Flat nested array recursively in Swift

Following task - make flat array from nested array. Because there are could be any levels of nested array we use recursive approach.

For example - given array [1, [2, [3, 4, 5]]]. We need to get this [1, 2, 3, 4, 5]

Standard Approach

Algorithm

1. In for loop go through the array
2. If element of array is int then add it to new array
3. If element of array is array too then make recursive invoke of function and send this array

We use Any type as type of input array because input array can hold as integers as array. They both are value types, so Any is suitable for us in this function.

Let's get coding.

import UIKit

let array: [Any] = [1, 2, [3]]

func makeFlatArray(_ array: [Any]) -> [Int] {
    var flatArray = [Int]()
    for item in array {
        if let item = item as? Int {
            flatArray.append(item)
        } else if let item = item as? [Any] {
            let result = makeFlatArray(item)
            flatArray += result
        }
    }
    return flatArray
}

print(makeFlatArray([1, 2, 3]))
// [1, 2, 3]
print(makeFlatArray([1, [2, 3, 4]]))
// [1, 2, 3, 4]
print(makeFlatArray([1, [2, [3, 4]]]))
// [1, 2, 3, 4]
print(makeFlatArray([[1], [2, [3, 4, [5]]]]))
// [1, 2, 3, 4, 5]

Using Generics

What if we want to use this function with array of any type. We should use Generics:

func makeFlatArrayGeneric<T>(_ array: [Any]) -> [T] {
    var flatArray = [T]()
    for item in array {
        if let item = item as? T {
            flatArray.append(item)
        } else if let item = item as? [Any] {
            let result: [T] = makeFlatArrayGeneric(item)
            flatArray += result
        }
    }
    return flatArray
}

Now we can use this function with any kind of array. With integer array as previous

let array: [Any] = [1, 2, [3], [4, [5]]]
let items: [Int] = makeFlatArrayGeneric(array)
// [1, 2, 3, 4, 5]

And with array of strings

let array: [Any] = ["A", "BB", ["CCC"], ["DD", ["EE"]]]
let items: [String] = makeFlatArrayGeneric(array)
// ["A", "BB", "CCC", "DD", "EE"]

Make Extension for Array 

What if we want this feature for any array as built in method. Let's create extension for Array Type. Here we use Swift method flatMap.

extension Array {
    
    func makeFlat() -> [Element] {
        let flatArray = self.flatMap { (element) -> [Element] in
            if let array = element as? Array {
                return array.makeFlat()
            }
            return [element]
        }
        return flatArray
    }
}

Result of using array extension.

let array: [Any] = ["A", "BB", ["CCC"], ["DD", ["EE"]]]
print(array.makeFlat())
// ["A", "BB", "CCC", "DD", "EE"]