perl eval to get single element array

Question

I'm trying to figure out how to use perl's eval to get a single element array. So far, I have:

use strict;
use warnings;
use Data::Dumper;

sub printit
{
    my $param = shift;
    my $thing = eval $param;
    if (ref($thing) =~ /HASH/ || ref($thing) =~ /ARRAY/) {
        print Dumper \$thing;
    } else {
        print $param . "\n";
    }
}
my $ip = "192.168.1.100";
my $ip_array = "[192.168.1.100]";
my $ip_array2 = "[192.168.1.100,]";
my $string = "{ a => 1, b => 2, c => 3}";
my $another_string = "[1, 2, 3 ]";

printit($ip);
printit($string);
printit($another_string);
printit($ip_array);
printit($ip_array2);

My output looks like:

[user]$ perl ~/tmp/cast.pl 
192.168.1.100
$VAR1 = \{
            'c' => 3,
            'a' => 1,
            'b' => 2
          };
$VAR1 = \[
            1,
            2,
            3
          ];
$VAR1 = \[
            "\x{c0}\x{a8}d"
          ];
$VAR1 = \[
            "\x{c0}\x{a8}d"
          ];

I think I'm getting a scalar ref for the last 2 print outs but I want an array with a single element like this:

$VAR1 = \[
            "192.168.1.100"
          ];

Are you sure that's what you're getting? I've just tried running your code, and got the desired output... — Sobrique
– Sobrique, Commented Apr 30, 2015 at 15:33
Activeperl 5.20.1 - guessing we might have a quote interpolation thing going on here. Does: my $ip_array = "['192.168.1.100']"; show the same problems? — Sobrique
– Sobrique, Commented Apr 30, 2015 at 15:34
print Dumper \$thing; should be print Dumper $thing;. Use \ when you want to dump a variable that's not a scalar. — ikegami
– ikegami, Commented Apr 30, 2015 at 15:43

mob · Accepted Answer · 2015-04-30 15:40:13Z

5

eval "[192.168.1.100]" is an array reference, not a scalar reference. The array reference contains one element, but it is not the string "192.168.1.100" as you might expect, because 192.168.1.100 is not quoted. Instead you are creating the version string 192.168.1.100.

The fix is to include a quote or quoting operator in your input.

my $ip_array = "['192.168.1.100']";
my $ip_array2 = "[qq/192.168.1.100/,]";

See "Version Strings" in perldata.

edited Apr 30, 2015 at 15:40

answered Apr 30, 2015 at 15:29

mob

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5 Comments

mob Over a year ago

Thanks @choroba for the edit -- I was having trouble finding the docs

Sobrique Over a year ago

Do you know if that's version specific? Because as noted - the OPs code works ok on my 5.20.2 perl. (Although granted, probably has other unintended consequences due to data typing)

ikegami Over a year ago

@Sobrique, Look again. You are getting [ 192.168.1.100 ] instead of the desired [ "192.168.1.100" ]

Sobrique Over a year ago

True. So it's presumably same result, but handling the output a bit better.

Mike Noeth Over a year ago

[user]$ perl -v This is perl, v5.10.0 built for x86_64-linux-thread-multi

ikegami · Accepted Answer · 2015-04-30 16:15:12Z

1

You say you want

[ "192.168.1.100" ]

but the code you pass to Perl is

[ 192.168.1.100 ]

Those are very different.

"192.168.1.100" creates the 13-character string 192.168.1.100.
192.168.1.100 is short for v192.168.1.100, and it creates the same 4-character string as chr(192).chr(168).chr(1).chr(100).

One of many ways of writing what you want:

my $ip_array = '["192.168.1.100"]';

answered Apr 30, 2015 at 16:15

ikegami

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