Re: "Constraint Error: Range Check" That Does Not Always Occur

[Matthew Heaney <matthewjheaney@earthlink.net>]

DarthBob88 <darthbob88@gmail.com> writes:

> procedure test is
> 	subtype Die is Integer range 1..6;
> 	--Declare that anything of type Die may be
> 	--an integer, value between 1 and 6, inclusive, and sod-all else.

That's fine.


> 	subtype Dice is Integer range 3*Die'First .. 3*Die'Last;
> 	--Declares that Dice may be anything from 3 to 18.

But you sum 4 dice; that's a min of 4 * 1 and a max of 4 * 6 -- why did you
make your Dice subtype too small?


> 	package Random_Die is new Ada.Numerics.Discrete_Random (Die);
> 	use Random_Die;
> 	--Random generator; produces an integer between 1 and 6

Right.


> 	sum : Integer := 0;
> 	G: Generator;
>      	D1 : Dice;
> 	D2 : Dice;
> 	D3 : Dice;
> 	D4 : Dice;
> 	--Declare four Dice

--or:   D1, D2, D3, D4 : Die;  -- DICE IS THE WRONG SUBTYPE
--      Sum : Dice;  -- subtype should be 4 .. 24

> begin
> 	Reset(G);
> 	D1 := Random(G);
> 	D2 := Random(G);
> 	D3 := Random(G);
> 	D4 := Random(G);
> 	--I Haven't found the error just yet, but a Constraint_Error
> 	--Occasionally comes up for the previous 4 lines
> 	--I haven't been able to find it online, but I presume
> 	--It's because the generator comes up with a number outside of [1,6]

The generator will return you a random number between 1 and 6 (inclusive).

Your Die subtype has a minimum bounds of 1, so the random number can (and will)
return values that include the values 1 and 2.  

Your Dice subtype (to which you assign the value returned by the generator) has
a minimum bound of 3.  So when the random number generator returns a value less
then 3, you get C_E during the assignment.

Just change the declarations of Dx to use subtype Die instead of Dice, and then
all is well.


> 	sum := D1 + D2 + D3 + D4;
> 	--Debugging
> 	Ada.Integer_Text_IO.Put(D1);
> 	Ada.Integer_Text_IO.Put(D2);
> 	Ada.Integer_Text_IO.Put(D3);
> 	Ada.Integer_Text_IO.Put(D4);
> 	Ada.Integer_Text_IO.Put(sum);
> end test;
> --compile Ada programs with gnat make <foo>.adb
>
> If it helps any, I'm using the Windows version of GNAT on XP. Thanks
> in advance.