python return within loop

Question

If I define:

def hasNoX_2(s):
    if type(s)!=str:
       return False
    for c in s:
        if c!='x' or c!='X':
           return True
    return False

and enter hasNoX_2('Xenon'), True is returned and I'm not sure why. Nothing is being returned in the first If statement, since it only returns False if s is NOT a string. So what is the for loop "looking" at when it says, "Hey, I don't see 'x' or 'X', so I can return True?"

because your logic tests if at least one letter is not x. Just run manually through your code and you'll see why. — Julien
– Julien, Commented Sep 13, 2017 at 0:28
it returns on first itteration, because evaluation of your if statement is True or False, which is obviously True — Yaroslav Surzhikov
– Yaroslav Surzhikov, Commented Sep 13, 2017 at 0:30
the code returns True if ANY character in the string is not an 'x' or 'X' — kdopen
– kdopen, Commented Sep 13, 2017 at 0:31
Actually, it will always return True - even for the string 'xxxxx'. the only string it will return False for is the empty string "" — kdopen
– kdopen, Commented Sep 13, 2017 at 0:34

kdopen · Accepted Answer · 2017-09-13 11:20:53Z

2

Let's focus just on the if statement

if c!='x' or c!='X':

This is a boolean expression with two terms

c != 'x' and
c != 'X'

As you use the or operator, if either term evaluates as True, then the entire expression evaluates as True. If the expression evaluates as True, then the body of the if statement (in this case return True) is executed.

Here is the standard truth table for an or expression (A or B)

 A or B     | A = False | A = True |
 ----------------------------------
| B = False | False     | True     |
| B = True  | True      | True     |

As you can see, the result is False only if both terms are False

Let's look at how your expression evaluates for each type of input

c                   | c != 'x' | c != 'X' | or expression |
----------------------------------------------------------
'x'                 | False    | True     | True          |
'X'                 | True     | False    | True          |
any other character | True     | True     | True          |

In short, your expression will always evaluate as True - and the if branch will always be taken.

So, the function will return True when it examines the first character in the provided string.

The only ways this function can return False are

If something other than a str is provided
If an empty string is provided - because the for loop is never entered.

answered Sep 13, 2017 at 11:20

kdopen

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1 Comment

g.z. Over a year ago

Thank you for taking the time to do this! This is very helpful.

Yaroslav Surzhikov · Accepted Answer · 2017-09-13 00:36:01Z

1

I suppouse you trying to achieve True if all chars in input string not equal 'x' or 'X'.

If so, you could simply use the following code:

def hasNoX_2(s):

    if isinstance(s, str):
        return 'x' not in s.lower()

    return False

print(hasNoX_2('Xenon'))
print(hasNoX_2('eenon'))

answered Sep 13, 2017 at 0:36

Yaroslav Surzhikov

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1 Comment

kdopen Over a year ago

This is valid if you have interpreted the OP's intent correctly. However, they are actually asking why their code always returns True. See my answer

Dhruv Mohan Datta · Accepted Answer · 2024-04-19 05:02:35Z

1

So basically you could scan the characters of the string one at a time till you find 'x' or 'X', and return false. In case you reach the end of the loop iterating over the characters of the string, it would be a good opportunity to return True then, as no 'x' or 'X' was found anyways.

so your new code would look something like this:

    def hasNoX_2(s):
       if type(s)!=str:
           return False
       for c in s:
           if c=='x' or c=='X':
               return False
       return True

edited Apr 19, 2024 at 5:02

answered Apr 19, 2024 at 4:59

Dhruv Mohan Datta

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Comments

jamiroquai93 · Accepted Answer · 2017-09-13 14:40:27Z

0

You iterate in your for loop through each character in the string. So it's looking at each character. Pretty much your loop is looking for when those if statement is true.

However using or if one of them is true the whole thing conditional evaluates to true so use and instead to make sure both statements are true

edited Sep 13, 2017 at 14:40

answered Sep 13, 2017 at 1:10

jamiroquai93

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3 Comments

kdopen Over a year ago

See my answer - the if statement evaluates as true for ALL characters.

jamiroquai93 Over a year ago

Right I see now that person would most likely have to use and to get both cases

kdopen Over a year ago

Probably .. but the function name is unhelpful. Does the function return True if there are "No X's in the string" or if there is "something other than X in the string"? That's why I just tried to focus on explaining the result they were seeing.

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