Accessing and displaying key-value pairs in Perl from subroutine - perl

Question

New to Perl. Got syntax errors in accessing key-value pairs from subroutine.
sub displayObj{
my %obj = shift; //the hash. It is a JSON object after decode_json()
my $field = shift; //the key we are searching. It is a string.
my $serialized = "{}"; //Initialization
foreach my $key (keys %obj){
if($key eq $field){
$serialized = $obj[{$field}];
}
}
print "$serialized\n";
}
It is giving me a compilation error in the if block.
I would like to know:
Should I use % or $ in my %obj?
How to access the key-value pair (also a JSON object) and assign it to $serialized using $obj?

Answer 1

I think you're trying to write a subroutine that displays the value of a hash element given its key
But you're missing the basic purpose of hashes: they're content-addressable. That means there's no need to iterate through all the keys of a hash comparing them to the string you're looking for, you can write just $hash{key} and the search will be done for you very quickly using a hashing system (hence the name of the data type). This is just the same as using $array[$i] to access an array element directly instead of looping over all of the indices of the array comparing each one to $i until you find the element you're looking for
If you're really dealing with objects then you shouldn't be accessing their internal data like this anyway. An object will have accessor methods that return the values you're supposed to be using; anything else is part of the internal working of the class and is meant to be private
The syntax error is in this line
$serialized = %obj[{$field}]
where it looks like you're trying to use both a hash key {...} and an array index [...]. That won't work
You don't show how you're calling your subroutine, but I imagine you're passing a reference to a hash, which is a scalar value and must be treated as such inside the subroutine
This program shows a working version of what I think you intended
use strict;
use warnings 'all';
my $obj = {
aa => 1,
cc => 2,
};
displayObj($obj, 'cc');
displayObj($obj, 'bb');
sub displayObj {
my $obj = shift; # The hash. It is a JSON object after decode_json()
my $field = shift; # The key we are searching. It is a string.
my $serialized = '{}'; # Initialization
foreach my $key ( keys %$obj ) {
if ( $key eq $field ) {
$serialized = $obj->{$field};
}
}
print "$serialized\n";
}
output
2
{}
But the loop can be removed altogether as I described, leaving
sub displayObj {
my ($obj, $field) = #_;
my $serialized = $obj->{$field} // '{}';
print "$serialized\n";
}
which produces exactly the same result. In fact there's little point in creating a subroutine to do this; you can write just
print $obj->{bb} // '{}', "\n"
with the same effect

Answer 2

I usually do this way:
sub displayObj{
my $objref = shift;
my %obj = %{$objref};
}