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From: PerlFAQ Server on 27 Feb 2010 12:00
This is an excerpt from the latest version perlfaq5.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .

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5.38: How do I select a random line from a file?

Short of loading the file into a database or pre-indexing the lines in
the file, there are a couple of things that you can do.

Here's a reservoir-sampling algorithm from the Camel Book:

srand;
rand($.) < 1 && ($line = $_) while <>;

This has a significant advantage in space over reading the whole file
in. You can find a proof of this method in *The Art of Computer
Programming*, Volume 2, Section 3.4.2, by Donald E. Knuth.

You can use the "File::Random" module which provides a function for that
algorithm:

use File::Random qw/random_line/;
my $line = random_line($filename);

Another way is to use the "Tie::File" module, which treats the entire
file as an array. Simply access a random array element.



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