How can I randomly sample the contents of a file?

By : kal

I have a file with contents


There are more than 2 million lines in the files. I want to randomly sample lines from the files and output 50K lines. Any thoughts on how to approach this problem? I was thinking along the lines of Perl and its rand function (Or a handy shell command would be neat).

Related (Possibly Duplicate) Questions:

By : kal


If you need to extract an exact number of lines:

use strict;
use warnings;

# Number of lines to pick and file to pick from
# Error checking omitted!
my ($pick, $file) = @ARGV;

open(my $fh, '<', $file)
    or die "Can't read file '$file' [$!]\n";

# count lines in file
my ($lines, $buffer);
while (sysread $fh, $buffer, 4096) {
    $lines += ($buffer =~ tr/\n//);

# limit number of lines to pick to number of lines in file
$pick = $lines if $pick > $lines;

# build list of N lines to pick, use a hash to prevent picking the
# same line multiple times
my %picked;
for (1 .. $pick) {
    my $n = int(rand($lines)) + 1;
    redo if $picked{$n}++

# loop over file extracting selected lines
seek($fh, 0, 0);
while (<$fh>) {
    print if $picked{$.};
close $fh;

From perlfaq5: "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:

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.

This video can help you solving your question :)
By: admin