Question

This is the end part of a script that takes 4 days information,
averages the previous three days, then subtracts todays value from the average
to get the variance.

The first example is correct.
However the second example if you subtract 0.00299268 from 0.002997575 equals -0.000004895. However List::Util is listing it as -4.89499999999955e-06.

I need to get it in regular notation.

use List::Util qw/sum/;
$todays_latency = $ecp_average[0];
$sum = sum $ecp_average[1] + $ecp_average[2] + $ecp_average[3]  + $ecp_average[4];

$average = $sum/$#ecp_average;
$variance = $todays_latency - $average ;

print "Todays listing is  $todays_latency\n";
print "The Average is $average\n";
print "Todays Variance from the average is $variance\n";
print "\n";

foreach(@ecp_average){

    print "$_\n";
}

print "\n";
 @ecp_average = ();
}

Output

Eislnd1
Todays listing is  0.00376258
The Average is 0.004412365
Todays Variance from the average is -0.000649785
0.00376258
0.00371207
0.00511266

Eislnd2
Todays listing is  0.00299268
The Average is 0.002997575
Todays Variance from the average is -4.89499999999955e-06
0.00299268
0.00301986
0.00297529

Answer 1

This is a rounding numerical representation issue.

From perldoc perlfaq4 : Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? :

---

For the long explanation, see David Goldberg’s “What Every Computer
Scientist Should Know About Floating-Point Arithmetic”
(http://web.cse.msu.edu/~cse320/Documents/FloatingPoint.pdf).

Internally, your computer represents floating-point numbers in binary.
Digital (as in powers of two) computers cannot store all numbers
exactly. Some real numbers lose precision in the process. This is a
problem with how computers store numbers and affects all computer
languages, not just Perl.

perlnumber shows the gory details of number
representations and conversions. To limit the number of decimal places
in your numbers, you can use the printf or sprintf function.

See Floating Point Arithmetic in perlop for more details.

printf "%.2f", 10/3;
my $number = sprintf "%.2f", 10/3;

---

In other words, if this is an issue, round it using sprintf:

$variance = sprintf '%.9f', $todays_latency - $average ;  # Rounded to 9 d.p.