Re: Fixed-point question

["Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>]

On 06/26/2015 11:21 AM, Patrick Noffke wrote:
> On Friday, June 26, 2015 at 11:56:44 AM UTC-5, Jeffrey R. Carter wrote:
>> On 06/26/2015 06:20 AM, Patrick Noffke wrote:
>>>
>>>    type Fixed_Type is delta 1.0 / 180.0 range 0.0 .. 65535.0 / 180.0;
>>
>> What is Fixed_Type'Small?
> 
> It is 1/180.

Not as you wrote it. By default, the small of an ordinary fixed-point type is a
power of two <= the delta. In this case, that would be 2**(-8) or less [ARM
3.5.9 (8/2)].

The introduction to this I got (in Ada 83) was

Cent : constant := 0.01;

type Money is delta Cent range -10_000.0 .. 10_000.0;

Amount : Money := 0.0;

Add_5 : for I in 1 .. 5 loop
   Amount := Amount + Cent;
end loop Add_5;

Text_IO.Put_Line (Item => Money'Image (Amount) );

which output 0.04, to most people's consternation.

This is because the small was 2**(-7), or 1/128. The multiple of that nearest to
0.01 is 1/128, so Amount ends up containing 5/128, or 0.0390625. Adding

for Money'Small use Cent;

gives the expected result.

-- 
Jeff Carter
"I did not rob a bank. If I'd robbed a bank, everything
would be great. I tried to rob a bank, is what happened,
and they got me."
Take the Money and Run
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