how to check if a hash is empty in perl

perl hash

88,506

Solution 1

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.

Solution 2

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.

Solution 3

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.")

Solution 4

"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.

View more solutions

88,506

Author by

Kiran K Telukunta

Updated on July 14, 2022

Comments

Kiran K Telukunta almost 2 years
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";} 
```
Borodin over 12 years

No it doesn't return "false", which would be a true value. It returns zero.
choroba almost 11 years

It seems much slower than keys %hash.
U. Windl almost 3 years

Good answer: What I was missing was the reference where the semantics are defined (it is not in perlop).
U. Windl almost 3 years

@choroba Do you also have an explanation and numbers (Perl version) for it?
choroba almost 3 years

@U.Windl: It has recently changed, so in the current Perl version(s), %hash in scalar context is equivalent to keys %hash.
U. Windl almost 3 years

Being paranoid I used scalar keys %hash == 0 as test. So !keys ... is basically doing a test on an array, not on a hash.
U. Windl almost 3 years

I agree: While thinking if (%hash) might work, I tried to find the specification of the semantics. Probably the confusing part is that if (@array) seems to be more clearly defined, but for symmetry it should work for hashes, too. Maybe that is why they changed it recently.
Schwern almost 3 years

@U.Windl Yes, keys %hash returns an array. scalar keys %hash returns an integer. Note that Perl functions are context aware and can optimize. !%hash is optimized to return a boolean. It does not iterate through its keys, it already has a count. The code is a nightmare, but here's the optimization. i = HvUSEDKEYS(hv);
Schwern almost 3 years

Seems !%h was optimized some more in 5.26. scalar keys performance was improved in 5.28.