Calculating a simple sum with Core Data

Question

I have a simple Core Data store with an entity Cost with an integer property "value". I want to sum up all amounts of the costs in my store, which is equivalent to the following sql statement:

SELECT sum(value) FROM costs

How do I do it the most efficient way in Cocoa Touch? By using Core Data? Or just get all cost entities and do the summation manually?

Answer 1

There are some special key value coding operators that work on arrays and sets, and one of them is @sum. If you fetch all the objects that you want to sum into a set called costs, and if the attribute that you want to sum for each object is value, you can then use the @sum operator like this:

float theSum = [costs valueForKeyPath:@"@sum.value"];

Answer 2

The best way is to use a fetch for specific values and supply a NSExpressionDescription with a sum: function.

When you execute the fetch you get a one element array containing a dictionary whose keys match the expression descriptions and whose values are the results of the expressions. In this case, you would get a sum key whose value would be the sum of the attributes given the expression.