Why can't I xor strings?

dataangel k04jg02 at kzoo.edu
Sat Oct 9 01:15:59 CEST 2004


Phil Frost wrote:

>On Fri, Oct 08, 2004 at 04:57:13PM -0400, dataangel wrote:
>  
>
>>Phil Frost wrote:
>>
>>    
>>
>>>^ is the bitwise xor operator. Performing bitwise operations on strings
>>>doesn't make much sense, so it's invalid. You can define a logical xor
>>>function like so:
>>>
>>>xor = lambda p, q: (p and not q) or (not p and q)
>>>
>>>or equivalently:
>>>
>>>xor = lambda p, q: bool(p) != bool(q)
>>>
>>>On Fri, Oct 08, 2004 at 04:19:22PM -0400, dataangel wrote:
>>>
>>>
>>>      
>>>
>>>>I wrote a function to compare whether two strings are "similar" because 
>>>>I'm using python to make a small text adventure engine and I want to it 
>>>>to be responsive to slight mispellings, like "inevtory" and the like. To 
>>>>save time the first thing my function does is check if I'm comparing an 
>>>>empty vs. a non-empty string, because they are to be never considered 
>>>>similar. Right now I have to write out the check like this:
>>>>
>>>> if str1 and str2:
>>>>     if not str1 or not str2:
>>>>         return 0
>>>>
>>>>Because python won't let me do str1 ^ str2. Why does this only work for 
>>>>numbers? Just treat empty strings as 0 and nonempty as 1.
>>>>
>>>>Regardless of whether this is the best implementation for detecting if 
>>>>two strings are similar, I don't see why xor for strings shouldn't be 
>>>>supported. Am I missing something? Inparticular, I think it'd be cool to 
>>>>have "xor" as opposed to "^". The carrot would return the resulting 
>>>>value, while "xor" would act like and/or do and return the one that was 
>>>>true (if any).
>>>>        
>>>>
>>Urk, ran into a little trouble with your code (the first one):
>>
>>    
>>
>>>>>xor("", "") == False
>>>>>          
>>>>>
>>False
>>    
>>
>>>>>xor("", "") == True
>>>>>          
>>>>>
>>False
>>
>>I'm a python newbie so I'm not sure how something can niether evaluate 
>>to be true nor false at the same time :P Bug?
>>    
>>
>
>
>Sorry, I didn't read your problem well enough. False xor False == False
>by definition; the problem is that xor is not what you want. In your
>example:
>
>if str1 and str2:
>  if not str1 or not str2:
>    return 0
>
>"return 0" will never be reached. In order for "str1 and str2" to be
>true, both strings must be not empty. Your test within that, "if not
>str1 or not str2" says "if either string is empty...", which isn't
>possible, since you just tested that neither is empty. What you probably
>want is this:
>
>if not (str1 and str2):
>  # at least one string is empty
>  
>
Sorry, was confusing what logical or does with what bitwise or does ;)

And the reason the xor example confused me was because == was testing if 
it was the false object, which is not what I wanted, I wanted to know if 
its boolean equivalent was (which if implicitly casts stuff to).




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