Beefy Boxes and Bandwidth Generously Provided by pair Networks
Don't ask to ask, just ask
 
PerlMonks  

How do I find out if a hash is empty?

( #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) { ...do stuff... }

Matt

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"; }

Please (register and) log in if you wish to add an answer



  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?
    Username:
    Password:

    What's my password?
    Create A New User
    Chatterbox?
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others musing on the Monastery: (9)
    As of 2015-07-29 22:28 GMT
    Sections?
    Information?
    Find Nodes?
    Leftovers?
      Voting Booth?

      The top three priorities of my open tasks are (in descending order of likelihood to be worked on) ...









      Results (269 votes), past polls