Integer和NSDecimalNumber的基本数学问题 - 二进制的操作数无效

Question

Man the simplest stuff always seems so crazy in Objective-C to me, anyways I need to do some basic subtraction and multiplication and am stumped.

I have:

client.PricingDiscount <-- this is an Integer 16 property on a CoreData NSManagedObject
sku.RetailPrice <-- this is a Decimal property on a CoreData NSManagedObject

I am simply trying to get a NSDecimalNumber to display like so:

NSDecimalNumber *showPrice = sku.RetailPrice * (100 - client.PricingDiscount);

I have tried a bunch of different forms of this and cannot figure out what the hell I am doing wrong here.

Answer 1

Standard operators cannot be used with NSDecimalNumber

NSDecimalNumber *one = [NSDecimalNumber decimalNumberWithDecimal:[[NSNumber numberWithInt:100] decimalValue]]; 
NSDecimalNumber *factor = [one decimalNumberBySubtracting:[NSDecimalNumber decimalNumberWithDecimal:[client.PricingDiscount decimalValue]]];
NSDecimalNumber *showPrice = [sku.RetailPrice decimalNumerByMultiplying:factor];

Answer 2

NSDecimalNumber is an object wrapper for a number--you are treating it like a C value. Instead, try:

float price = [sku.retailPrice floatValue] * (100 - [client.pricingDiscount floatValue]);
NSDecimalNumber *showPrice = [NSDecimalNumber numberWithFloat:price];

It's confusing, but NSNumber and NSDecimalNumber are Objective-C wrappers for C types, typically used for storage in container objects such as NSArray (or Core Data). NSInteger and NSDecimal , on the other hand, are C types ( NSInteger just maps to int ).

EDIT: falconcreek's answer is better for avoiding accuracy loss. When working with integers, though, you'll typically use unboxed C types.

Answer 3

NSDecimalNumber *showPrice = sku.RetailPrice * (100 - client.PricingDiscount);

NSDecimalNumber is a class. What you are doing here is calculating a value, and then assigning that to be the value of a pointer to an NSDecimalNumber object. This is bad mojo.

I haven't used NSDecimalNumber before, but if you need that particular object, what you want is something like this:

NSNumber* number = [NSNumber numberWithFloat:(sku.RetailPrice * (100 - client.PricingDiscount))];
NSDecimal* decimal = [number decimalValue];
NSDecimalNumber* showPrice = [NSDecimalNumber decimalNumberWithDecimal:decimal];