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:[email protected]>
Sent: 11 February 2023 04:20
To: perldl<mailto:[email protected]>;
perldl<mailto:[email protected]>
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 | [email protected] /\_/\__/
GPG: 791EB9EB, C949 3F81 6D9B 1191 9A16 C2DF 5F0A C52B 791E B9EB
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