What type has a variable containing a range to be?

Question

A for loop can be done in Ada using a range with a start and an end point:

for I in 0..10 loop
(...)
end loop;

I know, it's possible doing the for loop using two variables describing the range:

for I in range_start..range_end loop
(...)
end loop;

Is it also possible to store the range in one variable?, like:

for I in my_range loop
(...)
end loop;

Which type has the variable *my_range* to be?

Edit: Let's say I want to use this variable as a parameter in a subprogram: So the subprogram has this loop which iterates over the range. I'd rather use two variables describing the range instead of using generics, because generics would cause in higher effort. But I think using one variable describing the range would cause in a higher readability, that's why I'm asking that question.

Answer 1

If your variable is an array, then you can iterate over its range via:

for I in Arr_Var'Range loop
   ...
end loop;

If you're interested in iterating over the elements of a container, eg array, vector, map, etc., and don't care about the index, you can use generalized looping (Ada 2012 only):

for Elem of Container loop
   ...
end loop;

Answer 2

Use a range type, something along these lines:

type Range_Type is range -5 .. 10;
...
for A in Range_Type loop

See the complete example in here .

Answer 3

Hmmm. It looks like you may be (perhaps unknowingly) a devotee of Andrei Alexandrescu . He's argued strongly for languages to make ranges a first-class type, and the removal of "iterators" as a programming paradigm.

It would be interesting to see what an Ada implementation of this would look like. Unless I'm missing something, that's not the way Ada 2012 went.

Answer 4

If you want to pass around an object containing the range, you can wrap Start and End in a record. Inside the subprogram you can locally declare a new [sub]type bounded by the range.

type Range_Type is range TheRange.Start .. TheRange.end;

Answer 5

My count says you need 4 more lines to create a generic subprogram than a plain one:

generic                             -- extra
   type Bounds is (<>);             -- extra
procedure R;

procedure R is
begin
   for J in Bounds'Range loop
      null;
   end loop;
end R;

with R;
procedure P is
   type Rng is range 1 .. 10;       -- extra
   procedure A is new R (Rng);      -- extra
begin
   A;
end P;

which doesn't seem so bad.

But, usually an iteration is iterating over something (an array, ...): if so, you can derive the iteration range from the thing that's being iterated over.