurprised

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Marc B · Answer 2 · 2016-03-22T14:18:23.577

1

Because this:

if a or b == 10:

is executed as the equivalent of

temp = (a or b)
if (temp == 10):

YOu can NOT test multiple values against a single fixed value like that. You have to test each one individually, e.g. if you had

if (a == 10) or (b == 10):

the statement would have worked. Read the docs: https://docs.python.org/3/reference/expressions.html#operator-precedence

edited Mar 22 '16 at 14:18

answered Mar 22 '16 at 14:16

Marc B

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I think `a` evaluates to True and `b` is not evaluated. – Jérôme Mar 22 '16 at 14:18
yes. but `or` still has a higher priority than `==`, so you're testing the result of the `or` against `10`, not the individual `a` or `b` values. – Marc B Mar 22 '16 at 14:19

score 1 · Answer 3 · answered Mar 22 '16 at 14:16

1

a or b == 10

This evaluates to True as a evaluates to True

You must write

(a == 10) or (b == 10)

answered Mar 22 '16 at 14:16

Jérôme

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score 1 · Answer 4 · answered Mar 22 '16 at 14:17

1

if a or b == 10:

That doesn't do what you think it does. It will be evaluated as:

if (a) or (b == 10):

You most likely want:

if a == 10 or b == 10:

answered Mar 22 '16 at 14:17

Carcigenicate

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So any number other than "0" would return True if you wrote: if a: ... – Gav Mac Mar 22 '16 at 14:29
@GavMac Yes. That's correct. Look up the terms "truthy" and "falsey". – Carcigenicate Mar 22 '16 at 14:38
Thanks Carcigenicate! – Gav Mac Mar 22 '16 at 15:28
@GavMac No problem. Get lots of practice setting up conditions and understanding how truthy and falsey values work. You'll be using those concepts constantly. – Carcigenicate Mar 22 '16 at 16:06

score 1 · Answer 5 · answered Mar 22 '16 at 14:17

a or b == 10

This will first evaluate a in the context of a boolean expression, and if this is true the whole expression is just true. If it is false, then it will evaluate b == 10. It is different from a == 10 or b == 10.

Alternatively, you would just say

if 10 in [a, b]:

this will also do what you want to accomplish.

The use of "or" in evaluating integers

5 Answers5