having problems with a multi-conditional while statement

Ned Deily nad at acm.org
Wed Jan 7 01:42:26 CET 2009


In article 
<40a44d6b-c638-464d-b166-ef66496a0676 at l16g2000yqo.googlegroups.com>,
 "bowman.joseph at gmail.com" <bowman.joseph at gmail.com> wrote:

> Hi,
> 
> I'm trying to write a multi-conditional while statement, and am having
> problems. I've broken it down to this simple demo.
> 
> #!/usr/bin/python2.5
> 
> condition1 = False
> condition2 = False
> 
> while not condition1 and not condition2:
>     print 'conditions met'
>     if condition1:
>         condition2 = True
>     condition1 = True
> 
> 
> As I understand it, this should print 'conditions met' twice, however,
> it only prints it once. It seems that once condition1 is made true,
> the whole thing evaluates as true and stops the while loop.

Are you perhaps expecting that the "while" condition is tested at the 
end of the loop?   It's not; it is tested at the top of the loop, so, 
once the condition evaluates as false, the loop exits.  This can even 
result in zero trips:

>>> while False:
...     print "never"
... 
>>> 

Unwinding the snippet above:

>>> condition1 = False
>>> condition2 = False
>>> not condition1 and not condition2
True
>>> if condition1:
...     condition2 = True
... 
>>> condition1 = True
>>> not condition1 and not condition2
False

# -> while loop exits after 1 trip

-- 
 Ned Deily,
 nad at acm.org




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