Re: Top 10 Language Constructs (Ada)

[Bill Brennan <william.c.brennan@lmco.com>]

David Kristola wrote:

> The other day, i came across a message field in an interface document
> that i was coding against.  The field is 16 bits, with the high order
> bit being a sign bit, the next highest order bit has the value 2^1,
> the third highest bit is 2^0, and so on down to the low order bit,
> which is 2^(-13).  The note said that the field is 2's complement.
> No problem....
> 
>    type Plasma_Volts_Type is delta 0.00012207 range -4.0 .. 3.99987793;
>    for Plasma_Volts_Type'Size use 16;

Actually, this doesn't compile in GNAT, I presume because it rounds
Plasma_Volts_Type'Last to something which won't fit in 16 bits.

Besides the fact that this type definition isn't portable (for GNAT
anyway), 
it doesn't really reflect what you're trying to say.  Without your
description,
I would have no idea that 0.00012207 ~= 2^(-13) or that 
3.99987793 ~= 4 - 2^(-13).

Personally, I think it's clearer to state it as

   type Plasma_Volts_Type is
      delta  2#000.0_0000_0000_0001#     --  approx 0.00012207
      range -2#100.0_0000_0000_0000# ..  --        -4
             2#011.1_1111_1111_1111#;    --  approx 3.99987793

   for Plasma_Volts_Type'Size use 16;
 
which will compile, and more accurately expresses what is intended.

This ability to express literals fairly transparently using any
numerical
base of convenience is yet another reason to value Ada.  (though I'm not
sure it's important enough to go on my top 10 list.)

-- Bill Brennan