Correct way to find max in an Array in Swift

Given:

let numbers = [1, 2, 3, 4, 5]

Swift 3:

numbers.min() // equals 1
numbers.max() // equals 5

Swift 2:

numbers.minElement() // equals 1
numbers.maxElement() // equals 5

Update: This should probably be the accepted answer since maxElement appeared in Swift.

---

Use the almighty reduce:

let nums = [1, 6, 3, 9, 4, 6];
let numMax = nums.reduce(Int.min, { max($0, $1) })

Similarly:

let numMin = nums.reduce(Int.max, { min($0, $1) })

reduce takes a first value that is the initial value for an internal accumulator variable, then applies the passed function (here, it's anonymous) to the accumulator and each element of the array successively, and stores the new value in the accumulator. The last accumulator value is then returned.

With Swift 5, Array, like other Sequence Protocol conforming objects (Dictionary, Set, etc), has two methods called max() and max(by:) that return the maximum element in the sequence or nil if the sequence is empty.

---

#1. Using Array's max() method

If the element type inside your sequence conforms to Comparable protocol (may it be String, Float, Character or one of your custom class or struct), you will be able to use max() that has the following declaration:

@warn_unqualified_access func max() -> Element?

Returns the maximum element in the sequence.

The following Playground codes show to use max():

let intMax = [12, 15, 6].max()
let stringMax = ["bike", "car", "boat"].max()

print(String(describing: intMax)) // prints: Optional(15)
print(String(describing: stringMax)) // prints: Optional("car")

class Route: Comparable, CustomStringConvertible {

    let distance: Int
    var description: String { return "Route with distance: \(distance)" }

    init(distance: Int) {
        self.distance = distance
    }

    static func ==(lhs: Route, rhs: Route) -> Bool {
        return lhs.distance == rhs.distance
    }

    static func <(lhs: Route, rhs: Route) -> Bool {
        return lhs.distance < rhs.distance
    }

}

let routes = [
    Route(distance: 20),
    Route(distance: 30),
    Route(distance: 10)
]

let maxRoute = routes.max()
print(String(describing: maxRoute)) // prints: Optional(Route with distance: 30)

---

#2. Using Array's max(by:) method

If the element type inside your sequence does not conform to Comparable protocol, you will have to use max(by:) that has the following declaration:

@warn_unqualified_access func max(by areInIncreasingOrder: (Element, Element) throws -> Bool) rethrows -> Element?

Returns the maximum element in the sequence, using the given predicate as the comparison between elements.

The following Playground codes show to use max(by:):

let dictionary = ["Boat" : 15, "Car" : 20, "Bike" : 40]

let keyMaxElement = dictionary.max(by: { (a, b) -> Bool in
    return a.key < b.key
})

let valueMaxElement = dictionary.max(by: { (a, b) -> Bool in
    return a.value < b.value
})

print(String(describing: keyMaxElement)) // prints: Optional(("Car", 20))
print(String(describing: valueMaxElement)) // prints: Optional(("Bike", 40))

class Route: CustomStringConvertible {

    let distance: Int
    var description: String { return "Route with distance: \(distance)" }

    init(distance: Int) {
        self.distance = distance
    }

}

let routes = [
    Route(distance: 20),
    Route(distance: 30),
    Route(distance: 10)
]

let maxRoute = routes.max(by: { (a, b) -> Bool in
    return a.distance < b.distance
})

print(String(describing: maxRoute)) // prints: Optional(Route with distance: 30)