Hello, I'm trying to generate a file out of random data. I thought the best way would be to use rand() to get some randomness. But it looks like rand() is ridiculously slow compared to just write the same character into the file.
Let's have a look at the code first: 
use strict;
use IO::Handle;


if ($#ARGV < 1) 
    { 
        die("usage: <size_in_bytes> <file_name>\n"); 
    }
open(my $gen_file_h,">" . $ARGV[0]) or die "Can't open file for writin
+g\n";
$gen_file_h->autoflush(1);

superfast();

close($gen_file_h);
die();    


    
sub superfast
{
            #each time though the loop should be 1 meg
            for (1 .. $ARGV[1]) 
                {
                    #print 1 meg of Zs
                    print {$gen_file_h} "Z" x (1024*1024)
                }    
}

sub generateRandomFile
{
    my $final_size = $ARGV[1];
    for (my $mbytes = 0; $mbytes < $final_size; $mbytes ++) 
        {
            my $string="";
            my @chars = split(" ","a b c d e f g h i j k l m n o p q r
+ s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 
+- _ ! & ? = 0 1 2 3 4 5 6 7 8 9");
            for (1..1048576)
                {
                    my $rand = int(rand(68));
                    $string .= $chars[$rand];
                }
            print $gen_file_h $string;
        }
}

[download]
So the sub superfast() is just writing the character "Z" into the file and this is fast like hell. I could fill my SSD within seconds completely.
The sub generateRandomFile() is really slow. It takes several seconds to create for example a 10MB file.
Do you have any suggestion how I could speed up things?

BTW: The files don't need "secure" randomness, I just need files with a specific size and I'm using ActiveState Perl 5.6.1 635 (and I'm bound to this version) on Windows 7

Thanks in advance,
Stefan

In reply to Is rand() really that slow or am I just using it wrong? by adiuva

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