Populating a perl array of hashes with copies of a single hash

Question

I have searched and searched and I can't get any of the code I've found to work. I'm sorry if this is repeating old ground, but I've now spent 2 days trying to get these 10 lines to work and I am at my wits' end with no hair left :-(

I am running Perl 5.8.8.

I want to populate an array of hashes in Perl such that it contains multiple copies of a single hash variable I am updating. My code is here:

use strict;
use warnings;

my @array;

my %tempHash = (state => "apple", symbol => "54", memberId => "12345");
push(@array, \%tempHash);

%tempHash = (state => "tiger", symbol => "22", memberId => "12345");
push(@array, \%tempHash);

%tempHash = (state => "table", symbol => "37", memberId => "12345");
push(@array, \%tempHash);

printf("%p %p %p\n", $array[0], $array[1], $array[2]);

foreach my $entry (@array){
    printf("state: %s\n", $entry->{state});
    printf("memberId: %s\n", $entry->{memberId});
    printf("symbol: %s\n\n", $entry->{symbol});
}

This produces the following output:

1868954 18688d0 18688c4
state: table
memberId: 12345
symbol: 37

state: table
memberId: 12345
symbol: 37

state: table
memberId: 12345
symbol: 37

So it looks to me like the scalar values in the array are different. Yet the values in the hashes these scalars point to are all the same.

Thanks in advance for your help.

Answer 1

1) The code you posted doesn't work under use strict;, did you mean %tempHash and %hash are really the same variable?

2) If you use %s instead of %p, you'll get 3 identical HASH(0x1234abcd) strings, which means the contents of the array are indeed references to the same hash.

3) I would suggest creating a new anonymous hash each time:

#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my @array;
my %tempHash = (state => "apple", symbol => "54",memberId => "12345");
push(@array, { %tempHash });

%tempHash = (state => "tiger", symbol => "22", memberId => "12345");
push(@array, { %tempHash });

%tempHash = (state => "table", symbol => "37", memberId => "12345");
push(@array, { %tempHash });

print Dumper( \@array );

Answer 2

It sounds like you are fetching data a line at a time from a CSV file using Text::CSV.

Suppose your code is like this

my %tempHash;
my @array;

while (my $line = $csv->getline($fh)) {
  # Add values to %tempHash;
  push @array, \%tempHash;
}

then you could solve your problem very simply by declaring %tempHash insode the while loop

my @array;

while (my $line = $csv->getline($fh)) {
  my %tempHash;
  # Add values to %tempHash;
  push @array, \%tempHash;
}

because Perl creates a new lexical hash each time the block is entered

---

Update

If the data isn't necessarily complete after each input record, then write

my @array;
my $data = {};

while ( my $line = $csv->getline($fh) ) {
  # use information from $line to supplement $data
  if ($data is complete) {
    push @array, $data;
    $data = {};
  }
}

Answer 3

If you'd added use strict and use warnings to tor script, it would have told you what's wrong:

1st, your filling the hash temphash and store a reference to it. Next you create a new has, hash which you fill BUT NEVER USE! Instead, you add new references to temphash...

Answer 4

My bet is the push function does just not care about references. See perldoc:

Starting with Perl 5.14, push can take a scalar EXPR, which must hold a reference to an unblessed array. The argument will be dereferenced automatically. This aspect of push is considered highly experimental. The exact behaviour may change in a future version of Perl.

EDIT

You could try to not use push:

my @array;
my %tempHash = (state => "apple", symbol => "54", memberId => "12345");
$#array=4;
$array[0]=\%tempHash;
$array[1]=\%tempHash;
$array[2]=\%tempHash;
printf("%p %p %p\n", $array[0], $array[1], $array[2]);
foreach my $entry (@array){
    printf("state: %s\n", $entry->{state});
    printf("memberId: %s\n", $entry->{memberId});
    printf("symbol: %s\n\n", $entry->{symbol});
}

with same result as my perl interpreter just told me :-( (perl 5.14.2)