Reduce Examples

Let's start with a favorite of mine, the sum of a list of numbers:

[0, 1, 2, 3, 4].reduce(0, +)

// 10

+ is a valid combinator function as it will just add the lhs and the rhs and return the result, which is specifically the requirement reduce has.

Another example is building the product of a list of numbers:

[1, 2, 3, 4].reduce(1, *)

// 24

Or what about reversing a list:

[1, 2, 3, 4, 5].reduce([Int](), { [$1] + $0 })

// 5, 4, 3, 2, 1

Finally, something a tad more complicated. We'd like to partition a list based on a division criteria

typealias Acc = (l: [Int], r: [Int])

func partition(_ lst: [Int], criteria: (Int) -> Bool) -> Acc {

    return lst.reduce((l: [Int](), r: [Int]()), { (ac: Acc, o: Int) -> Acc in 

       if criteria(o) {

         return (l: ac.l + [o], r: ac.r)

       } else {

         return (r: ac.r + [o], l: ac.l)

       }

    })

}

partition([1, 2, 3, 4, 5, 6, 7, 8, 9], criteria: { $0 % 2 == 0 })

//: ([2, 4, 6, 8], [1, 3, 5, 7, 9])

The most interesting thing we're doing above is using a tuple as the accumulator. As you'll find out, once you start trying to incorporate reduce into your daily work-flow, tuples are a great way of quickly combining related data during a reduce operation.