Should I use NSDecimalNumber to deal with money?

by

Marcus Zarra has a pretty clear stance on this: “If you are dealing with currency at all, then you should be using NSDecimalNumber.” His article inspired me to look into NSDecimalNumber, and I’ve been very impressed with it. IEEE floating point errors when dealing with base-10 math have been irritating me for a while (1 * (0.5 – 0.4 – 0.1) = -0.00000000000000002776) and NSDecimalNumber does away with them.

NSDecimalNumber doesn’t just add another few digits of binary floating point precision, it actually does base-10 math. This gets rid of the errors like the one shown in the example above.

Now, I’m writing a symbolic math application, so my desire for 30+ decimal digit precision and no weird floating point errors might be an exception, but I think it’s worth looking at. The operations are a little more awkward than simple var = 1 + 2 style math, but they’re still manageable. If you’re worried about allocating all sorts of instances during your math operations, NSDecimal is the C struct equivalent of NSDecimalNumber and there are C functions for doing the exact same math operations with it. In my experience, these are plenty fast for all but the most demanding applications (3,344,593 additions/s, 254,017 divisions/s on a MacBook Air, 281,555 additions/s, 12,027 divisions/s on an iPhone).

As an added bonus, NSDecimalNumber’s descriptionWithLocale: method provides a string with a localized version of the number, including the correct decimal separator. The same goes in reverse for its initWithString:locale: method.

More Related Contents:

  1. How to get pixel data from a UIImage (Cocoa Touch) or CGImage (Core Graphics)?
  2. Cocoa: What’s the difference between the frame and the bounds?
  3. “Unknown class in Interface Builder file” error at runtime
  4. Formatting NSDate into particular styles for both year, month, day, and hour, minute, seconds
  5. How do copy and mutableCopy apply to NSArray and NSMutableArray?
  6. Strip Non-Alphanumeric Characters from an NSString
  7. How do I copy or move an NSManagedObject from one context to another?
  8. Cocoa Autolayout: content hugging vs content compression resistance priority
  9. Getting a list of files in a directory with a glob
  10. Rounding numbers in Objective-C
  11. `[super viewDidLoad]` convention
  12. Managing multiple asynchronous NSURLConnection connections
  13. Where to place the “Core Data Stack” in a Cocoa/Cocoa Touch application
  14. Fix warning “Capturing [an object] strongly in this block is likely to lead to a retain cycle” in ARC-enabled code
  15. Is there a proper way to handle overlapping NSView siblings?
  16. Quick Explanation of SUBQUERY in NSPredicate Expression
  17. If “a == b” is false when comparing two NSString objects
  18. Best way to implement Enums with Core Data
  19. Remove Sandboxing
  20. What is the best Core Image filter to produce black and white effects?
  21. swift: issue in converting string to double
  22. How can I duplicate, or copy a Core Data Managed Object?
  23. Correcting floating point numbers
  24. NSPrivateQueueConcurrencyType Not saving properly
  25. Get video NSData from ALAsset url iOS
  26. How to get a list of all open NSWindow from all running application?
  27. Key-Value-Observing a to-many relationship in Cocoa
  28. Looping using NSRange
  29. NSDate between two given NSDates
  30. Ignoring certificate errors with NSURLConnection

Leave a Comment