What's the best technique for handling US Dollar calculations in Perl?

Question

What's the best technique for handling US Dollar calculations in Perl?

Especially: the following needs to work:

$balance = 10;
$payment = $balance / 3; # Each payment should be 3.33. How best to round amount?
$balance -= $payment * 3;
# assert: $balance == .01

Answer 1

One common technique is to do all calculations in integer cents, then convert to dollars and cents for output. So your $10 balance would be represented by 1000 (cents), and dividing by three gives 333, or $3.33.

However, if you want to divide a $10 payment by three, you will need some way to end up with payments of $3.33, $3.33, and $3.34. This will be more up to your application logic and business rules than the arithmetic features of your language.

Answer 2

See Math::Currency.

Updated:

Assuming all payments adding up to the balance is desirable, I came up with the following script based on the points made by Greg Hewgill:

#!/usr/bin/perl

use strict;
use warnings;

use List::Util qw( sum );

my @balances = (10, 1, .50, 5, 7, 12, 3, 2, 8, 1012);

for my $balance (@balances) {
    my @stream = get_payment_stream($balance, 3);
    my $sum = sum @stream;
    print "$balance : @stream : $sum\n";
}

sub get_payment_stream {
    my ($balance, $installments) = @_;
    $balance *= 100;
    my $payment = int($balance / $installments);
    $installments -= 1;
    my $residual = $balance - int($payment * $installments);
    my @stream = (($payment) x $installments, $residual);
    return map { sprintf '%.2f', $_ / 100} @stream;
}

Output:

C:\Temp> p
10 : 3.33 3.33 3.34 : 10
1 : 0.33 0.33 0.34 : 1
0.5 : 0.16 0.16 0.18 : 0.5
5 : 1.66 1.66 1.68 : 5
7 : 2.33 2.33 2.34 : 7
12 : 4.00 4.00 4.00 : 12
3 : 1.00 1.00 1.00 : 3
2 : 0.66 0.66 0.68 : 2
8 : 2.66 2.66 2.68 : 8
1012 : 337.33 337.33 337.34 : 1012

Answer 3

use Math::Currency;

Not reinventing the wheel is a good thing :)