What does Element mean when writing an extension on Array?

like in this example:

extension Array {
    func reduce<T>(_ initial: T, combine: (T, Element) -> T) -> T {
        var result = initial
        for x in self {
            result = combine(result, x)
        }
        return result
    }
}

CodePudding user response：

The combine parameter is a function which takes a parameter of type T and Element. The Element is the actual Element of/in the array.

For example, this is an array of Int elements:

let arr = [1,2,5,77]

In reduce, initial is of type T. This is the staring value for the mapping you are about to perform. In combine, T is like your starting value for each subsequent step of combing the next Element to produce another value of type T which will be used as the next T in combine, and so and so forth until the entire array has been processed.

If you were using a default use of reduce such as:

arr.reduce(0,  )

You can see that in this case, T and Element would both be of the same type, Int.

However, I could have a custom object that the array is of, and my combine is defining how to get the running total. If you had something like this:

struct Thing {
   var val1: String
   var val2: Int
}

let thingArray = //...define some Things in an array

You could use reduce and define your own combine function to return the sum of all the val2 values. In this case, T would be an Int, and Element would be Thing.