如何使用 hash 切片进行“定义”

Question

I'm trying to learn Perl better, and learn hash slices.

Instead of 3 different if (defined statements, I'm trying to tidy the code to make it more maintainable and readable, but have come across the following conundrum:

#!/usr/bin/env perl

use strict;
use warnings FATAL => 'all';
use feature 'say';
use autodie ':all';
use Carp 'confess';
use DDP; # a.k.a. Data::Printer
use JSON 'decode_json';

my $hashref;
$hashref->{Jane} = decode_json('{"sex":"Female","Mortality_Status":"Alive", "latest_date":"2020-11-26","Hospitalized":"no","Risk_Status":"NA"}');
p $hashref; # pretty print the data
my @needed_terms = qw(age BMI sex);
if (defined @{ $hashref->{Jane} }{@needed_terms}) {
    say 'all terms are defined.'; # this is what it says, which is WRONG!!!
} else {
    say 'some terms are missing.'; # Jane is missing BMI and age, so the script should print here
}

I've read How to do sum of hash reference slice? and https://perlmonks.org/?node=References+quick+reference to no avail.

This person Jane is missing both age and BMI information, so the if (defined statement should say that some terms are missing, but is instead passing.

I get the same error whether I use @{ $hashref->{Jane} }{@needed_terms} or %{ $hashref->{Jane} }{@needed_terms}

I've also thought that maybe defined is returning how many terms of the slice are defined, but that isn't true.

How can I set if (defined statement on a hash slice properly?

Answer 1

This is a good place to use all from List::Util :

use List::Util qw(all);

if (all { exists $hashref->{Jane}{$_} } @needed_terms) {
    say 'all terms are defined.';
} else {
    say 'some terms are missing.'; # Jane is missing BMI and age, so the script should print here
}

It loops through all the needed terms and checks to see if each exists as key to the Jane hash.

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One of my favorite docs for Perl Data Structures is perldoc perldsc . It is more of a step-by-step tutorial than References quick reference .