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From: dewar@cs.nyu.edu (Robert Dewar)
Subject: Re: Emulating Modula-3's FOR loop
Date: 1996/07/07
Message-ID: <dewar.836753316@schonberg>#1/1
X-Deja-AN: 164198507
references: <4rocup$rvo@ia.mks.com>
organization: Courant Institute of Mathematical Sciences
newsgroups: comp.lang.ada
Date: 1996-07-07T00:00:00+00:00
List-Id: <comp.lang.ada>


David J. Fiander suggests for a loop jumping by N:

        begin
            while I <= Subscript'Last loop
                -- process
                I := I + N;
            end loop;
        exception
            when Constraint_Error => null;
        end;

This is definitely bad code. Using Constraint_Error to catch normal
flow of control situations as opposed to errors is a bad misuse of
the feature. Not only is it conceptually incorrect, but in practice
it can be disastrously inefficient (in many implementations, a
constraint error signal will involve a kernel trap with huge overhead).

A much better translation is

      loop
         -- process

         exit when I > Subscript'Last - N;
         I := I + N;
      end loop;

this should generate perfectly efficient code, since the expression
Subscript'Last - N is loop invariant. Note that this assumes you know
the loop will execute at least once. if not, the entire loop needs
to be protected by an appropriate test.