Perl rounding error again

Question

The example to show the problem:

• having a number 105;
• divide with 1000 (result 0.105)
• rouded to 2 decimal places should be: 0.11

Answer 1

In the old Integer math days of programming, we use to pretend to use decimal places:

N = 345
DISPLAY N        # Displays 345
DISPLAY (1.2) N  # Displays 3.45

We learned a valuable trick when attempting to round sales taxes correctly:

my $amount = 1.344;
my $amount_rounded = sprintf "%.2f", $amount + .005;
my $amount2 = 1.345;
my $amount_rounded2 = sprintf "%.2f", $amount2 + .005;
say "$amount_rounted   $amount_rounded2";  # prints 1.34 and 1.35

By adding in 1/2 of the precision, I display the rounding correctly. When the number is 1.344, adding .005 made it 1.349, and chopping off the last digit displays dip lays 1.344. When I do the same thing with 1.345, adding in .005 makes it 1.350 and removing the last digit displays it as 1.35.

You could do this with a subroutine that will return the rounded amount.

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Interesting...

There is a PerlFAQ on this subject. It recommends simply using printf to get the correct results:

use strict;
use warnings;
use feature qw(say);

my $number = .105;
say "$number";
printf "%.2f\n", $number;   # Prints .10 which is incorrect
printf "%.2f\n", 3.1459;    # Prins 3.15 which is correct

For Pi, this works, but not for .105. However:

use strict;
use warnings;
use feature qw(say);

my $number = .1051;
say "$number";
printf "%.2f\n", $number;   # Prints .11 which is correct
printf "%.2f\n", 3.1459;    # Prints 3.15 which is correct

This looks like an issue with the way Perl stores .105 internally. Probably something like .10499999999 which would be correctly rounded downwards. I also noticed that Perl warns me about using round and rounding as possible future reserved words.