On 1/22/2013 3:09 PM, Ethan Furman wrote:
On 01/22/2013 09:44 AM, Terry Reedy wrote:
Several people have trouble understanding Python's while-else and
for-else constructs. It is actually quite simple if one starts with
if-else, which few have any trouble with.
Start with, for example
if n > 0:
n -= 1
else:
n = None
The else clause is executed if and when the condition is false. (That
the code is useless is not the point here.) Now use pseudo-Python label
and goto statements to repeatedly decrement n
label: check
if n > 0:
n -= 1
goto: check
else:
n = None
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.
--
Terry Jan Reedy
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