What's the best technique for handling US Dollar calculations in Perl?
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13-09-2019 - |
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
Solution
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
OTHER TIPS
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.
use Math::Currency;
Not reinventing the wheel is a good thing :)