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

See my answer to a very similar question that offers up to 5 different ways to solve your problem. — Imanou Petit
– Imanou Petit, Commented Mar 9, 2017 at 23:58

Martin R · Accepted Answer · 2015-05-13 17:39:44Z

5

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)])
}

edited May 13, 2015 at 17:39

answered May 13, 2015 at 17:22

Martin R

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3 Comments

Libor Zapletal Over a year ago

Looks like what I was looking for. I forget mention that number of elements don't need to be multiple of 4 so thanks for first solution. One more thing. I have problem that I have result as ArraySlice<String> not [String]. How can I fix it?

Eendje Over a year ago

@MartinR Would it also be possible to do it with reduce and map together?

Martin R Over a year ago

@JacobsonTalom: It would be possible with reduce(), but then a "nested array" would be created in each call of the reduce closure, so that is probably less effective. Of course there might be another solution that I did not think of.

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