How to calculate number of rows of a referenced array in Perl - perl

Question

A part of my code calculates inverse of a matrix (generated previously in the code) with dimensions more than 300 X 300. I want to use the elements of the inversed matrix further in the code. Have used the below code for this, trying with only 5X5 matrix for testing:
use strict;
use warnings;
use Math::MatrixReal;
my #a=(); #a is the matrix obtained
$a[0][0]=0.18761134;
$a[0][1]=0.010779401; #Have hard-coded the values here till $a[4][4]
my $ref_a = \#a;
my $b = Math::MatrixReal->new_from_rows($ref_a);
my $b_inv = $b->inverse();
print "\n Inverse is\n",$b_inv; #prints correct inverse
print "\n\nTest printing elements\n";
print $$b_inv[0][1][1],"\n"; #prints the correct element
my $row_b=scalar(#{$b});
print "Number of rows in b: ",$row_b,"\n"; #prints 6
my $col_b=#{$$b[0]};
print "Columns in b: ",$col_b,"\n"; #prints 5
my $row_binv=scalar(#$b_inv);
print "Number of rows in b_inv: ",$row_binv,"\n"; #prints 3
my $col_binv=#{$$b_inv[0]};
print "Number of columns in b_inv ",$col_binv,"\n"; #prints 5
I am not able to understand
why the output of number of rows for both b and b_inv is wrong? How to get the correct value of number of rows?
That although the syntax of printing elements of a referenced array is $$b_inv[1][1], I get the correct output when I use $$b_inv[0][1][1]

Answer 1

You are creating a
Math::MatrixReal
matrix object, and then accessing it as a simple Perl array. Poking around inside a Perl object indiscriminately is wrong, and you must use the methods defined in the documentation
In particular, your statement
print $$b_inv[0][1][1],"\n"; # prints the correct element
accesses a three-dimensional array, and there is no way of knowing what the "correct element" should be for this without reading the code of the module
This modification sets up a 5 x 5 identity matrix (in future, please provide data that we can use to reproduce your results) and takes its inverse. The values output are derived using the object's methods as I described and are all correct. Note that the rows and columns are indexed from one instead of from zero that you would expect for Perl arrays
use strict;
use warnings 'all';
use Math::MatrixReal;
my #arr = (
[1, 0, 0, 0, 0],
[0, 1, 0, 0, 0],
[0, 0, 1, 0, 0],
[0, 0, 0, 1, 0],
[0, 0, 0, 0, 1],
);
my $ref_a = \#arr;
my $b = Math::MatrixReal->new_from_rows(\#arr);
my $b_inv = $b->inverse;
print "\nInverse is\n", $b_inv;
print "\n\nTest printing elements\n";
print $b_inv->element($_, $_), "\n" for 1 .. 5;
my ($row_b, $col_b) = $b->dim;;
print "Number of rows in b: $row_b\n"; # prints 5
print "Columns in b: $col_b\n"; # prints 5
my ($row_binv, $col_binv) = $b_inv->dim;;
print "Number of rows in b_inv: $row_binv\n"; # prints 5
print "Number of columns in b_inv $col_binv\n"; # prints 5
output
Inverse is
[ 1.000000000000E+000 0.000000000000E+000 0.000000000000E+000 0.000000000000E+000 0.000000000000E+000 ]
[ 0.000000000000E+000 1.000000000000E+000 0.000000000000E+000 0.000000000000E+000 0.000000000000E+000 ]
[ 0.000000000000E+000 0.000000000000E+000 1.000000000000E+000 0.000000000000E+000 0.000000000000E+000 ]
[ 0.000000000000E+000 0.000000000000E+000 0.000000000000E+000 1.000000000000E+000 0.000000000000E+000 ]
[ 0.000000000000E+000 0.000000000000E+000 0.000000000000E+000 0.000000000000E+000 1.000000000000E+000 ]
Test printing elements
1
1
1
1
1
Number of rows in b: 5
Columns in b: 5
Number of rows in b_inv: 5
Number of columns in b_inv 5