How to access a hash value by index stored in variable - perl

Question

I am working on some code to extract data stored in JSON format from web page. JSON data extracted and decode properly into hash.
The structure of JSON data is very complex, I wrote some assisting code/function which 'traverse' through the hash and helps finds 'index' (location) of a value of interest in the hash.
The 'find' function returns 'index' (location) of the data in the hash which stored in a variable.
I attempt to use this variable (stored 'index') in other operations but so far without any success.
Please see included simple demo snippet code for explanation of the problem.
Thank you,
Polar Bear
use strict;
use warnings;
use JSON qw(decode_json);
my $index;
my $slice;
my $data = decode_json( join '', <DATA> );
printf "TITLE: %-15s TIME: %5s TIMES: %5s FAVORITE: %s\n",
$data->{playList}[1]{title},
$data->{playList}[1]{time},
$data->{playList}[1]{played},
$data->{playList}[1]{favorite} ? "yes" : "no";
$index = '{playList}[1]';
$slice = $data{$index}; # does not pass 'use strict' compilation error
$slice = $data->{$index}; # empty slice
$slice = $data->$index; # Can't call method "{playList}[1]" on unblessed reference at
printf "TITLE: %-15s TIME: %5s TIMES: %5s FAVORITE: %s\n",
$slice->{title},
$slice->{time},
$slice->{played},
$slice->{favorite} ? "yes" : "no";
__DATA__
{
"playList": [
{
"title": "Song name 1",
"time": "3:25",
"played": "240",
"favorite": "1"
},
{
"title": "Song name 2",
"time": "4:12",
"played": "30",
"favorite": "0"
},
{
"title": "Song name 3",
"time": "2:56",
"played": "85",
"favorite": "0"
}
]
}
I expect to access data by utilizing 'index' stored in a variable but I could not find a way to achieve this result. See comments in the code for details.
NOTE: In real life the index looks as following
my $index = "{contents}{twoColumnBrowseResultsRenderer}{tabs}[0]{tabRenderer}{content}{sectionListRenderer}{contents}[0]{itemSectionRenderer}{contents}[0]{playlistVideoListRenderer}{contents}[0]{playlistVideoRenderer}{title}{accessibility}{accessibilityData}{label}";
SOLUTION:
I would like to extend my 'thank you' to Håkon Hægland and lordadmira for offered solution
use Data::Diver qw/Dive/; # or Data::DPath, etc
# Capture web page, extract data JSON, convert to hash, assign hash ref to $data
my $data = ...;
# Find index/location in the hash
#my $index = find($data, $value);
my $index = "{contents}{twoColumnBrowseResultsRenderer}{tabs}[0]{tabRenderer}{content}{sectionListRenderer}{contents}[0]{itemSectionRenderer}{contents}[0]{playlistVideoListRenderer}{contents}[0]{playlistVideoRenderer}{title}{accessibility}{accessibilityData}{label}";
$index =~ s/[{\[]//g; # throw away opening brackets
my #index = split /[}\]]/, $index; # split into array on closing brackets
pop #index for 1..8 # 8 levels to back up to
my $slice = Dive( $data, #index ); # extract hash slice of interest
# extract playlist
my $playlist = $slice->{playlistVideoListRenderer}{contents};
# go through playlist and extract information of our interest
foreach ( #$playlist ) {
my $video = $_->{playlistVideoRenderer};
printf "%s %8s %s\n",
$video->{videoId},
$video->{lengthText}{simpleText},
$video->{title}{simpleText};
}
Both of them referred me to use Data::Dive with help of this module I can do back up few levels from the depth of the hash and extract slice of interest.
It was learned that by utilizing this module the index in form of array is easier to work with. Due this factor I will alter my find function to return an index array.

Answer 1

You can use Data::Diver :
use Data::Diver qw( DiveVal );
use JSON qw(decode_json);
my $data = decode_json( join '', <DATA> );
my $slice = DiveVal( $data, qw( playList 1 ) );
printf "TITLE: %-15s TIME: %5s TIMES: %5s FAVORITE: %s\n",
$slice->{title},
$slice->{time},
$slice->{played},
$slice->{favorite} ? "yes" : "no";
Output:
TITLE: Song name 2 TIME: 4:12 TIMES: 30 FAVORITE: no

Answer 2

I would like to extend my 'thank you' to Håkon Hægland and lordadmira for offered solution
use Data::Diver qw/Dive/; # or Data::DPath, etc
my $data = ...;
my $index = "{contents}{twoColumnBrowseResultsRenderer}{tabs}[0]{tabRenderer}{content}{sectionListRenderer}{contents}[0]{itemSectionRenderer}{contents}[0]{playlistVideoListRenderer}{contents}[0]{playlistVideoRenderer}{title}{accessibility}{accessibilityData}{label}";
$index =~ s/[{\[]//g;
my #index = split /[}\]]/, $index;
pop #index for 1..8 # 8 levels to back up to
my $slice = Dive( $data, #index );
my $playlist = $slice->{playlistVideoListRenderer}{contents};
foreach ( #$playlist ) {
my $video = $_->{playlistVideoRenderer};
printf "%s %8s %s\n",
$video->{videoId},
$video->{lengthText}{simpleText},
$video->{title}{simpleText};
}
Both of them referred me to use Data::Dive with help of this module I can do back up few levels from the depth of the hash and extract slice of interest.
It was learned that by utilizing this module the index in form of array is easier to work with. Due this factor I will alter my find function to return an index array.

Answer 3

In the comments, you said you had a function that finds the an element in your JSON data structure and returns a "path" to that element, and that your question is about finding a higher-level container of that element.
If this was XML, I'd use an XPath to do both the search and finding the right container. But worry not, someone has devlopped an XPath-like language for JSON, and someone has provide this functionality via Perl module JSON::Path.

Answer 4

The problem is that your code is returning all of the keys and elements to find the given element but not the element value itself. The simplest answer is to return a reference to that located value.
... bunch of lookdown code
return \ $this_level->{the_key_I_want}
That way the caller can have direct read/write access to the leaf value.
If you want to use the list of keys and elements directly to access a deep value, you will have to do a string eval. This is NOT recommended unless you 1000% percent trust the data because some joker can name a hash key "; system qw{rm -rf /}".
$index = "{contents}{twoColumnBrowseResultsRenderer}{tabs}[0]{tabRenderer}{content}{sectionListRenderer}{contents}[0]{itemSectionRenderer}{contents}[0]{playlistVideoListRenderer}{contents}[0]{playlistVideoRenderer}{title}{accessibility}{accessibilityData}{label}"
$value = eval "\$HASH$index";
The right way to use a list like that is another lookdown function that takes the list of keys etc and accesses them one by one.
HTH
PS: I slightly misunderstood your original question. Just write in your code:
$slice = $data->{playList}[1];
2nd Edit to actually use $index:
use Data::Diver qw/Dive/; # or Data::DPath, etc
my $data = ...;
my $index = "{contents}{twoColumnBrowseResultsRenderer}{tabs}[0]{tabRenderer}{content}{sectionListRenderer}{contents}[0]{itemSectionRenderer}{contents}[0]{playlistVideoListRenderer}{contents}[0]{playlistVideoRenderer}{title}{accessibility}{accessibilityData}{label}";
my #index = split /[{}[\]]+/, $index;
shift #index;
pop #index for 1..2 # however many levels to back up to
my $slice = Dive( $data, #index );