How do I select a random key from a hash?

tadman has asked for the wisdom of the Perl Monks concerning the following question: ⭐ (hashes)

Replies are listed 'Best First'.
Re: How do I select a random key from a hash?⭐ by mirod (Canon) on Feb 05, 2001 at 20:57 UTC
The brute force solution: `/bin/perl -w use strict; my %h=( k1 => "v1", k2 => "v2", k3 => "v3"); my @keys= keys %h; my $k= $keys[int rand( @keys)]; print "random key: $k\n";'` [download] If you want to do this on a huge hash and you want to do lots of random access and updates then you should look at Building a Better Hash, a pretty clever way to solve this problem.	[reply] [d/l]
Re: How do I select a random key from a hash?⭐ by arhuman (Vicar) on Feb 05, 2001 at 21:43 UTC
What's wrong with this ?: `$lh=scalar keys %h; print ((keys %h)[int rand($lh)]);` [download] [Q&A Editor- Changed '`(split "/",scalar %h)[0]`' to "scalar keys %h" as noted by chipmunk.]	[reply] [d/l] [select]
Re: Answer: How do I select a random key from a hash? by chipmunk (Parson) on Feb 05, 2001 at 22:01 UTC
In a scalar context, %h returns a string consisting of the number of buckets used, a slash, and the number of buckets allocated. You could have a hash with 1000 keys and only 16 buckets. Thus, your "random key" is limited to one of the first few keys in the hash. Try `$lh = scalar keys %h;` instead.	[reply] [d/l]
Re: How do I select a random key from a hash?⭐ by MrNobo1024 (Hermit) on Mar 25, 2001 at 00:48 UTC
`((keys %hash)[int rand keys %hash])` You don't need scalar() because rand() automatically converts its argument to a scalar.	[reply] [d/l]
Re: Answer: How do I select a random key from a hash? by merlyn (Sage) on Mar 25, 2001 at 19:14 UTC
You also don't need int because indexing automatically int's it. {grin} But you are computing the keys twice, and that's expensive for large hashes. Perhaps this instead: `do { my @k = keys %hash; $k[rand @k] }` [download] -- Randal L. Schwartz, Perl hacker	[reply] [d/l]
Re: Re: Answer: How do I select a random key from a hash? by ysth (Canon) on Nov 04, 2003 at 10:09 UTC
`scalar keys %hash` doesn't actually compute the keys (except for tied hashes).	[reply] [d/l]
Re: How do I select a random key from a hash? by arhuman (Vicar) on Feb 05, 2001 at 21:58 UTC
`$lh=(split "/",scalar keys %h)[0]; print ((keys %h)[int rand($lh)]);` [download] [Q&A Editor- Changed "scalar %h" to "scalar keys %h" as noted by chipmunk.] Originally posted as a Categorized Answer.	[reply] [d/l]