Question

Two quick questions if I may, is this how I should go about taking two NSNumber objects, performing a calculation and ending up with a result that is also an NSNumber?

NSNumber *flux = [[NSNumber alloc] initWithDouble:100.0];
NSNumber *mass = [[NSNumber alloc] initWithDouble:3];

double intermediate = [flux doubleValue] / [mass doubleValue];
NSLog(@"INTER : %.20f", intermediate);

NSNumber *result = [[NSNumber alloc] initWithDouble:intermediate];
NSLog(@"RESULT: %@", result);

...
...

[flux release];
[mass release];
[result release];

Also looking at the results in console from NSLog, is there any loss of precision? I would assume there is not and what I am seeing is just display precision, but just curious?

INTER : 33.33333333333333570181
RESULT: 33.33333333333334

gary

Answer 1

(Tangential to your question but related)

NSNumber isn’t intended to do base-10 math with. It’s largely there to wrap and store numerical values. If you need to do real math, you want to use a NSDecimal.

NSDecimalNumber, an immutable subclass
of NSNumber, provides an
object-oriented wrapper for doing
base-10 arithmetic. An instance can
represent any number that can be
expressed as mantissa x 10^exponent
where mantissa is a decimal integer up
to 38 digits long, and exponent is an
integer from –128 through 127

Despite the fact that we call them “computers” our logic engines can’t do actual math so they have to fake it. When you get to the extremes of very large or very small magnitude numbers, that faking begins to show. That is why you need custom numerical classes that can hold more information than just a string of digits.

So, if you have any concerns about precision, use NSDecimal instead of NSNumber. NSDecimal is designed to perform precise calculations.

Edit01:

… how I should go about taking two
NSNumber objects, performing a
calculation and ending up with a
result that is also an NSNumber?

Strictly speaking, you should not use NSNumber for calculations. You will notice that NSNumber has no dedicated methods for doing math. You have to convert to scalar and then back again to an object. This causes a loss of precision and the precision can change depending on the hardware or the definitions of the scalars.

NSDecimal by contrast can precisely represent very precise numbers because it holds them abstractly. It has dedicated methods for performing precise mathematical operations.

Also looking at the results in console
from NSLog, is there any loss of
precision?

Yes, there is a loss of mathematical precision beyond just the formatting. Scalars have different precision depending on their type and size of the number they store.. At large magnitudes, this causes problems with precision. If you mix types, say a NSInteger and a NSUInteger, you get the maximal precision of the NSInteger.

You also run into all the old problems of using scalars.

If you ask an NSNumber object for its
value using a type that cannot hold
the value, you get back an erroneous
result—for example, if you ask for the
float value of a number created with a
double that is greater than FLT_MAX,
or the integer value of a number
created with a float that is greater
than the maximum value of NSInteger.

NSDecimal frees you from all these possible sources of error. It does precise mathematical calculations up to magnitudes way beyond what anyone would use in the real world.

I repeat: If precision is a concern, don’t use NSNumber or scalar.