Declare an array from Core Data Attribute Swift

Question

I have an Entity named: Transaction and an Attribute named: amount . I would like to declare an array from amount attribute that can using the summing an array sample code

sum = array_name.reduce(0,+)

I am trying to do this way to declare an array from Attribute

var amountArray = 
 (self.transactionsArray as NSArray).value(forKey: "amount") as! NSArray
sum = amountArray.reduce(0,+)

but it's not working and throw the error

Ambiguous reference to member '+'

Answer 1

Using Swift 4, @MartinR's suggestion should work. If it doesn't, you should provide more information about your code, maybe specifically about the details of Transaction , the type of the amount variable, and about the exact nature of transactionsArray .

Assuming that transactionsArray is declared as the non-optional [Transaction] and that amount is one of the Core Data numeric types,

let sum = transactionsArray.reduce(0) { $0 + $1.amount }

...is correct. You might make it slightly more explicit by declaring the type of sum :

let sum : Double = transactionsArray.reduce(0) { $0 + $1.amount }

(Make sure to use the same type as amount here, I'm just guessing Double ).

It's possible to get the sum with an NSArray and the @sum operator, for example

if let sum = (transactionsArray as NSArray).value(forKeyPath:"@sum.amount") as? Double {
    ....
}

The as? Double as? Double is needed because of the lack of type info on NSArray that @vadian mentioned.