Randomly select N lines from a file, on the fly

[blokhead]

Many people know the trick from perlfaq about how to choose a line uniformly at random from a file (or pipe), without knowing a priori how many lines are coming.

Here is a generalization of the method that chooses a random subset (without repetition) of N random lines from a file. The method only needs to keep N lines of the file in memory. It also preserves the ordering of lines. If the little script is named sample, you use it like this:

$ sample 10 somelongfile.txt  ## to get 10 random lines

$ some long command | sample 50 > mysample.txt

Proof of correctness is fairly straight-forward by induction.

Note: perlfaq recommends File::Random for the case of choosing 1 random line. And indeed, the random_line function in that module has an option to choose more than 1 line. However, it selects with repetition.

my $wanted = shift || 10;
my @got;

die "Invalid number of lines!\n" if $wanted < 1;

while (<>) {
  if (@got < $wanted) {
      push @got, $_;
  } elsif (rand($.) < $wanted) {
      splice @got, rand(@got), 1;
      push @got, $_;
  }
}

die "Not enough lines!\n" if @got < $wanted;
print @got;