blokhead's code suffers greatly in terms of performance, when the hash is big.

On my old PC, tachyon's code only took 2 seconds to run with a hash with 100_001 elements. My following version has the same performance as tachyon's, as you can see the idea is the same, instead of deleting, we simply form a new hash: (To test other solutions, simply plug them in between those two print time)

use Data::Dumper;

my $n = 5;
my %hash;
@hash{ 0..100_000 } = 0..100_000;
print time, "\n";
my %temp;
@temp{(keys(%hash))[0..4]} = (values(%hash))[0..4];
%hash = %temp;
print time, "\n";
print Dumper(\%hash);

[download]

Update:

When I was writing thhis post, both davido and TedPride's code were not there... I retested all solutions above on my new PC, all solutions other than blokhead's took 0 second to complete (< 1 second). blokhead's took 30 seconds.

OP's code took 0 second, but has a warning: "Use of uninitialized value in delete at b.pl line 7."

In reply to Re: Delete all hash keys except for n of them by pg
in thread Delete all hash keys except for n of them by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • 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, details, 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, summary, 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.