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;
-
Are you posting in the right place? Check out Where do I post X? to know for sure.
-
Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
<code> <a> <b> <big>
<blockquote> <br /> <dd>
<dl> <dt> <em> <font>
<h1> <h2> <h3> <h4>
<h5> <h6> <hr /> <i>
<li> <nbsp> <ol> <p>
<small> <strike> <strong>
<sub> <sup> <table>
<td> <th> <tr> <tt>
<u> <ul>
-
Snippets of code should be wrapped in
<code> tags not
<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
intervention).
-
Want more info? How to link
or How to display code and escape characters
are good places to start.
|