PEP-315 ("do" loop)

Dan Bishop danb_83 at yahoo.com
Wed Feb 18 02:44:38 CET 2004


Wayne Folta <wfolta at netmail.to> wrote in message news:<mailman.78.1077041293.31398.python-list at python.org>...
> > until x <==> while not x

> but at the same time my spider senses were telling me that surely a 
> language wouldn't reserve a keyword "until" to simply save someone from 
> typing "while not". (Of course, a language like perl might well do this, 

Microsoft QBasic did.  It allowed all four of the constructs

' pretest loops
DO WHILE condition...LOOP
DO UNTIL condition...LOOP
' posttest loops
DO...LOOP WHILE condition
DO...LOOP UNTIL CONDITION

> At the very least I'd say that you need to choose a word other than 
> "while" to avoid this kind of confusion. For example, "do ... until 
> ..." loop where "until" <==> "while not" so that you know immediately 
> what's going on. Otherwise, when you see a "while", you're never sure 
> what kind of loop it is until you look above it and determine it's not 
> a "do while".

I agree.

In a similar discussion a while (no pun intended) back, I suggest
adding a keyword like "posttest" to the current while loop syntax. 
Unfortunately, it wouldn't easily extend to the "loop and a half"
construct.



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