Swift - array of strings to multiple subarrays with constant number of strings

Question

Let's say I have this array of strings:

let Vehicles = ["Aeroplane", "Bicycle", "CarVehicle", "Lorry", "Motorbike", "Scooter", "Ship", "Train"]

What I want is this result:

let resultArray = [["Aeroplane", "Bicycle", "CarVehicle", "Lorry"], ["Motorbike", "Scooter", "Ship", "Train"]]

I know I could do this by for but I want to use Higher Order functions in Swift. I mean functions like map, reduce, filter. I think it's possible to do this way and it could be better. Can anyone help me with this? Thanks

Answer 1

A possible solution with map() and stride() :

let vehicles = ["Aeroplane", "Bicycle", "CarVehicle", "Lorry", "Motorbike", "Scooter", "Ship", "Train"]
let each = 4

let resultArray = map(stride(from: 0, to: vehicles.count, by: each)) {
    vehicles[$0 ..< advance($0, each, vehicles.count)]
}

println(resultArray)
// [[Aeroplane, Bicycle, CarVehicle, Lorry], [Motorbike, Scooter, Ship, Train]]

The usage of advance() in the closure guarantees that the code works even if the number of array elements is not a multiple of 4 (and the last subarray in the result will then be shorter.)

You can simplify it to

let resultArray = map(stride(from: 0, to: vehicles.count, by: each)) {
    vehicles[$0 ..< $0 + each]
}

if you know that the number of array elements is a multiple of 4.

Strictly speaking the elements of resultArray are not arrays but array slices. In many cases that does not matter, otherwise you can replace it by

let resultArray = map(stride(from: 0, to: vehicles.count, by: each)) {
    Array(vehicles[$0 ..< advance($0, each, vehicles.count)])
}