NSDecimalNumber for finances - ios

Question

I'm building an app that deals with money and I've been using floating point arithmetic up until now, but I've learned that it's better to use NSDecimalNumber.
I want to make sure that I've understood it correctly, so here goes:
Imagine some worker, earning 20.57$/hour. This information is provided by the user. I did it like this before:
#property (nonatomic) float hourlyRate;
NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
numberFormatter.numberStyle = NSNumberFormatterDecimalStyle;
NSNumber *hourlyRate = [numberFormatter numberFromString:self.rateTextField.text];
settingsObject.hourlyRate = [hourlyRate floatValue];
But I've now changed it to:
#property (nonatomic) NSDecimalNumber *hourlyRate;
settingsObject.hourlyRate = (NSDecimalNumber *)[NSDecimalNumber decimalNumberWithString:self.rateTextField.text locale:[NSLocale currentLocale]];
Is this the correct way to read NSDecimalNumbers from string?
Say this person enters a workplace at 10:01. I save that information like so:
[[NSUserDefaults standardUserDefaults] setObject:[NSDate date] forKey:#"start"];
Once the person is finished, the start time is read from NSUserDefaults like so:
NSDate *start = [[NSUserDefaults standardUserDefaults] objectForKey:#"start"];
The duration is calculated like so:
NSTimeInterval interval = [[NSDate date] timeIntervalSinceDate:start] / 3600;
NSDecimalNumber *earned = [settingsObject.hourlyRate decimalNumberByMultiplyingBy:[[NSDecimalNumber alloc] initWithFloat:interval]];
is this the correct and most efficient way, while keeping precision?
Thanks!

Answer 1

Is this the correct way to read NSDecimalNumbers from string?
Yes. However, you shouldn't need to cast the result. So it should just look like this:
settingsObject.hourlyRate = [NSDecimalNumber decimalNumberWithString:self.rateTextField.text locale:[NSLocale currentLocale]];
And the NSLocale part isn't necessary, unless you're using a non-current locale:
settingsObject.hourlyRate = [NSDecimalNumber decimalNumberWithString:self.rateTextField.text];
I don't think this function likes nil values, so you would want to make sure that rateTextField has a non-nil value first:
if ([self.rateTextField hasText]) {
settingsObject.hourlyRate = [NSDecimalNumber decimalNumberWithString:self.rateTextField.text];
}
else {
settingsObject.hourlyRate = [NSDecimalNumber zero];
}
is this the correct and most efficient way, while keeping precision?
Yes, your conversion of the interval to an NSDecimal number and then the multiplication looks good to me.

Answer 2

First you need an answer for this question:
How accurate do you want to be - or do you need to be?
Will your app generate invoices, or will it serve as the basis for invoices? If yes, you may have to stay away from floating point numbers. You may also have to think about how duration is actually used in calculations. There may be regulations for certain business areas or professions, which define how to measure / invoice time.
Not really accurate:
If you do not need to be really accurate, I would continue using floating numbers until you hit obstacles.
Accurate:
1. Money:
Store money as integers as cents.
2. Time:
Determine time slots and store them as integers (maybe minutes).
3. Calculations:
Calculate using integers. Never use floats.
Details:
When you have an input like 25.07 (I used a different number than your 20.57 to explain cents rendering) store it as 2507. Do your calculations in cents. For displaying convert to $ or $/hrs. For example:
int amount = 2507;
int amountDollars = amount / 100;
int amountCents = amount % 100;
NSString *output = [NSString stringWithFormat:#"%d.%02d $/hrs", amountDollars, amountCents];
NSLog(#"output = %#", output);
// prints:
// output = 25.07 $/hrs
For time chose a suitable base unit (minute / second / five minutes / ten minutes / half an hour?) and store that as an integer.
Lets assume you invoice in units of 10 minutes. You probably then bill the next 10 minutes, when they are begun. So if 11 minutes are in the record, you have to calculate 20 minutes. Similar issues exist for minutes - assume these values:
Actual Time | "Integer Time"
10:09:59 | 10:09
10:10:01 | 10:10
Difference:
00:00:02 | 00:01
If you just use actual time using floating points the first difference amounts to almost nothing - the second amounts to one minute. If your times are shown in the report as 10:09 and 10:10, people will wonder, why in the first case the amount is almost nothing.
Doing financial and time calculations is hard if you need to be really accurate. So it boils down to this: First find out, how accurate you need to be. (Note that "accurate" may have weird meanings and implications when entering the realm of regulations, laws etc.)