Hi Ed,
Thanks for looking at this. It was not too important, but it was
surprising, as I was chaining pdl methods (...->det->approx(0) to
detetct singular matrices) and I was getting somewhat strange error messages
as det was sometimes an ndarray and other times a scalar.
Regards,
Luis
On Sat, Feb 11, 2023 at 04:49:39AM +0000, Ed . wrote:
> Hi Luis,
>
> The key (if you’ll forgive the pun) is the behaviour of the “det” function,
> which is (at this writing) defined in Basic/MatrixOps/matrixops.pd as:
>
> sub det {
> my($x) = shift;
> my($opt) = shift;
> $opt = {} unless defined($opt);
> my($lu,$perm,$par);
> if(exists ($opt->{lu}) and (ref $opt->{lu} eq 'ARRAY')) {
> ($lu,$perm,$par) = @{$opt->{lu}};
> } else {
> ($lu,$perm,$par) = lu_decomp($x);
> $opt->{lu} = [$lu,$perm,$par]
> if(exists($opt->{lu}));
> }
> ( (defined $lu) ? $lu->diagonal(0,1)->prodover * $par : 0 );
> }
>
> You will see the last line returns a Perl scalar 0 if the $lu came back
> undefined, otherwise it will do a prodover on the diagonal, which returns a
> PDL ndarray. Here is my slightly-modified version of your script:
>
> use strict; use warnings; use feature 'say';
> use PDL;
> my $m1=pdl[[-2,-2,-2], [-1,-2,-2], [0,-2,-2]];
> my $m2=pdl[[-2,-2,-2],[-1,-1,-2],[0,0,-2]];
> for($m1, $m2){
> my $det = $_->det(my $opt={lu=>undef});
> say "Got det=$det (Ref=@{[ref $det]}) lu=(@{$opt->{lu}}) for Matrix=$_";
> }
>
> I will look in a while at why the lu_decomp is behaving differently for the
> two matrices. It is almost certainly a bug.
>
> Best regards,
> Ed
>
> From: Luis Mochan<mailto:moc...@icf.unam.mx>
> Sent: 11 February 2023 04:20
> To: perldl<mailto:pdl-general@lists.sourceforge.net>;
> perldl<mailto:pdl-de...@lists.sourceforge.net>
> Subject: [Pdl-devel] inconsistent determinant
>
> I found a curious situation when running pdl, as illustrated in the
> following example. I build two singular 3x3 matrices and calculate
> their determinant, which is zero. However, in one case I get a PDL
> scalar and in the other I get a Perl scalar. I don't know why the
> different behaviour:
>
> # Program
> use v5.36;
> use PDL;
> my $m1=pdl[[-2,-2,-2], [-1,-2,-2], [0,-2,-2]];
> my $m2=pdl[[-2,-2,-2],[-1,-1,-2],[0,0,-2]];
> for($m1, $m2){
> say "Matrix=$_", "Det=", $_->det, "\nRef=", ref $_->det;
> }
>
>
> #Results:
> Matrix=
> [
> [-2 -2 -2]
> [-1 -2 -2]
> [ 0 -2 -2]
> ]
> Det=0
> Ref=PDL
> Matrix=
> [
> [-2 -2 -2]
> [-1 -1 -2]
> [ 0 0 -2]
> ]
> Det=0
> Ref=
>
> I'm currently running PDL v2.081 under Perl 5.36 in Linux/testing.
>
> Best regards,
> Luis
>
>
>
> --
>
> o
> W. Luis Mochán, | tel:(52)(777)329-1734 /<(*)
> Instituto de Ciencias Físicas, UNAM | fax:(52)(777)317-5388 `>/ /\
> Av. Universidad s/n CP 62210 | (*)/\/ \
> Cuernavaca, Morelos, México | moc...@fis.unam.mx /\_/\__/
> GPG: 791EB9EB, C949 3F81 6D9B 1191 9A16 C2DF 5F0A C52B 791E B9EB
>
>
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>
--
o
W. Luis Mochán, | tel:(52)(777)329-1734 /<(*)
Instituto de Ciencias Físicas, UNAM | fax:(52)(777)317-5388 `>/ /\
Av. Universidad s/n CP 62210 | (*)/\/ \
Cuernavaca, Morelos, México | moc...@fis.unam.mx /\_/\__/
GPG: 791EB9EB, C949 3F81 6D9B 1191 9A16 C2DF 5F0A C52B 791E B9EB
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