Re: Ada vs Fortran for scientific applications

[nospam@see.signature (Richard E Maine)]

Jan Vorbr�ggen <jvorbrueggen@not-mediasec.de> wrote:

> >>If I have a native complex type, the language implementor is free to arrange
> >>the real and imaginary part in memory as he sees fit,
> > Although Fortran does restrict the implementation, so the langauge
> > implementer is not fully free.
> 
> Is that through sequence association - i.e., I can assume (conceptually)
> an EUIVALENCE of two reals, the real and imaginary part in that order, to
> be in place for a complex variable?

Yes. And it is a bit more than conceptually. The standard is pretty
explicit about it.... well at least the f77 standard is. I'm failing to
find quite the same words in the f2003 standard. I have no real doubt
what the answer to an interp request on it would be (based on
compatibility with f77), but I'm not finding the explicit words in
f2003. Can anyone pint out where I might have missed them?

In f2003 we have, in the section on complex type (4.4.3)

  "The values of a complex type are ordered pairs of real values. The
   first real value is called the real part. The second real value is
   called the imaginary part."

But these words talk about values - not about representations. This
sounds to me like a description of a mathematical concept. I find words
elsewhere saying that complex occupies two consecutive numeric storage
units, which says something about representation, but I can't find the
words to say what those two storagte units have to look like.

In contrast, f77 says, in its section on complex type (4.6)

  "The representation of a complex datum is in the form of an ordered
   pair of real data. The first of the pair represents the real part
   of the complex datum and the second represents the imaginary part.
   Each part has the same degreee of approximation as for a real datum.
   A complex datum has two consecutive numeric storage units in a
   storage sequence: the first storage unit is the real part and the
   second storage unit is the imaginary part."

While the wods about an ordered pair look pretty similar, the f77
version has the word "representation", while the f2003 version doesn't.
That makes them say different things acccording to me, although I'm sure
the difference is unintentional. The f77 version seems to me to say the
same thing twice, but the f2003 version dropped it down to zero  times.
Section 17.1 of f77 says the same thing a third time, but f2003 doesn't
seem to have that copy either.

This change in wording happened between f77 and f90.

I see that even the f77 version doesn't quite seem to say what I'm sure
was intended. Again, I might have missed it, but...

F77 talks about the storage units being the real and imaginary parts,
and it says that each part has "the same degree of approximation" as for
a real datum. But I don't see that it says it has the same
representation as a real datum. Now by the time you are tied down that
much, I can't see why the representation wouldn't be the same in
practice, but I don't say that this says it actually has to be. I'm sure
that is the intent, particularly given the words in 17.2(11) and (12).
Those words in section 17 don't say what values the partially associated
entities become defined with, but there are no other plausible candidate
values and it would be quite unlike the usual practice of the standard
to say that they become defined, but to omit any mention of what value
they become defined with. Sometimes things get processor-dependent
values, but the standard at least tends to say that such values are
processor dependent rather than just saying that there is a value, but
being silent about what it might be.

-- 
Richard Maine                     | Good judgment comes from experience;
email: my first.last at org.domain| experience comes from bad judgment.
org: nasa, domain: gov            |       -- Mark Twain