Achieving Mutability When Mixing Primitives and Cocoa Collections

Question

Okay, I think I might be over-complicating this issue but I truly am stuck. Basically, I am trying to model a weight set, specifically an olympic weight set. So I have the bar which is 45 lbs, then I have 2 weights of 2.5 lbs, 4 of 5 lbs, and then 2 of 10, 25, 35, and 45 respectively. This makes a total of 300 lbs.

bar = 45 lbs
2 of 2.5
4 of 5
2 of 10
2 of 25
2 of 35
2 of 45

I want to model this weight set so that I have this information: the weight and the quantity of weights I have. I know I could hard-code this but I eventually want to let the user enter how many of each weight they may have.

Anyways, originally I thought I could simply have an NSDictionary with the key being the weight, such as 35, and the value being the quantity.

Naturally I cannot store primitives in an NSDictionary or other Cocoa collection, so I have to encapsulate each integer in an NSNumber. However, the point of my modeling this weight set is so that I can simulate the use of certain weights. For example, if I use a 35 lbs. weight that takes 2 off (one for each side), so I have to go and edit the value for the 35 lbs. weight to reflect the fact that I have deducted 2 from the quantity.

This involves the tedious task of unboxing the NSNumber, converting back to a primitive, doing the math, and then re-boxing into an NSNumber and assigning that new result to the appropriate location in the NSDictionary. After searching around a bit, I confirmed my initial premonition that this was not a good idea.

So I have a couple questions. First of all, is there a better way of modeling a weight set aside from using a dictionary-style solution? If not, what is the suggested way to go about doing this? Do I have to leave the cocoa-realm and resort to using some sort of C++ STL template such as a map?

I have seen some information on NSDecimalNumber, should I just use that?

Like I said, I wouldn't be surprised if I am over-complicating this. I would really appreciate any help, thanks.

EDIT: I am beginning to think that the weight set 'definition' as described should indeed be immutable, as it is a definition after all. Then when I use a certain weight, I can add to some sort of tally. The thing is that the tally will also be some form of collection whose values I will be modifying (adding to), so that I can correlate it to the specific weight. So I guess I am in the same problem.

I think where I am trying to get at is creating a 'clone' so to speak of the weight set definition which I can easily modify (to simulate the usage of individual weights).

Sorry, I'm burned out.

Answer 1

There's nothing wrong with storing your numbers in an NSDictionary! The question you referenced was referring to complicated, frequent math. Converting from NSNumber and back is slow compared to simple int addition, but is still super-fast compared to human perception. I think your dictionary idea is EDIT: not as good as Chuck's NSCountedSet idea. :)

Answer 2

Storing this in a dictionary isn't a natural fit. I think the best approach would be to make a Weight class that represents the weights, and stick them in an NSCountedSet. You can get the individual kinds of Weight and the counts for each kind, and you can get the weight of the whole set with [weightSet valueForKeyPath:@"@sum.weightInPounds"] (assuming the Weights have a weightInPounds property that represents how heavy they are).

You could use NSNumbers in the NSCountedSet and sum them with @sum.integerValue if you wanted, but it seems a bit awkward to me. At any rate, NSCountedSet is definitely a more natural collection than an NSDictionary for storing — well, a counted set.