comparing booleans

[Gerrit Holl]

Hi,

is it proper to compare booleans? It is possible, of course, because
they're compatible with numbers, but booleans aren't truly numbers. I'm
tempted to write:

return cmp(self.extend s, other.extends)

instead of

if self.extends and not other.extends:
return 1
else:
return -1
# I've already verified self.extends != other.extends

....but somehow comparing Booleans doesn't feel right...
Is my feeling correct?

(Hmm, makes me wonder, for booleans, are != and ^ equal?)

Gerrit.

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[Erik Max Francis]

Re: comparing booleans

Gerrit Holl wrote:
[color=blue]
> is it proper to compare booleans? It is possible, of course, because
> they're compatible with numbers, but booleans aren't truly numbers.
> I'm
> tempted to write:
>
> return cmp(self.extend s, other.extends)[/color]

Even if you're seriously worried about the correctness of comparing
Booleans, you can always explicitly turn them into integers:

return cmp(int(self.ex tends), int(other.exten ds))
[color=blue]
> (Hmm, makes me wonder, for booleans, are != and ^ equal?)[/color]

Easily checked:
[color=blue][color=green][color=darkred]
>>> for x in (True, False):[/color][/color][/color]
.... for y in (True, False):
.... print x, y, x != y, x ^ y
....
True True False False
True False True True
False True True True
False False False False

--
__ Erik Max Francis && max@alcyone.com && http://www.alcyone.com/max/
/ \ San Jose, CA, USA && 37 20 N 121 53 W && &tSftDotIotE
\__/ Weakness of attitude becomes weakness of character.
-- Albert Einstein

[Dang Griffith]

Re: comparing booleans

On Wed, 28 Jan 2004 16:17:22 -0800, Erik Max Francis <max@alcyone.co m>
wrote:
[color=blue]
>Gerrit Holl wrote:
>[color=green]
>> is it proper to compare booleans? It is possible, of course, because
>> they're compatible with numbers, but booleans aren't truly numbers.
>> I'm
>> tempted to write:
>>
>> return cmp(self.extend s, other.extends)[/color]
>
>Even if you're seriously worried about the correctness of comparing
>Booleans, you can always explicitly turn them into integers:
>
> return cmp(int(self.ex tends), int(other.exten ds))
>[color=green]
>> (Hmm, makes me wonder, for booleans, are != and ^ equal?)[/color]
>
>Easily checked:
>[color=green][color=darkred]
>>>> for x in (True, False):[/color][/color]
>... for y in (True, False):
>... print x, y, x != y, x ^ y
>...
>True True False False
>True False True True
>False True True True
>False False False False[/color]

It's not always the same, as shown here:
[color=blue][color=green][color=darkred]
>>> True != True != True[/color][/color][/color]
False[color=blue][color=green][color=darkred]
>>> True ^ True ^ True[/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>> True != False != True != True[/color][/color][/color]
False[color=blue][color=green][color=darkred]
>>> True ^ False ^ True ^ True[/color][/color][/color]
True

Not that I've never used this (cool) Python syntax in practice,
but I thought it was worth mentioning that using != and ^ in
boolean expressions does not always give the same result. It does
give the same result when there are only two operands.

Interestingly, and I'm not sure why:[color=blue][color=green][color=darkred]
>>> (True != True) != True[/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>> True != (True != True)[/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>> True != True != True[/color][/color][/color]
False

--dang

[Duncan Booth]

Re: comparing booleans

Dang Griffith <noemail@noemai l4u.com> wrote in
news:eaa90cc908 c7a217f0d6475e4 1c7622d@news.te ranews.com:
[color=blue]
> Interestingly, and I'm not sure why:[color=green][color=darkred]
>>>> (True != True) != True[/color][/color]
> True[color=green][color=darkred]
>>>> True != (True != True)[/color][/color]
> True[color=green][color=darkred]
>>>> True != True != True[/color][/color]
> False[/color]

You can chain comparison operators in Python. e.g.

a < b < c

is the same as:

(a < b) && (b < c)

except that if b is an expression the first form evaluates it exactly once
whereas the second form evaluates it either once or twice.

You can use any comparison operators in this form, so your True!=True!=Tru e
is just shorthand for:

(True!=True) && (True!=True)

which is in turn equivalent to:

False && (True!=True)

and 'False && anything' gives False.

[Erik Max Francis]

Re: comparing booleans

Dang Griffith wrote:
[color=blue]
> Not that I've never used this (cool) Python syntax in practice,
> but I thought it was worth mentioning that using != and ^ in
> boolean expressions does not always give the same result. It does
> give the same result when there are only two operands.[/color]

This is because of operator chaining, which only exists a special case
for the relational operators (==, !=, <, <=, >, >=). It's not actually
a difference in the truth table; it's because chaining operators behave
differently than other operators.

--
__ Erik Max Francis && max@alcyone.com && http://www.alcyone.com/max/
/ \ San Jose, CA, USA && 37 20 N 121 53 W && &tSftDotIotE
\__/ The work will teach you how to do it.
-- (an Estonian proverb)

[zde]

Re: comparing booleans

Erik Max Francis wrote:
[color=blue]
> This is because of operator chaining, which only exists a special case
> for the relational operators (==, !=, <, <=, >, >=). It's not actually
> a difference in the truth table; it's because chaining operators behave
> differently than other operators.[/color]

Unfortunately, this mis-feature works for other operators as well:
Since 'in' operator usually has higher precedence than '==', it's
pretty annoying to see:

Python 2.3.3 (#2, Jan 4 2004, 12:24:16)[color=blue][color=green][color=darkred]
>>> 'a' in 'abc'[/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>> 'a' in 'abc' == True[/color][/color][/color]
False[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]