The two major methods :-

Prefetch the line locations and then randomly pick one has an even distribution so the chance of picking a short like is the same as a long line.

Randomly picking a possition and moving to the head/end of the current record will be distorted by the length of the record. Longer records will have a higher probability o being picked than short records.

However you can combine the methods if you are using fixed length records.

my $record_length=200;               # set the record length
open my $fh,'<',$filename or die $!; # open handle
my $size = -s $fh;                   # get the file size
my $records=$size/$record_length;    # workout the number of records
seek $fh, rand($records)*$record_length, 0;# move to a random record
my $randomline = <$fh>;              # read that record
close $fh;                           # Close handle

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Technically ypu should lock the file so it does not change beween getting its length and getting the random record but generally that is not a massive issue. Hope this helps

UnderMine

In reply to Probablilty of reading a random line from a file by UnderMine
in thread reading a random line from a file by Anonymous Monk

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