I disagree. You can do random access with ASCII so long as you write fixed length records, but it does take about 50% longer to do the conversion. It's the conversion that chews up most of the time, so pack() can be a substantial time saver.

In the test below, writing floats is 10 times faster than writhing ascii. The file is half the size, so that smaller size only accounts for a factor of 2 speed-up. A factor of 5 comes from not doing the ascii conversion. 

use warnings;
use strict;

use Time::HiRes qw( gettimeofday tv_interval );

my $size = 1000;
my $last_idx = $size-1;
my $float_value;


my $t0;
$t0 = [gettimeofday];

open ASC, '>', "asciinum.txt"  or die "can't open ascii file";
for my $row (0..$last_idx) {
   my @result = ();
   my $row_len = $row+1;
   $float_value = $row/10_000 + 1/100_000_000;
   for my $col (0..$last_idx) {
      #$float_value = $row/10_000 + $col/100_000_000;
      push @result, $float_value;
   }
   print ASC join(',', @result);
}
print STDERR "Ascii write took -- ",tv_interval ( $t0 ),"\n";
close ASC;

$t0 = [gettimeofday];

open FLOAT, '>', "floatnum.txt"  or die "can't open float file";
for my $row (0..$last_idx) {
   my @result = ();
   my $row_len = $row+1;
   $float_value = $row/10_000 + 1/100_000_000;
   for my $col (0..$last_idx) {
      #$float_value = $row/10_000 + $col/100_000_000;
      push @result, $float_value;
   }
   print FLOAT pack("F$row_len", @result);
}
print STDERR "Float write took -- ",tv_interval ( $t0 ),"\n";
close FLOAT;

# Gave the following results on my fairly slow machine, with $size = 1
+000
#  Ascii write took -- 15.078125
#  Float write took -- 2.778736

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In reply to Re^2: Issue on covariance calculation by rodion
in thread Issue on covariance calculation by Mandrake

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