[Rd] Fortran BLAS giving bad results

[Berwin A Turlach]

G'day Emmanuel,

On Sat, 11 Jan 2014 14:08:41 -0800
Emmanuel Sharef <esharef at gmail.com> wrote:

> Example: this program creates two double vectors, takes the dot
> product with ddot, and prints the result:
> 
> test.f:
>       subroutine testddot(n,x,y)
>       integer i,n
>       double precision x,y,d1(n),d2(n)
>       do 100 i=1,n
>         d1(i)=x
>         d2(i)=y
>  100  continue
>       print *,ddot(n,d1,1,d2,1)
>       end
> 
> Compiling
> R CMD SHLIB test.f

Mmh, FORTRAN77 does not have dynamical memory allocation (though I have
been told that many compilers implement it as an extension to the
standard).  So I am surprised that this code works.  It is certainly
not portable (i.e. a compiler that implements the FORTRAN77 standard
strictly would barf on this code).

Also, output to * in FORTRAN code dynamically linked to R is
discouraged, see chapter 5.7 (Fortran I/O) in the "Writing R Extension"
manual.  From memory, many years ago one could do so (on linux and
SUN machines atleast), then it stopped working.  I am surprised that it
works again.

But neither of these two issues are the reason for your strange
results.  You would have pinpointed your actual problem easily if you
would have used my favourite none-FORTRAN77 feature (which is
implemented by most compilers as an extension), namely having "implicit
none" as the first line in each function/subroutine. (The FORTRAN77
compliant version, AFAIK, was to write "IMPLICIT CHARACTER (A-Z)" into
the first line of all functions/subroutines.)

Essentially, your routine testddot does not know what kind of result
ddot returns and by the implicit type definition rules of FORTRAN77 it
expects an INTEGER back.  The fact that a DOUBLE PRECISION value is
returned when an INTEGER is expected creates some confusion.  Somewhere
at the start of your routine you have to add a "DOUBLE PRECISION ddot"
statement.

HTH.

Cheers,

	Berwin

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