type declaration and storage requirements

type declaration and storage requirements

[Russ]

Ada allows me to specify the number of digits and the range of a
floating-point type, or the delta and range of a fixed-point type.
This gives more control than, say, C/C++, which only allows me to
specify single or double precision. What bothers me, however, is that
I apparently need to do some homework of my own to determine if my
declaration will require one or two words for storage (for a
particular word size).

Suppose I have a particular range in mind, and I just want to get the
maximum number of digits I can get for, say, a 32-bit word. Is there a
simple way to determine how many digits to specify? If I specify one
digit too many, can it step the storage requirement from one to two
words? I'm no expert, but it seems to me that what you really want to
be able to specify is not the number of digits but rather the total
number of bytes to be used.

Also, suppose I specify one digit too many for single-word storage. I
presume I will get double-word storage and nearly double the precision
I asked for. If I then port the program to another machine with a
different word size, I may get less precision, which means that the
program may produce different results. Am I missing something here?

I guess Ada takes the view that storage requirements are secondary to
the application's actual precision requirements. That's certainly
valid to a point, I guess. But sometimes precision requirements are
not so precise themselves, and jumping from one to two words might be
undesirable, particularly if the type is to be used extensively.

Re: type declaration and storage requirements

[martin.m.dowie]

"Russ" <18k11tm001@sneakemail.com> wrote in message
news:bebbba07.0206021321.a3bbfac@posting.google.com...
> Ada allows me to specify the number of digits and the range of a
> floating-point type, or the delta and range of a fixed-point type.
> This gives more control than, say, C/C++, which only allows me to
> specify single or double precision. What bothers me, however, is that
> I apparently need to do some homework of my own to determine if my
> declaration will require one or two words for storage (for a
> particular word size).
[snip]

I think the argument here is - "Why do you care?"

The only time you care about such things are when you are defining
interfacing
and usually there will be an IDD to define such things (or else there isn't
and,
yes, you may be the person assigned to think about such size issues - but
that
is not a language issue).

Re: type declaration and storage requirements

[Dale Stanbrough]

In article <hfyK8.40339$wd3.6272305@news6-win.server.ntlworld.com>,
 "martin.m.dowie" <martin.m.dowie@ntlworld.com> wrote:

> I think the argument here is - "Why do you care?"
> 
> The only time you care about such things are when you are defining
> interfacing
> and usually there will be an IDD to define such things (or else there isn't
> and,
> yes, you may be the person assigned to think about such size issues - but
> that
> is not a language issue).

I'm not sure if i've got the wrong end of the stick here, but it seems
to me that it -is- a language issue, because the Ada designers have
deemed to put it -into- the language.

You can argue about whether it is the right decision (and you seem to
be arguing against that). I would disagree - for some languages it makes
sense to go to the trouble to include this information, as it may make
writing portable programs easier.

Dale

Re: type declaration and storage requirements

[martin.m.dowie]

"Dale Stanbrough" <dale@cs.rmit.edu.au> wrote in message
news:dale-09A130.10404303062002@its-aw-news.its.rmit.edu.au...
> In article <hfyK8.40339$wd3.6272305@news6-win.server.ntlworld.com>,
[snip]
> I'm not sure if i've got the wrong end of the stick here, but it seems
> to me that it -is- a language issue, because the Ada designers have
> deemed to put it -into- the language.

Sorry, could have been clearer. IDD's are language independent was
what I was driving at.


> You can argue about whether it is the right decision (and you seem to
> be arguing against that). I would disagree - for some languages it makes
> sense to go to the trouble to include this information, as it may make
> writing portable programs easier.

No - I totally want be be able to control underlying representation when I'm
defining messages that must go over particular interfaces (e.g. ethernet,
1553,
etc).

But for types that are solely for use to the internals of my program 99
times
out of a 100 I don't care, provided that I've defined my 'digits' or 'delta'
or
'ranges' correctly.

I will admit that I use record representations less and less as the years go
by
and just do explicit byte array => internal representations by hand (with
the
help of a few generics). I've always found this area to be one of the most
troublesome as far as portability goes...

Re: type declaration and storage requirements

[Dale Stanbrough]

martin.m.dowie wrote:

> But for types that are solely for use to the internals of my program 99
> times
> out of a 100 I don't care, provided that I've defined my 'digits' or 'delta'
> or
> 'ranges' correctly.
> 
> I will admit that I use record representations less and less as the years go
> by
> and just do explicit byte array => internal representations by hand (with
> the
> help of a few generics). I've always found this area to be one of the most
> troublesome as far as portability goes...

:-)

i've always wondered (based on the problems that many of the language
designers have had formally explaining chapter 13 features) just whether
Ada currently sits on the right side of the cost/benefit relationship.


Dale

Re: type declaration and storage requirements

[Russ]

"martin.m.dowie" <martin.m.dowie@ntlworld.com> wrote in message news:<hfyK8.40339$wd3.6272305@news6-win.server.ntlworld.com>...
> "Russ" <18k11tm001@sneakemail.com> wrote in message
> news:bebbba07.0206021321.a3bbfac@posting.google.com...
> > Ada allows me to specify the number of digits and the range of a
> > floating-point type, or the delta and range of a fixed-point type.
> > This gives more control than, say, C/C++, which only allows me to
> > specify single or double precision. What bothers me, however, is that
> > I apparently need to do some homework of my own to determine if my
> > declaration will require one or two words for storage (for a
> > particular word size).
> [snip]
> 
> I think the argument here is - "Why do you care?"

Why do I care about how much storage I use? Maybe I'm just obsessive,
but I would like to know if I am using single or double precision. Is
that so unreasonable?

> The only time you care about such things are when you are defining
> interfacing
> and usually there will be an IDD to define such things (or else there isn't
> and,
> yes, you may be the person assigned to think about such size issues - but
> that
> is not a language issue).

I don't care what kind of "issue" you want to label it. I just want to
know whether I am using single or double precision. And what is the
point of specifying the number of digits if the compiler is just going
to choose one or two words? If I choose less than 7 digits, I probably
get single precision, but if I choose 7 (8?) or more I probably get
double precision.

It's like going to a bank where they ask you, "What is the minimum
amount of money you wish to withdraw?" If you ask for $100 or less,
you get $100, but if you ask for more than $100, you get $200. What is
the point of pretending that you get to specify the amount precisely
if you don't? Wouldn't it make more sense to just ask if you want $100
or $200?

Re: type declaration and storage requirements

[AG]

"Russ" <18k11tm001@sneakemail.com> wrote in message
news:bebbba07.0206022258.5653ecac@posting.google.com...
> "martin.m.dowie" <martin.m.dowie@ntlworld.com> wrote in message
news:<hfyK8.40339$wd3.6272305@news6-win.server.ntlworld.com>...
> > "Russ" <18k11tm001@sneakemail.com> wrote in message
> > news:bebbba07.0206021321.a3bbfac@posting.google.com...
> > > Ada allows me to specify the number of digits and the range of a
> > > floating-point type, or the delta and range of a fixed-point type.
> > > This gives more control than, say, C/C++, which only allows me to
> > > specify single or double precision. What bothers me, however, is that
> > > I apparently need to do some homework of my own to determine if my
> > > declaration will require one or two words for storage (for a
> > > particular word size).
> > [snip]
> >
> > I think the argument here is - "Why do you care?"

If that were the case the argument would be - "why is it in the language?"
Presumably, the must be a reason ...

>
> Why do I care about how much storage I use? Maybe I'm just obsessive,
> but I would like to know if I am using single or double precision. Is
> that so unreasonable?

It's been some time since I had to really dig into the Ada 83 concept of
model and safe numbers as defined for both float and fixed types. But I seem
to recall that at least *some* changes in 95 standard in that area were
exactly
due to the fact that some model numbers forced a double-word implementation
where a single-word would suffice. Anyone with a beter knowledge of the
issue
would care to comment? [I'm serious about that - would be really interesting
and, perhaps, illuminating in this context]

>
> > The only time you care about such things are when you are defining
> > interfacing

Or, maybe, when you are defining a matrix about 2Gb large. Or, more likely,
when you care about some obscure math properties of the numbers.

> > yes, you may be the person assigned to think about such size issues -
but
> > that
> > is not a language issue).

It is the language issue exactly (or maybe it's Annex). So what do you do if
you *are* the person assigned to think about such issues? Sort of need to
know what the language says, right?

>
> I don't care what kind of "issue" you want to label it. I just want to
> know whether I am using single or double precision. And what is the
> point of specifying the number of digits if the compiler is just going
> to choose one or two words? If I choose less than 7 digits, I probably
> get single precision, but if I choose 7 (8?) or more I probably get
> double precision.

Well, again, I didn't need to use real numbers in Ada since 83 model but
as I recall they were defined as a minimum power of two which satisfies the
requirements with no regard or even a concept of a single or double
precision.
[And yes, there were custom-sized deltas too]

Re: type declaration and storage requirements

[martin.m.dowie]

"Russ" <18k11tm001@sneakemail.com> wrote in message
news:bebbba07.0206022258.5653ecac@posting.google.com...
> > [snip]
> >
> > I think the argument here is - "Why do you care?"
>
> Why do I care about how much storage I use? Maybe I'm just obsessive,
> but I would like to know if I am using single or double precision. Is
> that so unreasonable?

No, but I tend to just try and ensure I've got the
ranges/digits/delta/alignments
ok and let the compiler choose what is most optimal. Even in the embedded
systems I work in, actual underlying representations of internal
types/objects
comes way down my list of 'this to check for inefficiencies'.

I'll repeat my caveat - "except when I'm defining messages for external
interfaces"



> I don't care what kind of "issue" you want to label it. I just want to
> know whether I am using single or double precision. And what is the
> point of specifying the number of digits if the compiler is just going
> to choose one or two words? If I choose less than 7 digits, I probably
> get single precision, but if I choose 7 (8?) or more I probably get
> double precision.

If you choose 7 digits the compiler will give you a size that will
accomodate
that. My understanding is that it would be free to give you double precision
if it deemed that the most efficient for the particular target.


> It's like going to a bank where they ask you, "What is the minimum
> amount of money you wish to withdraw?" If you ask for $100 or less,
> you get $100, but if you ask for more than $100, you get $200. What is
> the point of pretending that you get to specify the amount precisely
> if you don't? Wouldn't it make more sense to just ask if you want $100
> or $200?

No I don't think it is like that. My view of Ada typing is that it offers an
'abstract view' rather than absolute underlying representations (unless that
is what you specify). The compiler will always ensure that your minimum
requirements are met but that it is free to 'actually' implement it in prett
y
much anyway it sees fit.

Re: type declaration and storage requirements

[Larry Kilgallen]

In article <bebbba07.0206022258.5653ecac@posting.google.com>, 18k11tm001@sneakemail.com (Russ) writes:
> "martin.m.dowie" <martin.m.dowie@ntlworld.com> wrote in message news:<hfyK8.40339$wd3.6272305@news6-win.server.ntlworld.com>...
>> "Russ" <18k11tm001@sneakemail.com> wrote in message
>> news:bebbba07.0206021321.a3bbfac@posting.google.com...
>> > Ada allows me to specify the number of digits and the range of a
>> > floating-point type, or the delta and range of a fixed-point type.
>> > This gives more control than, say, C/C++, which only allows me to
>> > specify single or double precision. What bothers me, however, is that
>> > I apparently need to do some homework of my own to determine if my
>> > declaration will require one or two words for storage (for a
>> > particular word size).
>> [snip]
>> 
>> I think the argument here is - "Why do you care?"
> 
> Why do I care about how much storage I use? Maybe I'm just obsessive,
> but I would like to know if I am using single or double precision. Is
> that so unreasonable?

I haven't followed the entire discussion, but I presume this is for
use in portable programming where one cannot just look at the machine
and compiler documentation.

If you specified single or double precision, what would you do on
VAX, where you have a choice of three precisions ?  Do you always
mean "smallest" and "largest" when you say single and double ?
What would you say to get the other one - "middle" ?  If that
is the case, what happens when you move that program from VAX
to some other system where there are only two choices ?  What
would happen to those where you specified "middle"  is that
a "middle if you have it but otherwise smallest" or would you
prefer "middle if you have it but otherwise largest"?

Of course dedicated VAX fans remember that there are two different
64-bit floating point types, D and G.  The different between them
is their mantissa and exponent, math thingies that have to do with
range and precision supported by the hardware.  Now we could all
specify floating point types in terms of the VAX precisions, F,
G, D and H, but that raised complications when Alpha came along
with S and T floating point types, so on Alpha VMS supports 5
types (I forget which one they dropped).  So even specifying you
want 32 bit or 64 bit floating point types does not nail it down.

Re: type declaration and storage requirements

[Gautier]

Russ:

> It's like going to a bank where they ask you, "What is the minimum
> amount of money you wish to withdraw?" If you ask for $100 or less,
> you get $100, but if you ask for more than $100, you get $200. What is
> the point of pretending that you get to specify the amount precisely
> if you don't? Wouldn't it make more sense to just ask if you want $100
> or $200?

In that case you choose Float or Long_Float and you are happy, don't you ?
Well, are there specifications about Float and Long_Float in the Manual ?

BTW, your bank dialogue can be simulated by trying to compile with
some precision, see what the compiler finds and adjust according
to a precision/storage trade-off convenient to you.

________________________________________________________
Gautier  --  http://www.mysunrise.ch/users/gdm/gsoft.htm

NB: For a direct answer, address on the Web site!

Re: type declaration and storage requirements

[Jeffrey Carter]

Russ wrote:
> 
> Ada allows me to specify the number of digits and the range of a
> floating-point type, or the delta and range of a fixed-point type.
> This gives more control than, say, C/C++, which only allows me to
> specify single or double precision. What bothers me, however, is that
> I apparently need to do some homework of my own to determine if my
> declaration will require one or two words for storage (for a
> particular word size).

You should define your own types when the precision and range are
application defined and important, and the specific representation is
not.  "digits X" only specifies a minimum precision; the actual
representation may give you more. If the representation is important and
the precision is not, then you probably need the types defined in
package Interfaces.

-- 
Jeff Carter
"Oh Lord, bless this thy hand grenade, that with it thou
mayst blow thine enemies to tiny bits, in thy mercy."
Monty Python and the Holy Grail

Re: type declaration and storage requirements

[Simon Wright]

18k11tm001@sneakemail.com (Russ) writes:

> Suppose I have a particular range in mind, and I just want to get
> the maximum number of digits I can get for, say, a 32-bit word. Is
> there a simple way to determine how many digits to specify? If I
> specify one digit too many, can it step the storage requirement from
> one to two words? I'm no expert, but it seems to me that what you
> really want to be able to specify is not the number of digits but
> rather the total number of bytes to be used.

If your requirement is to specify a size for the values, say

  type My_Short_Float_Type is new Float;
  type My_Long_Float_Type is new Long_Float;

or

  subtype My_Short_Float_Type is Float;
  subtype My_Long_Float_Type is Long_Float;

which are pretty likely to map to C float and double.

If on the other hand your requirement is to specify the precision your
algorithm requires, you really do need

  type My_Float_Type is digits 7;

and if it turns out that that needs 8 bytes (ie double), so be it.

Re: type declaration and storage requirements

[Martin Dowie]

"Simon Wright" <simon@pushface.org> wrote in message
news:x7vptz79vr7.fsf@pushface.org...
> If your requirement is to specify a size for the values, say
>
>   type My_Short_Float_Type is new Float;
>   type My_Long_Float_Type is new Long_Float;
>
> or
>
>   subtype My_Short_Float_Type is Float;
>   subtype My_Long_Float_Type is Long_Float;
>
> which are pretty likely to map to C float and double.

or use Interfaces.C.C_Float and Interfaces.C.Double to be sure
to match C, or Interfaces.Integer_16, Interfaces.Integer_32 to
avoid even mentioning C ;-)


> If on the other hand your requirement is to specify the precision your
> algorithm requires, you really do need
>
>   type My_Float_Type is digits 7;
>
> and if it turns out that that needs 8 bytes (ie double), so be it.

Amen!

Re: type declaration and storage requirements

[Simon Wright]

"Martin Dowie" <martin.dowie@nospam.baesystems.com> writes:

> or use Interfaces.C.C_Float and Interfaces.C.Double to be sure to
> match C, or Interfaces.Integer_16, Interfaces.Integer_32 to avoid
> even mentioning C ;-)

I was going to mention that but the last 2 are integer types which
doesn't match the OP's reference to "digits"!

Re: type declaration and storage requirements

[David Thompson]

Russ <18k11tm001@sneakemail.com> wrote :
> Ada allows me to specify the number of digits and the range of a
> floating-point type, or the delta and range of a fixed-point type.
> This gives more control than, say, C/C++, which only allows me to
> specify single or double precision. ...

C and C++ don't have fixed-point at all, although in C++
you could cobble up a perhaps-templated class (= ADT)
that does a tolerable approximation except for literals.

For floating-point, there's a more-or-less workaround:
#include <float.h>
...
#if FLT_DIG >= 11 /* single is good enough */
typedef float mytype;
#elif DBL_DIG >= 11 /* single isn't but double is */
typedef double mytype;
#elif LDBL_DIG >= 11 /* not double but 'extended' */
typedef long double mytype;
#else
#error We lose, no suitable floating type
#endif

In C++, you can also use std::numeric_traits<>,
but only at 'compile time' not in the preprocessor.
Which Ada-trained folks may prefer anyway <G>
except that it can't alter a function 'signature'
i.e. the choice of dummy/formal/parameter types.

And of course neither C nor C++ has user-set ranges
(or subtypes) distinct from the underlying/base type's range,
though again in C++ you could write a class to do it, but
only at runtime cost, since the uneuphoniously-named
'non-type template parameters' can't be floating-point.
Well, unless you (can) inline everything and the compiler
can determine/prove no aliasing, I think that works but
I'd have to try it out to be certain.

(Aside:  'C/C++' is even more flamebaitful in comp.lang.c
then 'ADA' is here.  C is _not_ strictly a subset of C++,
though it is _close_ by any reasonable measure.)

--
- David.Thompson 1 now at worldnet.att.net