Replicating Array.reduce() method - swift

Question

I am trying to replicate the Array.reduce() method in my custom class and realised that it uses Result as type. Just could not understand that is the Result type been created as Enum or is it something else.
import Foundation
public class MyArray {
private var arr: [Int] = []
internal static var instance: MyArray?
private init() {}
public static func getInstance() -> MyArray {
if self.instance == nil {
self.instance = MyArray()
}
return self.instance!
}
public func insert(value val: Int) {
arr.append(val)
}
/*************** Custom reduce like function ***************/
public func perform(_ initialResult: Int, _ nextPartialResult: (Int, Int) -> Int) -> Int {
var result = initialResult
for element in arr {
result = nextPartialResult(result, element) // calling the closure
}
return result
}
}
Now accessing the MyArray class from outside
var arr1 = MyArray.getInstance()
arr1.insert(value: 1)
arr1.insert(value: 2)
arr1.insert(value: 4)
arr1.insert(value: 3)
arr1.insert(value: 2)
arr1.insert(value: 5)
arr1.insert(value: 2)
arr1.insert(value: 2)
// :Complex calculations left for user to implement
var result = arr1.perform(0) {
return $0 + ( $1 * $1)
}
print("Complex calculation in elements of MEMBER array of arr1: \(result)")
// :Just another way of writing the above closure
result = arr1.perform(0) { (result, num1) -> Int in
return result + ( num1 * num1)
}
print("Complex calculation in elements of MEMBER array of hello arr1: \(result)")
// :Simple calculations
print("Factorial of elements in MEMBER array of arr1: \(arr1.perform(1, *))")
print("Sum of elements in MEMBER array of arr1: \(arr1.perform(0, +))")
The problem is that I have to define my perform() function with one particular type at a time ( Int or String or Double etc ). I am trying to create my function to work with any type just like the reduce() function.
I am not able to understand how to define the Result type in my class and then use it in my function !!
I understand that Result type is not a part of standard library in swift.

Answer 1

The standard reduce function makes use of generics. See the Generics chapter in the Swift book.
func reduce<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Element) throws -> Result) rethrows -> Result
It has two generic types: Result and Element. Element comes from the type of the values in the collection and Result comes from the result type of the reduced value.
So your first step is to use the identical signature in your own perform function.
But in doing so you will discover that you now need to make your MyArray class also based on a generic instead of being hardcoded to work only with Int.
And in attempting to do that you will discover that you can't define MyArray to be generic and support the singleton pattern at the same time. So you need to remove instance and getIntance().
The end result becomes:
public class MyArray<Element> {
private var arr: [Element] = []
public init() {}
public func insert(value val: Element) {
arr.append(val)
}
/*************** Custom reduce like function ***************/
public func perform<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Element) -> Result) -> Result {
var result = initialResult
for element in arr {
result = nextPartialResult(result, element)
}
return result
}
}
With this in place, your first example becomes:
var arr1 = MyArray<Int>()
arr1.insert(value: 1)
arr1.insert(value: 2)
arr1.insert(value: 4)
arr1.insert(value: 3)
arr1.insert(value: 2)
arr1.insert(value: 5)
arr1.insert(value: 2)
arr1.insert(value: 2)
// :Complex calculations left for user to implement
var result = arr1.perform(0) {
return $0 + ( $1 * $1)
}
print(result)
And this outputs the desired result of 67.
In the end, it works but if you'll notice, there's no point to this MyArray class (other than a learning exercise). Just use Array.