pick N random lines from a file

[David Garamond]

I'm trying to extend the Perl cookbook recipe on how to pick a random 
line from a file:

 #!/usr/bin/perl
 rand($.) < 1 && ($line = $_) while <>;
 print $line;
for picking up to N random lines from a file:

------------start code--------------
#!/usr/bin/perl
die "Usage: $0 <N>, where N is the number of lines to pick\n"
  if @ARGV < 1;
$N = shift @ARGV;
@pick = ();
while (<>) {
  if (@pick < $N) {
    push @pick, $_;
    ($r1, $r2) = (rand(@pick), rand(@pick));
    ($pick[$r1], $pick[$r2]) = ($pick[$r2], $pick[$r1]);
  } else {
    rand($.) <= $N and $pick[rand(@pick)] = $_;
  }
}
print @pick;
-------------end code---------------
Could anyone verify if the algorithm is correct?

Thanks in advance,
--
dave


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