Perl: Array of hashes - Get the element with id in hash - perl

Question

I have a Perl data structure, loaded from a json, with Data::Dumper looking like this:
$VAR1 = {
'Stat' => [
{
'statCfgFile' => 'statcfg_0001.json',
'statid' => 1,
'status' => 'running',
'something' => 'other'
},
{
'statCfgFile' => 'statcfg_0002.json',
'statid' => 2,
'status' => 'running'
'something' => 'other'
}
]
}
From another dataset, I get a statid to read the hash, but the given id does not match the array key, therefore I need to go into deep, reading the statid property.
Of course, I could loop through the array, but with a large array list, that might hit performance.
Is there a way to directly access the array element by the statid stored in the hash?
As the data model is in my hands and currently in development:
Would it be better not to use an array, but also a hash with the statid as the naming element?

Answer 1

The short answer is no, you can't pull a value out of an array based on a criteria without iterating over the array. But there are ways to write it without using a loop.
Say the data structure is $data and you want to pull hashes where statid equals $id_to_find.
my #matching_hashes = grep {$_->{statid} = $id_to_find}} #{$data->{Stat}};
If only the first hash matching the ID is relevant, you could use the core module List::Util function first, which does the same thing as grep but only returns the first match instead of a list. It would be faster than grep because it stops iterating over the array as soon as it finds one match.
use List::Util 'first';
my $matching hash = first {$_->{statid} = $id_to_find}} #{$data->{Stat}};
As you indicated, using hash lookups is much faster than list operations. You could create a hash index of array adresses. Note this still requires you iterate over the array once.
# create the index;
my %index;
my #array = #{$data->{Stat}};
for my $address (0..$#array) { # "$#array" is the last element of #array
my $hash = $array[$address];
my $id = $hash->{statid};
$index{$id} = $address; # now you can use an ID to get the array address
}
# use the index
my $id_to_find = 42;
my $wanted_array_address = $index{$id_to_find};
my $matching_hash = $data->{Stat}->[$wanted_array_address];

Answer 2

If the statid field is unique then I suggest you use something more like this
{
1 => 'statcfg_0001.json',
2 => 'statcfg_0002.json',
}
The hash includes more data than shown. I extend the example
Then you need a hash instead of just a string value for each statid
{
1 => {
statCfgFile => 'statcfg_0001.json',
statid => 1,
status => 'running',
something => 'other',
},
2 => {
statCfgFile => 'statcfg_0002.json',
statid => 2,
status => 'running',
something => 'other',
},
}