Comments on: Cocoa Tutorial: Don’t Be Lazy With NSDecimalNumber (Like Me)

By: mj1531

mj1531 — Fri, 17 Apr 2009 22:13:27 +0000

Nevermind, I figured it out. The precision refers to the number of digits after the decimal point and the calculation of the mantissa should read:

mantissa = (yourPrimitive * 10^precision)

Thanks for this informative post.

By: mj1531

mj1531 — Fri, 17 Apr 2009 21:14:03 +0000

In converting primitives to the mantissa/exponent format, you mention precision. Is this the same value as the scale in NSDecimalNumberBehaviors, or is it a printf-style float-precision specifier (like 4.3 for 4 places before the decimal point and 3 after)?

By: Marcus Zarra

Marcus Zarra — Mon, 30 Jun 2008 05:53:02 +0000

dpenny,

Yes they are designed for currency use and the proper rounding of decimal points. They solve the issues with floating point math.

By: dpenny

dpenny — Mon, 30 Jun 2008 02:02:51 +0000

I cam across this article just as I was writing a small app the deals with currency as well. It seems that historically wisdom has said to always convert any currency value to a whole integer before doing any calculations with, does NSDecimalNumber alleviate the need for doing this?

By: Marcus Zarra

Marcus Zarra — Mon, 28 Apr 2008 21:18:08 +0000

I would not recommend even doing that. There is a reason that the NSDecimalNumber is configured differently and has an unusual constructor. Ignoring that seems foolish just to save a couple of characters typing.

Definitely sounds like the wrong kind of lazy to me :)

By: jediknil

jediknil — Fri, 25 Apr 2008 22:31:00 +0000

Ah, OK. In that case -initWithDouble: and friends are still valid initializers (with no warnings), and they of course give you NSDecimalNumbers.

On the other hand, for doing base-10 arithmetic, you're probably right not to use -initWithDouble:, since NSDecimalNumber's internal representation uses base-10 exponents, while primitive floats and doubles of course use base-2 exponents, and there could be some (slight) loss of precision.

For integers, though, I'd still prefer [[NSDecimalNumber alloc] initWithInteger:100] to [[NSDecimalNumber alloc] initWithMantissa:1 exponent:2 isNegative:NO].

By: Marcus Zarra

Marcus Zarra — Fri, 25 Apr 2008 14:39:02 +0000

jediknil,

Since NSDecimalNumber *test = [NSDecimalNumber numberWithDouble:9.5]; will produce a warning it is not recommended. When dealing with NSDecimalNumber objects it is much safer to use the recognized initializers and avoid the ones from its NSNumber parent class.

By: jediknil

jediknil — Fri, 25 Apr 2008 04:36:13 +0000

NSDecimalNumber also supports the various +numberWith(Primitive): methods that come with NSNumber. And yes, they correctly return an NSDecimalNumber.

By: Sebastian Ahlman

Sebastian Ahlman — Fri, 25 Apr 2008 04:24:00 +0000

Thanks for the tip!

Great blog BTW. I recently started programming with Cocoa and I have found CIMGF to be a valuable resource. Keep it up and I will keep reading!