Perl: Hash within Array within Hash

Question

I am trying to build a Hash that has an array as one value; this array will then contain hashes. Unfortunately, I have coded it wrong and it is being interpreted as a psuedo-hash. Please help!

my $xcHash       = {};
my $xcLine;

#populate hash header

$xcHash->{XC_HASH_LINES} = ();

#for each line of data
    $xcLine       = {};

    #populate line hash

    push(@{$xcHash->{XC_HASH_LINES}}, $xcLine);

foreach $xcLine ($xcHash->{XC_HASH_LINES})
    #psuedo-hash error occurs when I try to use $xcLine->{...}

Including use strict; use warnings; would have revealed the problem for you. — Ether
– Ether, Commented Nov 19, 2010 at 17:44

Richard Simões · Accepted Answer · 2010-11-19 17:22:06Z

3

$xcHash->{XC_HASH_LINES} is an arrayref and not an array. So

$xcHash->{XC_HASH_LINES} = ();

should be:

$xcHash->{XC_HASH_LINES} = [];

foreach takes a list. It can be a list containing a single scalar (foreach ($foo)), but that's not what you want here.

foreach $xcLine ($xcHash->{XC_HASH_LINES})

should be:

foreach my $xcLine (@{$xcHash->{XC_HASH_LINES}})

answered Nov 19, 2010 at 17:22

Richard Simões

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1 Comment

Ether Over a year ago

"forward-declaring" $xcHash->{XC_HASH_LINES} is not necessary at all though, as it will be autovivified on first use.

ysth · Accepted Answer · 2010-11-19 17:16:11Z

3

foreach $xcLine ($xcHash->{XC_HASH_LINES})

should be

foreach $xcLine ( @{ $xcHash->{XC_HASH_LINES} } )

See http://perlmonks.org/?node=References+quick+reference for easy to remember rules for how to dereference complex data structures.

answered Nov 19, 2010 at 17:16

ysth

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Comments

Community · Accepted Answer · 2020-06-20 09:12:55Z

2

Golden Rule #1

use strict;
use warnings;

It might seem like a fight at the beginning, but they will instill good Perl practices and help identify many syntactical errors that might otherwise go unnoticed.

Also, Perl has a neat feature called autovivification. It means that $xcHash and $xcLine need not be pre-defined or constructed as references to arrays or hashes.

The issue faced here is to do with the not uncommon notion that a scalar can hold an array or hash; it doesn't. What it holds is a reference. This means that the $xcHash->{XC_HASH_LINES} is an arrayref, not an array, which is why it needs to be dereferenced as an array using the @{...} notation.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Nov 19, 2010 at 17:26

Zaid

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2 Comments

David Beckman Over a year ago

Thanks! I do include strict & warnings in my code; I just omitted it here for brevity.

Zaid Over a year ago

@David : Then you must've posted the code from memory. The code you've posted would not run under the strict and warnings pragmas.

ennuikiller · Accepted Answer · 2010-11-19 17:10:26Z

0

Here's what I would do:

my %xcHash;

for each line of data:

push @{$xcHash{XC_HASH_LINES}},$xcLine;

answered Nov 19, 2010 at 17:10

ennuikiller

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1 Comment

Zaid Over a year ago

While this is valid advice, it doesn't address the issue at hand

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