Re: Local vs global variables in ADA

[Jeffrey Carter <jrcarter@acm.org>]

Fraser Wilson wrote:
> 
> On the "while" comment: are you (or were they) referring to the fact
> that a while loop can be expressed as a loop/exit when/end loop
> construction, or is there something about while loops that should be
> avoided?  Isn't there a preference hierarchy from for loop to while
> loop to loop?  I.e. use a for loop whenever possible, failing that a
> while loop, and as a last resort a loop with an exit.  OK, and as a
> last last resort use a goto.

The point made by Ichbiah, Barnes, and Firth at the Ada Launch was that 
while loops express the condition for continuation, not exit, and this 
is difficult for beginners to understand, and they usually require the 
use of negative logic (while not End_Of_File), which is often more 
difficult to understand than positive logic; that a loop with an exit as 
its first statement expresses the condition for exit, and this is easy 
for everyone to understand, and it usually uses positive logic (exit 
when End_Of_File). For these reasons they thought the latter form should 
be prefered.

There are as many preference hierarchies as there are people creating 
preference hierarchies. Here's one: use assembler whenever possible, 
failing that use C, failing that use a C derivative, failing that use 
another language other than Ada, and use Ada when forced to at gunpoint. 
I don't happen to agree with it, but I suspect that there are those who 
do. Just because you've seen someone's preference hierarchy doesn't mean 
you should follow it.

I use the rule that you should use the appropriate form of a loop for 
the iteration that's required. If you accept the arguments of "the 
founders", while is never the appropriate form.

-- 
Jeff Carter
"We burst our pimples at you."
Monty Python & the Holy Grail