How can I print a multidimensional array in Perl? - arrays

Question

I am trying to print a multidimensional array (matrix) in Perl passing reference to array to subroutine.
Here is my code:
sub print_matrix(\#) {
my $array = shift;
for my $i ( 0 .. $#{ $array } ) {
my $row = $array[$i];
for my $j ( 0 .. $#($row) ) {
print $array[$i][$j];
}
}
}

Answer 1

Borodin tells you what was wrong with your code.
Now consider this module: Data::Dumper (available on CPAN). You can use this module to print any data structure: arrayref of arrayrefs (what you called a matrix), hashref of hashrefs, arrayref of hashrefs, hashref of hashrefs, or any other combination of these structures for as many dimensions as you want. Of course, if you have too many dimensions, it could lead to a confusing output.
My point is, some time ago, I was asked in an interview how I would implement this module. I thought it was a very clever question. I had to think a little because I use the module often but never bothered to figure how it works. It is in fact very simple. Imagine in your subroutine you receive a reference but you don't actually know what kind of reference it is (scalarref, arrayref, hashref, etc.), how would you determine what it is? If you have multiple possibilities, what would you do to cover all of them? Have you thought of creating a recursive function?
So, to solve your problem quickly, if you just want to print your matrix for debugging purpose, use Data::Dumper. Otherwise, if you want to do something more complex and wish to cover multiple cases, try to create a recursive function.
Here's a Data::Dumper example:
my $arrayref = [
[ qw/ a b c d / ],
[ qw/ e f g h / ],
[ qw/ i j k l / ],
];
use Data::Dumper;
print Dumper $arrayref;
And here's the result you will get:
$VAR1 = [
[
'a',
'b',
'c',
'd'
],
[
'e',
'f',
'g',
'h'
],
[
'i',
'j',
'k',
'l'
]
];
Each "row" of your matrix is printed as a list of elements, separated by a comma (and a new line), inside a pair of brackets. Be careful, if you pass it an array, it will print each elements one by one, and you will lose the "dimensions". If you only have an array, you have to pass it as a reference like this:
print Dumper \#array;
I hope this helps.

Answer 2

Using plain print is OK when all you have are single letter entries in your matrix, but a module like Text::Table can make it much easier to produce tidy output. For example,
#!/usr/bin/env perl
use strict;
use warnings;
use Text::Table;
my #matrix = map {
[ map sprintf('%.2f', -500 + rand(1000)), 1 .. 5 ]
} 1 .. 5;
my $mat = Text::Table->new;
$mat->load(#matrix);
print $mat;
Output:
-7.73 -83.85 -351.18 21.06 320.40
174.83 238.29 91.16 361.43 213.04
446.43 -4.82 322.81 10.38 -436.62
-128.05 195.68 199.05 288.39 115.30
-251.19 -329.35 244.13 -428.25 454.64

Answer 3

You can print a two-dimensional Perl array very simply with something like this
use strict;
use warnings;
my #arr_2d = (
[ qw/ a b c d / ],
[ qw/ e f g h / ],
[ qw/ i j k l / ],
);
print_2d(\#arr_2d);
sub print_2d {
my ($matrix) = #_;
print "#$_\n" for #$matrix;
}
output
a b c d
e f g h
i j k l
Update
Here's a working version of your own code. You weren't using array references properly and had parentheses where there should have been braces. This version also prints a space after each element and a newline after each row.
sub print_matrix {
my $array = shift;
for my $i ( 0 .. $#{ $array } ) {
my $row = $array->[$i];
for my $j ( 0 .. $#{ $row } ) {
print $array->[$i][$j], ' ';
}
print "\n";
}
}