Understanding while...else...

René Klačan rene.klacan at gmail.com
Wed Jan 23 12:03:32 CET 2013


>
> # Version 1
> while condition:
>     # stuff
> else:
>     # other stuff
>
> # Version 2
> if condition:
>     while condition:
>         # stuff
> else:
>     # other stuff
>

they wouldnt be equivalent if #staff in version did not cointain "break"
statement and this is common mistake

On Wed, Jan 23, 2013 at 12:03 PM, René Klačan <rene.klacan at gmail.com> wrote:

> they wouldnt be equivalent if #staff in version 1 did not cointain "break"
> statement and this is common mistake
>
>
> On Wed, Jan 23, 2013 at 1:39 AM, Oscar Benjamin <
> oscar.j.benjamin at gmail.com> wrote:
>
>> On 22 January 2013 23:41, Terry Reedy <tjreedy at udel.edu> wrote:
>> > On 1/22/2013 3:09 PM, Ethan Furman wrote:
>> >>
>> >> On 01/22/2013 09:44 AM, Terry Reedy wrote:
>> >>>
>> [SNIP]
>> >>> The else clause is executed if and when the condition is false.
>> >>> Now use a real Python while statement to do the *same
>> >>> thing*.
>> >>>
>> >>> while n > 0:
>> >>>    n -= 1
>> >>> else:
>> >>>    n = None
>> >>
>> >>
>> >> I understand how it works (although it did take a while for it to sink
>> >> in); my gripe, and probably why it is misunderstood so often, is that
>> >> nine times out of ten when I /want/ to use a while-else or for-else I
>> >> only want the true/false check /once/, at the beginning of the loop.
>> >
>> >
>> > I do not understand what you are saying. There already is only one
>> > true/false check, at the beginning of the loop. If you only want the
>> check
>> > *performed* once, you would use if-else. But I presume you know this.
>>
>> I think he meant that he would use the else clause more often if it
>> had the semantics so that the two blocks below were equivalent:
>>
>> # Version 1
>> while condition:
>>     # stuff
>> else:
>>     # other stuff
>>
>> # Version 2
>> if condition:
>>     while condition:
>>         # stuff
>> else:
>>     # other stuff
>>
>> So he wants a convenient way to execute code only if the loop
>> performed zero iterations. I think that often when people are confused
>> about the else clause on while loops it is because they expect this
>> behaviour (which would also be useful). The same confusion arises with
>> for loops where people expect the else clause to execute if the
>> iterable was empty so that these would be equivalent:
>>
>> # Version 1
>> for x in iterable:
>>     # stuff
>> else:
>>     # other stuff
>>
>> # Version 2
>> iterated = False
>> for x in iterable:
>>     iterated = True
>>     # stuff
>> if not iterated:
>>     # other stuff
>>
>>
>> Oscar
>> --
>> http://mail.python.org/mailman/listinfo/python-list
>>
>
>
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