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iphonecore-datanspredicate

Use NSPredicate to add up all values

I have a program which uses Core Data. I was kinda cheating when adding the numeric values from each entity in a loop. I read about using NSPredicate to filter the data, but I don't know how to manipulate the data or how the results are even stored. Thanks.

Solution

  • You can do it in two steps, if that fits your requirements.

    1. First filter your data with your NSPredicate and keep it all in an NSArray.
    2. Then use a compound operator with Key-Value Coding to get the sum.

    Below is an example of how it can be done. To make it self-contained, a hard-coded array is used instead of Core Data:

    // In reality this array would be the result of a Core Data query:
NSArray *numbers = @[@{@"number":@3},
                     @{@"number":@2}];

NSNumber *sum = [numbers valueForKeyPath:@"@sum.number"];

    The trick here is the @sum compound operator. You can read about it (and another couple of similar operators) here.