Core Data Storing NSDecimalNumber with Precision - ios

Question

I'm receiving and parsing JSON and storing the data into Core Data. Some of the data is currency being stored as NSDecimalNumber, but some of these values have a higher precision than two decimal places.
For instance, if I get a value from the service such as 8.2399999995 I would like to store this in Core Data as 8.24. Is there any way to set up my model to a two decimal place precision? Or do I need to manually round each value after it's stored?
UPDATE
Leijonien thanks for the information. I tried to doing that and I'm having some trouble saving the formatted value. I checked the JSON and searched Google and it turns out I'm getting a clean value from the service. RESTKit is the problem....https://github.com/RestKit/RestKit/issues/1405.
However, I've created a category on one of my NSManagedObject classes, overridden the setter for the attribute I want, formatted the value, but I still see the long decimal value in my db. Here's my code.
- (void)setAmount:(NSDecimalNumber *)amount {
NSDecimalNumberHandler *round = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundPlain
scale:2
raiseOnExactness:NO
raiseOnOverflow:NO
raiseOnUnderflow:NO
raiseOnDivideByZero:YES];
NSDecimalNumber *newAmount = [amount decimalNumberByRoundingAccordingToBehavior:round];
NSLog(#"%#", newAmount);
[self setPrimitiveValue:newAmount forKey:#"amount"];
}
What's weird is that when newAmount prints to the console it's in the format 8.24 like I want, but when I check the db it's saved as 8.2399999995. Am I doing something wrong here?

Answer 1

CoreData will just store the value you pass, so if you need only 2 digits, you should round the value yourself. Probably better to round the value before it's stored, so you only have to do it once per result.

Answer 2

As it turns out, the problem is not RESTKit. In fact, the problem appears to be Core Data. This is why printing to the console in my setter method printed the correctly formatted number, but the number has the wrong precision in the db. The best remedy for this situation was to override the getter method and format the number there, after it has been pulled from Core Data. The following seems to work...let me know if you've found anything else that works.
- (NSDecimalNumber*)amount {
[self willAccessValueForKey:#"amount"];
NSDecimalNumber* unroundedAmount = [self primitiveValueForKey:#"amount"];
[self didAccessValueForKey:#"amount"];
NSDecimalNumberHandler *round = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundPlain
scale:2
raiseOnExactness:NO
raiseOnOverflow:NO
raiseOnUnderflow:NO
raiseOnDivideByZero:YES];
NSDecimalNumber *roundedAmount = [unroundedAmount decimalNumberByRoundingAccordingToBehavior:round];
return roundedAmount;
}

Answer 3

One thing to watch out for is that some (most?) decimal numbers can't be stored in floating-point binary in exactly the same way we think of them. There's alot to read on it out there, but I like the detailed approach of this one: http://www.cprogramming.com/tutorial/floating_point/understanding_floating_point_representation.html.
So for your number, the .24 can't be represented in floating-point because the binary can only give that the decimal component of a number is made up of .5, .25, .125, ... (that might be missing some special rules, but it's the general idea). So to represent .24 it uses as many of the lower values to get as close as it can.
NSDecimalNumber was set up to round it, but if you don't tell it to round anything you'll see the same 8.23999... value there too.