Quick way to sum a property of all objects within an NSSet?

Question

I thought there was a way to quickly ask a NSSet to poll its members and return a sum of say an NSInteger property in each of its objects, but I may very well be confusing this with the Mac OS X side of things. Does this exist in Cococa Touch?

The closest thing I can find is objectEnumerator, whereby I suppose I could rifle through each object and increment my own variable. Does the better way exist?

Nuncio

Are you talking about NSArrayController arrangedObjects.@sum. ... stuff? I'm not sure how to connect it with NSSet and how to get count of specific class members ... I'll hv to read up on that. – Murrumbidgee

Answer 1

If you're trying to find the sum of a given property (theIntegerPropertyToSum) for each member of an array/set-derived class that's KVC-compliant (theSet), you can do the following:

NSNumber* theSum = [theSet valueForKeyPath:@"@sum.theIntegerPropertyToSum"];

Dissect

This is great! Where, btw, is it documented? I can't seem to find anything in/around the definition of "valueForKeyPath:" in the KVC Protocol Reference... – Nuncio

they are called key value coding collection operators and the document is hard to find... developer.apple.com/library/ios/#documentation/cocoa/conceptual/… – Kast

This code is not maintenance friendly and will confuse people down the line. – Olsen

This is a cool feature of Objective C. One cannot do this in C++. – Woolgrower

Answer 2

Why not use plain objective C (2)?

NSInteger theSum = 0;
for (id obj in theSetOrArray)
    theSum += obj.theIntegerPropertyToSum;

Disadvantage: If you like to count lines, then this looks longer. Are there other disadvantages - I wonder what happens with the KVC method with objects that don't have the required 'theIntegerPropertyToSum' property?

Advantage: I would bet that this debugs and performance tests easier. Plus when someone else reads your code in a year or two they will know what is going on here - whether they have ever seen a line of objective C or not, this looks like what is actually happening.

Olsen

Your answer implies that you've not used or read the documentation on KVC -- you might want to read about it before saying it shouldn't be used. In your "plain objective C (2)" example, what happens when theIntegerPropertyToSum is undefined, or one value of theIntegerPropertyToSum is -1 and another is 1.2? What if it's nil? – Dissect

I use KVC all the time - to let the UI communicate with the code. The disadvantage of [theSet valueForKeyPath:@"@sum.theIntegerPropertyToSum"] is that is a language construct that does not show at a glance what is going on. Any javascript C, C# programmer seeing the valueForKeyPath statement will not understand it - and be sucked into a time consuming lesson in order to figure out what one line of code does. Never use language specific constructs unless you need them. Reminds me of STL C++ code, which is unreadable to all but STL experts. – Olsen

Sounds like you use KVO all the time, not KVC. – Dissect

"Never use language specific constructs unless you need them."?! That's the dumbest thing I've heard all week. Sure, why bother learning anything? (By the way, a C programmer wouldn't know what your fast enumeration for loop there is, either.) (Also, KVC is a framework feature, not language -- notice the argument is just a plain old string.) – Liard