CODEWITHSUNDEEP
perlhash
Kiran K Telukunta
Kiran K Telukunta

Reputation: 2234

how to check if a hash is empty in perl

I use the following code to check if the hash is empty. Is there a better method and is this safe to use?

if (!keys %hash) { print "Empty";}

Upvotes: 63

Views: 102185

Answers (4)

DVK
DVK

Reputation: 129403

Simpler:

if (!%hash) { 
    print "Empty";
}

! imposes a scalar context, and hash evaluated in a scalar context returns:

  • false if there are zero keys (not defined in the documentation but experimentally returns 0)

Depending on the version of Perl, either of the following:

  • A string signifying how many used/allocated buckets are used for >0 keys, which will of course be NOT false (e.g. "3/6"). (Non-empty string evaluate to true)

  • The number of keys in the hash (as explained in perldata: "As of Perl 5.25 the return was changed to be the count of keys in the hash. If you need access to the old behavior you can use "Hash::Util::bucket_ratio()" instead.")

Upvotes: 22

A Person
A Person

Reputation: 1112

"Better" is a subjective term. However I would argue that code that is easier to understand can be described as "better". For this reason I conclude that !keys %hash is better, because everybody writing perl code will know what this code does and that it works. !%hash is something at least I would have to look up to ensure if it really works or only looks like it would work. (The reason being that the return value of a hash in scalar context is rather confusing while an arrays behavior in scalar context is well known and often used.)

Also, !keys %hash is safe.

So no, there is no better or safer way to check if a hash is empty.

Upvotes: 2

Schwern
Schwern

Reputation: 164799

There was a bug which caused tied hashes in scalar context to always return false. The bug was fixed in 5.8.5. If you're concerned with backwards compatibility that far back I would stick with if( !keys %hash ). Otherwise use if( !%hash ) as recommended by others.

Upvotes: 28

TLP
TLP

Reputation: 67900

if (%hash)

Will work just fine.

From perldoc perldata:

If you evaluate a hash in scalar context, it returns false if the hash is empty. If there are any key/value pairs, it returns true; more precisely, the value returned is a string consisting of the number of used buckets and the number of allocated buckets, separated by a slash.

Upvotes: 105

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