for x'address or variant

for x'address or variant

[kat-Zygfryd]

I have a tiny problem, I want to have a structure:
type P is access X;
type X is record
    next: array(1..2) of P;
    left: P;
    right: P;
end record;
wherein the left component uses next(1)'address
and right component uses next(2)'address,
unfortunately "for x'address use" statements are
not allowed within record declarations, and
variant records don't allow me to access both
next and left/right exchangably. do you know a way
to work this out?

Zygfryd @gnat3.15

Re: for x'address or variant

[James S. Rogers]

"kat-Zygfryd" <6667@wp.pl> wrote in message
news:b7u1jh$p6k$1@news.onet.pl...
> I have a tiny problem, I want to have a structure:
> type P is access X;
> type X is record
>     next: array(1..2) of P;
>     left: P;
>     right: P;
> end record;
> wherein the left component uses next(1)'address
> and right component uses next(2)'address,
> unfortunately "for x'address use" statements are
> not allowed within record declarations, and
> variant records don't allow me to access both
> next and left/right exchangably. do you know a way
> to work this out?

You want a record that holds two values.
The problem is that you want each value aliased by
two fields.

My first question is "why". If next(1) will always be
the same as left, and next(2) will always be the same
as right, then what do you gain by having two names
for the same value in a single record?

What are you trying to achieve? I do not see how your
intended design is any better than

type P is access X;
type X is record
   left : P;
   right : P;
end record;

Of course, this simple design also reveals that X is a node
of a doubly linked list of nothing. In other words, the node
contains no data except its position in a list. Such a data
structure is generally useless.

Jim Rogers

Re: for x'address or variant

[Steve]

First:  If you're a newbie to Ada, you're taking the wrong approach.  If
that is the case, please elaborate a little more on the problem you're
trying to solve, maybe we can help with a different approach.

Second: If you're not a newbie and are showing a small subset of the problem
without the gory details, there is a pragma defined for the intent of
interfacing with C called "Unchecked_Union" that would allow you to do this.
The pragma is not currently (as far as I know) supported by the standard,
but is available with Gnat and ObjectAda (maybe more).  The Win32Ada
bindings have an example of the use of this pragma.

Steve
(The Duck)

"kat-Zygfryd" <6667@wp.pl> wrote in message
news:b7u1jh$p6k$1@news.onet.pl...
> I have a tiny problem, I want to have a structure:
> type P is access X;
> type X is record
>     next: array(1..2) of P;
>     left: P;
>     right: P;
> end record;
> wherein the left component uses next(1)'address
> and right component uses next(2)'address,
> unfortunately "for x'address use" statements are
> not allowed within record declarations, and
> variant records don't allow me to access both
> next and left/right exchangably. do you know a way
> to work this out?
>
> Zygfryd @gnat3.15
>
>

Re: for x'address or variant

[Simon Wright]

"kat-Zygfryd" <6667@wp.pl> writes:

> "Nick Roberts" <nickroberts@blueyonder.co.uk> wrote in message
> news:b7uhb9$4lb72$1@ID-25716.news.dfncis.de...

> > I hope this solves the problem. Say if it doesn't.
> of course it does, but I wanted it to be done
> a little more good-looking, thanks anyway.

Nick's solution looks pretty good to me, in general anything which
uses arrays indexed by (1..2) rather than some problem-specific index
like Nick's, or which relies on 'Address, is at best suspect!

Re: for x'address or variant

[Nick Roberts]

"kat-Zygfryd" <6667@wp.pl> wrote in message
news:b7u1jh$p6k$1@news.onet.pl...

> I have a tiny problem, I want to have a structure:

> type P is access X;
> type X is record
>     next: array(1..2) of P;
>     left: P;
>     right: P;
> end record;

> wherein the left component uses next(1)'address
> and right component uses next(2)'address,
> unfortunately "for x'address use" statements are
> not allowed within record declarations, and
> variant records don't allow me to access both
> next and left/right exchangably. do you know a way
> to work this out?

Declare the following:

    type Node;
    type Node_Ref is access Node;
    type Leaf_Direction is (Left, Right);
    type Leaf_Pair is array (Leaf_Direction) of Node_Ref;
    type Node is
        record
            Payload: ...;
            Leaves: Leaf_Pair;
        end record;

Now you can reference, for example, the left leaf an object X1 of type Node
by:

    X1.Leaves(Left).all

You can cycle through all the leaves of X1 by:

    for L in Leaf_Direction loop
        ... X1.Leaves(L).all ...
    end loop;

I hope this solves the problem. Say if it doesn't.

--
Nick Roberts
Jabber: debater@charente.de [ICQ: 159718630]

Re: for x'address or variant

[kat-Zygfryd]

"Nick Roberts" <nickroberts@blueyonder.co.uk> wrote in message
news:b7uhb9$4lb72$1@ID-25716.news.dfncis.de...
> Declare the following:
>
>     type Node;
>     type Node_Ref is access Node;
>     type Leaf_Direction is (Left, Right);
>     type Leaf_Pair is array (Leaf_Direction) of Node_Ref;
>     type Node is
>         record
>             Payload: ...;
>             Leaves: Leaf_Pair;
>         end record;
>
> Now you can reference, for example, the left leaf an object X1 of type
Node
> by:
>
>     X1.Leaves(Left).all
>
> You can cycle through all the leaves of X1 by:
>
>     for L in Leaf_Direction loop
>         ... X1.Leaves(L).all ...
>     end loop;
>
> I hope this solves the problem. Say if it doesn't.
>
> --
> Nick Roberts
> Jabber: debater@charente.de [ICQ: 159718630]

of course it does, but I wanted it to be done
a little more good-looking, thanks anyway.

Zygfryd

Re: for x'address or variant

[kat-Zygfryd]

"Simon Wright" <simon@pushface.org> wrote in message
news:x7v1xzxjg86.fsf@smaug.pushface.org...
> "kat-Zygfryd" <6667@wp.pl> writes:
> Nick's solution looks pretty good to me, in general anything which
> uses arrays indexed by (1..2) rather than some problem-specific index
> like Nick's, or which relies on 'Address, is at best suspect!

If you say so, I just thought that using a component
would be at least an instruction or two faster than
indexing an array [but I need the array solution also
in some places].
Anyway it could be useful to enable use of unrestricted
variants in Ada 2005, I guess just that programmers using
the feature would need to know what they are doing.
(Which is quite certain since Ada programmers are
very careful, geneticly ;P )

example:

record
    case not_needed_selector is
    when sth => B32: Integer;
    when sth => B8_1,B8_2,B8_3,B8_4:Byte;
    when sth => array(1..4) of Byte;
    end case;
end record;

or something alike, no more explicit typecasts needed in such case

Zygfryd

Re: for x'address or variant

[Robert A Duff]

"kat-Zygfryd" <6667@wp.pl> writes:

> If you say so, I just thought that using a component
> would be at least an instruction or two faster than
> indexing an array ...

Than indexing an array by a static index value?  I doubt it.

Seems like you ought to prove (by measuring, and reading the assembly
code) that this is a real problem before you start playing low-level
games like this for efficiency reasons.

- Bob

Re: for x'address or variant

[kat-Zygfryd]

"Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message
news:wccadel9cw7.fsf@shell01.TheWorld.com...
> Seems like you ought to prove (by measuring, and reading the assembly
> code) that this is a real problem before you start playing low-level
> games like this for efficiency reasons.
>
> - Bob

Perhaps you are right, I just wanted to make my
most commonly used algorithms 100% as efficient
as I can. I don't really know what does the compiler
optimize and what does it not.

Zygfryd

Re: for x'address or variant

[John R. Strohm]

"kat-Zygfryd" <6667@wp.pl> wrote in message
news:b7v2qr$bto$1@news.onet.pl...
> "Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message
> news:wccadel9cw7.fsf@shell01.TheWorld.com...
> > Seems like you ought to prove (by measuring, and reading the assembly
> > code) that this is a real problem before you start playing low-level
> > games like this for efficiency reasons.
> >
> > - Bob
>
> Perhaps you are right, I just wanted to make my
> most commonly used algorithms 100% as efficient
> as I can. I don't really know what does the compiler
> optimize and what does it not.

Dude, the rule is FIRST you make your code CORRECT, absolutely positively
CORRECT, and *THEN* you worry about making it fast.

Re: for x'address or variant

[kat-Zygfryd]

"John R. Strohm" <strohm@airmail.net> wrote in message
news:52C30E21B2695ADA.9A5568A9039EC23D.34356CD468622AAC@lp.airnews.net...
> Dude, the rule is FIRST you make your code CORRECT, absolutely positively
> CORRECT, and *THEN* you worry about making it fast.

I'm rewriting the code from a C version from a book,
DUDE.

Zygfryd

Re: for x'address or variant

[Robert A Duff]

"kat-Zygfryd" <6667@wp.pl> writes:

> I'm rewriting the code from a C version from a book,

Please show us what the C code looks like.

> DUDE.
> 
> Zygfryd

- Bob

Re: for x'address or variant

[kat-Zygfryd]

"Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message
news:wccvfx82egs.fsf@shell01.TheWorld.com...
> "kat-Zygfryd" <6667@wp.pl> writes:
>
> Please show us what the C code looks like.
> - Bob

It's LIBAVL 2.0.1 PDF doc, you can find it
and download. However I didn't mean that
those structures where used in this C code,
just that it is correct code (I guess, version
2.0.1 after all).

Zygfryd

PS.
I am deeply unsatisfied because I posted
a question to aquire an answer, not for my
coding style/skills to be discussed, as
some people tend to do.

Re: for x'address or variant

[tmoran]

> I am deeply unsatisfied because I posted
> a question to aquire an answer, not for my
> coding style/skills to be discussed, as
Why should we spend time answering what are clearly the wrong questions?

Re: for x'address or variant

[Robert A Duff]

"kat-Zygfryd" <6667@wp.pl> writes:

> "Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message
> news:wccvfx82egs.fsf@shell01.TheWorld.com...
> > "kat-Zygfryd" <6667@wp.pl> writes:
> >
> > Please show us what the C code looks like.
> > - Bob
> 
> It's LIBAVL 2.0.1 PDF doc, you can find it
> and download. However I didn't mean that
> those structures where used in this C code,
> just that it is correct code (I guess, version
> 2.0.1 after all).

I was trying to help, but I need to understand what you're trying to do.
I'm not willing to download and read all the LIBAVL code.
I looked for it briefly, and it's too much hassle.

I'm guessing you've got some C code, and you want to know how to do
similar things in Ada.  So post some simple examples in C.
If that's what you're after, please explain.

> Zygfryd
> 
> PS.
> I am deeply unsatisfied because I posted
> a question to aquire an answer, not for my
> coding style/skills to be discussed, as
> some people tend to do.

I'm not interested in discussing your *skills*.
But you seem to be asking about Ada *style*, so...

- Bob

Re: for x'address or variant

[kat-Zygfryd]

"Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message
news:wccfzob6384.fsf@shell01.TheWorld.com...
> I was trying to help, but I need to understand what you're trying to do.
> I'm not willing to download and read all the LIBAVL code.
> I looked for it briefly, and it's too much hassle.
>
> I'm guessing you've got some C code, and you want to know how to do
> similar things in Ada.  So post some simple examples in C.
> If that's what you're after, please explain.

Not really, I have code in C which is easily translatable to Ada,
I just wanted to know how to perform the thing I was talking
about (it isn't nessessary for this code, just a bit helpful)

> I'm not interested in discussing your *skills*.
> But you seem to be asking about Ada *style*, so...
>
> - Bob

Oh I didn't mean you, but mr. Strohm.

Zygfryd

Re: for x'address or variant

[Robert A Duff]

"kat-Zygfryd" <6667@wp.pl> writes:

> Not really, I have code in C which is easily translatable to Ada,

OK, given *that* statement I would think you're done.  But you say:

> I just wanted to know how to perform the thing I was talking
> about (it isn't nessessary for this code, just a bit helpful)

The "thing you were talking about" was overlaying integer record
fields with other fields that are arrays of integers.  I have no idea
why you want to do that (at first you said, speed, but it's probably not
faster), so I'm really unsure what you're trying to accomplish, so I
can't say how one would do it in Ada.

- Bob

Re: for x'address or variant

[John R. Strohm]

"kat-Zygfryd" <6667@wp.pl> wrote in message
news:b81jc5$4u7$1@news.onet.pl...
> "Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message
> news:wccfzob6384.fsf@shell01.TheWorld.com...
> > I was trying to help, but I need to understand what you're trying to do.
> > I'm not willing to download and read all the LIBAVL code.
> > I looked for it briefly, and it's too much hassle.
> >
> > I'm guessing you've got some C code, and you want to know how to do
> > similar things in Ada.  So post some simple examples in C.
> > If that's what you're after, please explain.
>
> Not really, I have code in C which is easily translatable to Ada,
> I just wanted to know how to perform the thing I was talking
> about (it isn't nessessary for this code, just a bit helpful)
>
> > I'm not interested in discussing your *skills*.
> > But you seem to be asking about Ada *style*, so...
> >
> > - Bob
>
> Oh I didn't mean you, but mr. Strohm.

I stand by my statement.  Until you have correct, RUNNING code, in your
target language, it is premature to worry about "efficiency".  You should
give thought to choosing the appropriate algorithm, but trying to make
micro-optimizations before the code is known to be correct and working is
the wrong approach.

Furthermore, many years of study have shown conclusively that human
programmers are very poor at finding the real "hot spots" where optimization
can really help.  Profiling the actual execution time, mapping out where the
program is actually spending the cycles, is absolutely critical.