Emulating Modula-3's FOR loop

Emulating Modula-3's FOR loop

[David J. Fiander]

I just ran across something minor, but annoying.  Suppose I've
got an array, indexed by the type Subscript, and I want to
operate on every n'th element of the array.  In Modula-3, I'd say
something like

	FOR i FROM n TO last BY n do
	    (* process the array elements *)
	END;

Now, if last is not divisible by n, this will still work, without
raising a Constraint_Error (or the modula equivalent).  But in
Ada, the obvious translation

	procedure Operate(Arr: in out Array(Subscript) of Elt;
			  N: in Subscript) is
	    I: Subscript := N;
	begin
	    while I <= Subscript'Last loop
	        -- Process the array
	        I := I + N;
	    end loop;
	end Operate;

will raise a Constraint_Error on the last addition.  Is there a
better way to deal with this than

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

Re: Emulating Modula-3's FOR loop

[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.