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From: Rob Solomon <usenet@drrob1-noreply.com>
Newsgroups: comp.lang.ada
Subject: float confusion
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Date: Tue, 14 Jul 2009 18:46:01 -0400
Xref: g2news2.google.com comp.lang.ada:7058
Date: 2009-07-14T18:46:01-04:00
List-Id: <comp.lang.ada>

I ran this simple example thru GNAT and I'm confused by the result.

-- conclusions from this pgm
-- integer type does not raise overflow constraint_error
-- natural and positive types do raise overflow constraint_error when
-- overflow is by multiplication, but not by addition.

with Ada.Text_IO;
with Ada.Integer_Text_IO;
with Ada.Float_Text_IO;
use Ada.Text_IO;
use Ada.Integer_Text_IO;
use Ada.Float_Text_IO;


   procedure SimpleCompute is

   X : Integer := 1;
   c : Integer := 0;  -- counter
   N : Natural := 1;
   P : Positive := 1;
   F : Float := 1.0;

   begin
   put_line("X C N P F");
   loop
    put(X);
    card32_IO.put(u2);
    put(c,3);
    put(n);
    put(' ');
    put(p);
    put(' ');
    put(f,15,0,0);
    New_Line;
    X := X * 2;
    n := n * 2;
    p := p + p;
    f := f * 2.0;
    c := c +1;
    exit when c > 40;
   end loop;
   exception
      when constraint_error =>
         n := 1;
   end simplecompute;

My confusion is that float'digits is 6 and long_float'digits is 15 (I
already checked this).

When I run this code, float grows to the full 13 digits before I stop
it.  Why is that?

And interestingly, I get overflow errors from the *2 statement but not
from self+self.  I wonder if this occurs because this kind of overflow
is needed in the computation of random numbers.

Thx