For some reason, certain Doubles in my Swift app are giving me trouble when converting to NSNumber, while some are not. My app needs to convert doubles with 2 decimal places (prices) to NSNumbers so they can be stored and retrieved using Core Data. For example, a few particular prices such as 79.99 would evaluate to 99.98999999999999 unless specifically formatted using NSNumber's doubleValue method.
Here selectedWarranty.price = 79.99 as shown in debugger
// item.price: NSNumber?
// selectedWarranty.price: Double?
item.price = NSNumber(double: selectedWarranty.price!)
I programmed some print statements to show how the conversion works out
Original double: 79.99
Converted to NSNumber: 79.98999999999999
.doubleValue Representation: 79.99
Can somebody explain if there is a reason why the initializer cannot surely keep 2 decimal places for every number? I would really like to store the prices in Core Data like they should be. Formatting every time it is displayed doesn't sound very convenient.
UPDATE: Converted Core Data object to type NSDecimalNumber through data model, 79.99 and 99.99 no longer a problem, but now more manageable issue with different numbers...
Original double: 39.99
Converted to NSDecimalNumber: 39.99000000000001024
Firstly, you're confusing some terms. 79.98999999999999
is higher precision than 79.99
(it has a longer decimal expansion), but lower accuracy (it deviates from the true value).
Secondly, NSNumber does not store neither 79.99
nor 79.98999999999999
. It stores the magnitude of the value according to the IEEE 754
standard. What you're seeing is likely the consequence of the printing logic that's applied to convert that magnitude into a human readable number. In any case, you should not be relying on Float
or Double
to store values with a fixed precision. By their very nature, they sacrifice precision in order to gain a longer range of representable values.
You would be much better off representing prices as an Int
of cents, or as an NSDecimalNumber
.
Please refer to Why not use Double or Float to represent currency?