Transform Array Ref to Hash Ref

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

I'm trying to convert an AoA returned by DBI->fetchall_arrayref into a HoA.

My AoA returned by fetchall_arrayref looks like this:

I want to take the second element in each array and make it a hash key. If the second element is undef, then I want to change it to '0'. Also I want to take the first element in every array and push it into the array referenced by the hash ( which is the second element in the array).

Here's my try at a test script to do this, what's wrong with it? I see that there's something wrong with pushing to an array slice.

!/usr/bin/perl -w


use strict;
use Data::Dumper;

my @one = qw(95 undef);
my @two = qw(100 2);
my @three = qw(105 undef);
my @four = qw(100 2);

my @array = \@one;
push @array, \@two;
push @array, \@three;
push @array, \@four;

print Dumper @array;

my %hash;

foreach my $ary ( @array ) {
        if ( @$ary[1] eq undef ) { @$ary[1] = '0' }
                $hash{@$ary[1]} = push @$ary[0];
}
print Dumper %hash;
[download]

Comment on Transform Array Ref to Hash Ref - Where's my error? Select or Download Code

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Re: Transform Array Ref to Hash Ref - Where's my error? by Errto (Vicar) on Feb 14, 2006 at 01:31 UTC
What you've got there is pretty close, but you've got the syntax for the push part a bit backwards. With push, you always specify the array to be pushed onto first, and you don't use the assignment operator: `my %hash; foreach my $ary ( @array ) { if ( !defined($$ary[1]) ) { $$ary[1] = '0' } push @{$hash{$$ary[1]}}, $$ary[0]; } print Dumper \%hash;` [download] I also changed your @$ usage to $$. `@$ary[1]` is an array slice, which is not appropriate to use when you are only dealing with single elements, as is the case here. I also introduced the defined function, which is the only valid way to check whether a value is defined or not (string comparison to undef will succeed for the empty string '' and throw a warning). By the way, as a personal preference I like to use -> for single element lookups in hash or array references because I think it is clearer. So everywhere you have `$$ary[1]` you can instead say `$ary->[1]`. Forgot one other thing. I changed `print Dumper %hash` to `print Dumper \%hash` because otherwise Dumper will treat your hash as simply a list of values and not as an actual hash. Yet one more. I aligned the two statements inside the `foreach` loop because they are in fact part of the same block. In the OP a reader might think that push statement was part of the if block, which it's not.	[reply] [d/l] [select]
Re^2: Transform Array Ref to Hash Ref - Where's my error? by davidrw (Prior) on Feb 14, 2006 at 02:12 UTC
No need (and probably shouldn't ... though i see that OP did) to modify the input w/the `$$ary[1] = '0'` statement .. `my %hash; foreach my $row ( @array ) { my $k = defined($row->[1]) ? $row->[1] : 0; push @{ $hash{$k} }, $row->[0]; }` [download] or a more condensed version: `my %hash; push @{ hash{ defined($_->[1]) ? $_->[1] : 0 } }, $_->[0] for @array;` [download]	[reply] [d/l] [select]
Re^3: Transform Array Ref to Hash Ref - Where's my error? by ayrnieu (Beadle) on Feb 14, 2006 at 07:20 UTC
Even more condensed: `my %h; push @{$h{$_->[1]\|\|0}}, $_->[0] for @a;` [download]	[reply] [d/l]
Re: Transform Array Ref to Hash Ref - Where's my error? by McDarren (Abbot) on Feb 14, 2006 at 01:12 UTC
I want to take the second element in each array and make it a hash key. That's not going to work, because hash keys must be unique. Well, it will "work", but you're going to clobber a lot of your data.	[reply]