Re: Boolean confusion
On 2007-05-09, Greg Corradini <[EMAIL PROTECTED]> wrote:
>
> Hello all,
> I'm having trouble understanding why the following code evaluates as it
> does:
>
>>>> string.find('020914A','.') and len('020914A') > 10
> True
>>>> len('020914A') > 10 and string.find('020914A','.')
> -1
>
> In the 2.4 Python Reference Manual, I get the following explanation for
> the
> 'and' operator in 5.10 Boolean operations:
> " The expression x and y first evaluates x; if x is false, its value is
> returned; otherwise, y is evaluated and the resulting value is returned."
>
> Based on what is said above, shouldn't my first expression (
> string.find('020914A','.') and len('020914A') > 10) evaluate to
> false b/c my 'x' is false? And shouldn't the second expression evaluate to
> True?
>The find method doesn't return a boolean, but returns the index where
>the substring was found with -1 indicating it wasn't found. If you just
>want to check wether one string is a substring of an other, use the in
>operator.
>>> '.' in '020914A' and len('020914A') > 10
False
>>> len('020914A') > 10 and '.' in '020914A'
False
Thank you Diez and Antoon for demystifing this problem. I see where I've
been going wrong.
--
View this message in context:
http://www.nabble.com/Boolean-confusion-tf3715438.html#a10393765
Sent from the Python - python-list mailing list archive at Nabble.com.
--
http://mail.python.org/mailman/listinfo/python-list
Re: Boolean confusion
Thank you Diez and Antoon for demystifing this problem. I see where I've been
going wrong.
Diez B. Roggisch-2 wrote:
>
> Greg Corradini wrote:
>
>>
>> Hello all,
>> I'm having trouble understanding why the following code evaluates as it
>> does:
>>
>>>>> string.find('020914A','.') and len('020914A') > 10
>> True
>>>>> len('020914A') > 10 and string.find('020914A','.')
>> -1
>>
>> In the 2.4 Python Reference Manual, I get the following explanation for
>> the 'and' operator in 5.10 Boolean operations:
>> " The expression x and y first evaluates x; if x is false, its value is
>> returned; otherwise, y is evaluated and the resulting value is returned."
>>
>> Based on what is said above, shouldn't my first expression (
>> string.find('020914A','.') and len('020914A') > 10) evaluate to
>> false b/c my 'x' is false? And shouldn't the second expression evaluate
>> to
>> True?
>
> The first evaluates to True because len(...) > 10 will return a boolean -
> which is True, and the semantics of the "and"-operator will return that
> value.
>
> And that precisely is the reason for the -1 in the second expression.
>
> y=-1
>
> and it's just returned by the and.
>
> in python, and is implemented like this (strict evaluation
> nonwithstanding):
>
> def and(x, y):
> if bool(x) == True:
>return y
> return x
>
> Diez
> --
> http://mail.python.org/mailman/listinfo/python-list
>
>
--
View this message in context:
http://www.nabble.com/Boolean-confusion-tf3715438.html#a10393705
Sent from the Python - python-list mailing list archive at Nabble.com.
--
http://mail.python.org/mailman/listinfo/python-list
Re: Boolean confusion
On 2007-05-09, Greg Corradini <[EMAIL PROTECTED]> wrote:
>
> Hello all,
> I'm having trouble understanding why the following code evaluates as it
> does:
>
string.find('020914A','.') and len('020914A') > 10
> True
len('020914A') > 10 and string.find('020914A','.')
> -1
>
> In the 2.4 Python Reference Manual, I get the following explanation for the
> 'and' operator in 5.10 Boolean operations:
> " The expression x and y first evaluates x; if x is false, its value is
> returned; otherwise, y is evaluated and the resulting value is returned."
>
> Based on what is said above, shouldn't my first expression (
> string.find('020914A','.') and len('020914A') > 10) evaluate to
> false b/c my 'x' is false? And shouldn't the second expression evaluate to
> True?
The find method doesn't return a boolean, but returns the index where
the substring was found with -1 indicating it wasn't found. If you just
want to check wether one string is a substring of an other, use the in
operator.
>>> '.' in '020914A' and len('020914A') > 10
False
>>> len('020914A') > 10 and '.' in '020914A'
False
--
Antoon Pardon
--
http://mail.python.org/mailman/listinfo/python-list
Re: Boolean confusion
Greg Corradini wrote:
>
> Hello all,
> I'm having trouble understanding why the following code evaluates as it
> does:
>
string.find('020914A','.') and len('020914A') > 10
> True
len('020914A') > 10 and string.find('020914A','.')
> -1
>
> In the 2.4 Python Reference Manual, I get the following explanation for
> the 'and' operator in 5.10 Boolean operations:
> " The expression x and y first evaluates x; if x is false, its value is
> returned; otherwise, y is evaluated and the resulting value is returned."
>
> Based on what is said above, shouldn't my first expression (
> string.find('020914A','.') and len('020914A') > 10) evaluate to
> false b/c my 'x' is false? And shouldn't the second expression evaluate to
> True?
The first evaluates to True because len(...) > 10 will return a boolean -
which is True, and the semantics of the "and"-operator will return that
value.
And that precisely is the reason for the -1 in the second expression.
y=-1
and it's just returned by the and.
in python, and is implemented like this (strict evaluation nonwithstanding):
def and(x, y):
if bool(x) == True:
return y
return x
Diez
--
http://mail.python.org/mailman/listinfo/python-list
Boolean confusion
Hello all,
I'm having trouble understanding why the following code evaluates as it
does:
>>> string.find('020914A','.') and len('020914A') > 10
True
>>> len('020914A') > 10 and string.find('020914A','.')
-1
In the 2.4 Python Reference Manual, I get the following explanation for the
'and' operator in 5.10 Boolean operations:
" The expression x and y first evaluates x; if x is false, its value is
returned; otherwise, y is evaluated and the resulting value is returned."
Based on what is said above, shouldn't my first expression (
string.find('020914A','.') and len('020914A') > 10) evaluate to
false b/c my 'x' is false? And shouldn't the second expression evaluate to
True?
Thanks for your help
Greg
--
View this message in context:
http://www.nabble.com/Boolean-confusion-tf3715438.html#a10393362
Sent from the Python - python-list mailing list archive at Nabble.com.
--
http://mail.python.org/mailman/listinfo/python-list
