Map, Filter, Reduce Quick Reference

MapFilterReduce.md

Swift Guide to Map Filter Reduce

Next time you find yourself looping over a collection check if you could use map, filter or reduce:

1. map returns an Array containing results of applying a transform to each item.
2. filter returns an Array containing only those items that match an include condition.
3. reduce returns a single value calculated by calling a combine closure for each item with an initial value.

map Full:

let squares3 = values.map({
  (value: Double) -> Double in
  return value * value
})

map Quicker:

let squares4 = values.map {value in value * value}

map Quickest:

let squares5 = values.map { $0 * $0 }

---

filter:

let even = digits.filter { $0 % 2 == 0 }

---

reduce Full:

let names = ["alan","brian","charlie"]
let csv = names.reduce("===") {text, name in "\(text),\(name)"}
// "===,alan,brian,charlie"

reduce quick:

let codes = ["abc","def","ghi"]
let text = codes.reduce("", +)
// "abcdefghi"

---

flatMap:

let results = [[5,2,7], [4,8], [9,1,3]]
let allResults = results.flatMap { $0 }
// [5, 2, 7, 4, 8, 9, 1, 3]

Using FlatMap on an optional:

let input: Int? = Int("8")
let passMark: Int? = input.flatMap { $0 > 5 ? $0 : nil }
// 8

Using CompactMap on a sequence with a closure that returns an optional:

Note that this use of flatMap was renamed to compactMap in Swift 4.1 (Xcode 9.3). It provides a convenient way to strip nil values from an array:

let keys: [String?] = ["Tom", nil, "Peter", nil, "Harry"]
let validNames = keys.compactMap { $0 }
validNames
// ["Tom", "Peter", "Harry"]

let counts = keys.compactMap { $0?.count }
counts
// [3, 5, 5]