Noob: Loops and the 'else' construct

[MRAB]

On Oct 19, 4:11 am, "Gabriel Genellina" <gagsl-... at yahoo.com.ar>
wrote:
> En Thu, 18 Oct 2007 23:44:27 -0300, Ixiaus <parnel... at comcast.net>
> escribió:
>
> > I have just come across a site that discusses Python's 'for' and
> > 'while' loops as having an (optional) 'else' structure.
>
> > At first glance I interpreted it as being a bit like the 'default'
> > structure in PHP's switch block... But the switch block isn't a loop,
> > so, I am now confused as to the reason for using 'else' with the for
> > and while loops...
>
> > A few quick tests basically show that statements in the else structure
> > are executed at the fulfillment of the loop's expression (ie, no
> > break).
>
> A `while` loop tests a condition: if it evaluates to true, keep cycling;
> if it is false, stop. The `else` clause is executed when the condition is
> false, as in any `if` statement. If you exit the loop by using `break`,
> the `else` part is not executed (because you didn't get out of the loop by
> determining the condition falseness)
>
> You can think of a `for` loop as meaning `while there are remaining
> elements to be iterated, keep cycling` and the `else` clause applies when
> there are no more elements. A `break` statement does not trigger the else
> clause because the iteration was not exhausted.
>
> Once you get the idea, it's very simple.
>
It's useful when you want to search for an item and to do something if
you don't find it, eg:

for i in items:
    if is_wanted(i):
        print "Found it"
        break
else:
    print "Didn't find ir"