control structures (was "Re: Sins")

[William Tanksley]

On Sun, 09 Jan 2000 11:35:58 -0500, Colin J. Williams wrote:
>skaller wrote:
>> Gordon McMillan wrote:

>> > That's the reason that Python has the "for
>> > .. else" construct that William Tanksley
>> > hates so much.

>> >  for i in X:
>> >    if X[i]==e:
>> >      # do the "found" thing
>> >      break
>> >  else:
>> >    print "Not Found"

>> That doesn't work. The 'do the "found" thing'
>> is in the wrong place: it must be outside the loop,
>> since it is usually the 'normal continuation'
>> of the routine.

>Aside from that.  It seems to me that the weakness of the
>   for ...
>   else 
>structure is that the else clause is executed whether the 
>'for' succeeds or not. The Language Reference Manual has:

>	"The expression list is evaluated once; it should yield a     
>sequence. The suite is then executed once for each item in the 
>sequence, in the order of ascending indices. Each item in turn 
>is assigned to the target list using the standard rules for 
>assignments, and then the suite is executed. When the items are 
>exhausted (which is immediately when the sequence is empty), the 
>suite in the else clause, if present, is executed, and the loop 
>terminates."

In case you were wondering, the reason Gordon mentions me above is that I
said the same thing you're about to.  I still agree with you; but neither
one of us is going to live to see the change we've proposed.

A better choice would be to advocate some other way of doing what's needed
-- labelled breaks would work superbly.  Even gotos would be better
(although I hope they're not the method chosen :-).

By the way, I think your reading of the current rules is wrong.  The else
clause will not be executed in case of a break -- it doesn't always get
executed, as you imply.

The 'try' statement has an 'else' clause with a similar mistake in
definition.  Argh.

>More useful would be to execute the else clause only if the sequence is
>empty.  This might be expressed as:

>    for x in L:
>	...
>    otherwise ...

I agree.

>Colin W.

-- 
-William "Billy" Tanksley, in hoc signo hack