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How do I find out if a hash is empty?

by Pug (Monk)
on Jun 11, 2002 at 22:34 UTC ( #173677=categorized question: print w/replies, xml ) Need Help??
Contributed by Pug on Jun 11, 2002 at 22:34 UTC
Q&A  > hashes

Answer: How do I find out if a hash is empty?
contributed by ysth

Preferably with  if (!keys %hash) { print "Empty" }.

This has the drawback of actually going through fetching all the keys for a tied hash, but the alternative (if (!%hash) { print "Empty" }) doesn't work at all for tied hashes.

If tied hashes aren't a concern, either way will do, but there is a difference to be aware of: the keys version will reset the hash's iterator (see each) while the other way will leave it unchanged.

Answer: How do I find out if a hash is empty?
contributed by gbarr

In a scalar context a hash yields a string like "1/4"

This is the number of buckets in use and the total number of buckets in the hash. When used in a numeric context it will result in the first number.

If the hash is empty, perl will just give "0"

But to prevent this generation of a string and then conversion to a number you can do

scalar keys %hash

You say beware ties. Well with ties everything goes out the window as there is no knowing if the tied package will behave in a usual fashion, ie it could arrange for keys to always return an empty list, but still return values for accesses. MJD has a package Interpolation that does this.

Answer: How do I find out if a hash is empty?
contributed by mojotoad

Beware the Ties of March!!!

In my innagural post I pointed out that a hash in scalar context will not behave as expected when tied, although it's tasty syntax: if (%my_hash) { ... do stuff ... }

I feel that's a bug and should be addressed.

As a bulletproof workaround, always use keys in scalar context thusly: if (scalar keys %my_hash) { stuff... }


Answer: How do I find out if a hash is empty?
contributed by particle

a hash in scalar context returns (somewhat uselessly -- except for now,) the number of slots filled out of the number of slots allocated. so scalar %hash will equal 0 if no slots are allocated.

print 'empty' unles scalar %hash;
Answer: How do I find out if a hash is empty?
contributed by Pug

This seems to work.. As to why? I don't know.

my %t; if (!%t) { print "Empty"; } else { print "Not Empty\n"; }

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