Duplicating array contents

dda has asked for the wisdom of the Perl Monks concerning the following question:

Hello Monks,

Please look at the code example:

#!/usr/bin/perl -w

use strict;

my @a = (1,2,3,4);
my $p = \@a;

print "before:\n";
foreach (@{$p}) { print "element $_\n"; }

@a = ();

print "after:\n";
foreach (@{$p}) { print "element $_\n"; }
[download]

What if I want to keep the old contents somewhere? I found that changing the line

my $p = \@a;
[download]

my $p = [@a];
[download]

does the trick. Is it a correct way?

--dda

Comment on Duplicating array contents Select or Download Code

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Re: Duplicating array contents by jdporter (Paladin) on Dec 16, 2002 at 15:10 UTC
Well, you understand that `$p = \@a;` makes $p be a reference to the array @a, right? That means if you alter @a in any way, @$p is altered as well, and vice versa. If your point is really to create a new, indpendent copy of the contents of an array, you would simply do this: `@b = @a;` [download] You may indeed do `$p = [ @a ];` with the understanding that $p is now a reference to a copy of @a. jdporter ...porque es dificil estar guapo y blanco.	[reply] [d/l] [select]
(jeffa) Re: Duplicating array contents by jeffa (Bishop) on Dec 16, 2002 at 15:12 UTC
Without knowing what you are doing, saying that your 'trick' is or is not a correct way is next to impossible. If you want to keep the old contents, you have to make a copy. If you want to work with a reference to the array, you have to account for the fact that those contents can be altered. `my $p = \@a;` [download] Now $p is a reference to @a. You can change the contents of @a via $p. `my $p = [@a];` [download] $p is still a reference, but a reference to an anonymous array that contains the contents of @a. You can change the contents of $p, but that will not affect @a (as long as @a didn't contain any references ... we won't go there). You could also copy @a like so: `my @p = @a;` [download] Instead of merely printing the contents out, i find using Data::Dumper to be much more helpful: `use strict; use warnings; use Data::Dumper; my @a = (1,2,3,4); my @copy = @a; my $ref = \@a; my $refcopy = [@a]; @a = (); print "orginal: ", Dumper \@a; print "array copy: ", Dumper \@copy; print "reference: ", Dumper $ref; print "ref copy: ", Dumper $refcopy;` [download] UPDATE: This is Perl, so instead of using the cluttered: `foreach (@{$p}) { print "element $_\n"; }` [download] use the more readable (IMHO at least): `print "element $_\n" for @$p;` [download] Have fun. :) jeffa L-LL-L--L-LL-L--L-LL-L-- -R--R-RR-R--R-RR-R--R-RR B--B--B--B--B--B--B--B-- H---H---H---H---H---H--- (the triplet paradiddle with high-hat)	[reply] [d/l] [select]
Re^2: Duplicating array contents by Aristotle (Chancellor) on Dec 16, 2002 at 19:29 UTC
Or since this is Perl, `print map "element $_\n", @$p;` `;-)` Makeshifts last the longest.	[reply] [d/l]
Re: Duplicating array contents by BrowserUk (Patriarch) on Dec 16, 2002 at 15:12 UTC
That is one way to take a copy of an array. With that line added to your your program, it is effectively equivalent to: `#!/usr/bin/perl -w use strict; my @a = (1,2,3,4); my @p = @a; print "before:\n"; foreach (@p) { print "element $_\n"; } @a = (); print "after:\n"; foreach (@p) { print "element $_\n"; }` [download] Examine what is said, not who speaks.	[reply] [d/l]