I want to learn more about functional programming. I decided to start with an "easy" example: getting the Cartesian product of two arrays, as in this code:
let xArray = [1, 2, 3]
let yArray = [4, 5, 6]
var pairs: [(Int, Int)] = []
for x in xArray {
for y in yArray {
pairs.append((x, y))
}
}Now, many hours later, I still haven't figured out how to do this in a functional way. Perhaps I should stick to declarative programming.
I am not sure to understand what you are really looking for.
Are you looking for a notation "a la map()" as
(array1, array2).cartesian
Are you looking for a recursive construction of cartesian ?
Do you know zip function ?
let xArray = [1, 2, 3]
let yArray = [4, 5, 6]
let zArray = zip(xArray, yArray)
for z in zArray { print(z) }yields
(1, 4)
(2, 5)
(3, 6)
It will even work if not same soze arrays.
With
let xArray = [1, 2, 3]
let yArray = [4, 5, 6, 7]
let zArray = zip(xArray, yArray)
for z in zArray { print(z) }you get
(1, 4)
(2, 5)
(3, 6)
I want to get the array [(1, 4), (1, 5), (1, 6), (2, 4), (2, 5), (2, 6), (3, 4), (3, 5), (3, 6)].
You could also define an operator for cartesian product:
infix operator ⊗
extension Array where Element : Any {
static func ⊗(left:[Element], right:[Element]) -> [(Element, Element)] {
var pairs: [(Element, Element)] = []
for x in left {
for y in right {
pairs.append((x, y))
}
}
return pairs
}
}
let cartArray = xArray ⊗ yArray
print(cartArray)gives
[(1, 4), (1, 5), (1, 6), (2, 4), (2, 5), (2, 6), (3, 4), (3, 5), (3, 6)]
It will work for any values:
let x2Array = [1, 2, 3]
let y2Array = ["A", "B", 6] as [Any]
let cartArray = x2Array ⊗ y2Array
print(cartArray)[(1, "A"), (1, "B"), (1, 6), (2, "A"), (2, "B"), (2, 6), (3, "A"), (3, "B"), (3, 6)]
Note:
That can probably be written with map(), but it rapidly becomes hard to read…
How is this?
let xArray = [1, 2, 3]
let yArray = [4, 5, 6]
let product = xArray.map { xElem in yArray.map {yElem in (xElem, yElem)}}.flatMap{$0}
print(product) //->[(1, 4), (1, 5), (1, 6), (2, 4), (2, 5), (2, 6), (3, 4), (3, 5), (3, 6)]Nice.
Beautiful! I'd gotten the first part of that, but didn't realize that flatMap would combine the three arrays.