Re: Emulating Modula-3's FOR loop

[dewar@cs.nyu.edu (Robert Dewar)]

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.