CODEWITHSUNDEEP
perlhashgrep
KeyPi
KeyPi

Reputation: 526

How does the grep operator work?

I have an array and an hash. I'd like to match the key of the hash with the elements of the array.

I found an expression to grep all of the keys which matches with the array. It works, but I'm not sure that I've understood how it works. Can you explain what does it happen after grep?

Array @text and hash %sys are already defined.

my @new_array = grep { exists $sys{$_} ? $sys{$_} : 0 } @text;

Upvotes: 0

Views: 118

Answers (1)

Neil Slater
Neil Slater

Reputation: 27207

The code you found is over-complicated. It could/should be*:

my @new_array = grep { $sys{$_} } @text;

The grep function processes a list - in this case the contents of @text array - into a new list that only includes elements where the expression in the middle evaluates to a true value. The default value $_ is set to each list item in turn so that the expression can take account of the list contents.

The expression in your example includes a ternary operator ? which evaluates to the item before the : if the first part is true, or the value after it if false. So, in your case, it checks whether each key is in the %sys hash, and evaluates to a lookup of the hash entry if it exists, or 0 (a false value) if it does not. Functionally inside a grep this should be identical to just $sys{$_} because undef that you get from looking up the value of a non-existing key is also a false value.

* However, there is a caveat - if %sys was tied to a Perl class and had some hash functions overridden, different hash methods are called for exists and hash lookup. So in that scenario there can be a difference either in performance or behaviour.

Upvotes: 2

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