Should Perl hashes always contain values?

Question

I had an earlier question that received the following response from the noted Perl expert, Perl author and Perl trainer brian d foy:

[If] you're looking for a fixed sequence of characters at the end of each filename. You want to know if that fixed sequence is in a list of sequences that interest you. Store all the extensions in a hash and look in that hash:

    my( $extension ) = $filename =~ m/\.([^.]+)$/;
    if( exists $hash{$extension} ) { ... }

You don't need to build up a regular expression, and you don't need to go through several possible regex alternations to check every extension you have to examine.

Thanks for the advice brian.

What I now want to know is what is the best practice in a case like the above. Should one only define the keys, which is all I need to achieve what's described above, or should one always define a value as well?

Answer 1

It's usually preferable to set a defined value for every key. The idiomatic value (when you don't care about the value) is 1.

my %hash = map { $_ => 1 } @array;

Doing it this way makes the code the uses the hash slightly simpler because you can use $hash{key} as a Boolean value. If the value can be undefined you need to use the more verbose exists $hash{key} instead.

That said, there are situations where a value of undef is desirable. For example: imagine that you're parsing C header files to extract preprocessor symbols. It would be logical to store these in a hash of name => value pairs.

#define FOO 1
#define BAR

In Perl, this would map to:

my %symbols = ( FOO => 1, BAR => undef);

In C a #define defines a symbol, not a value -- "defined" in C is mapped to "exists" in Perl.