Swift：使用 for-in 循环，找到 Int 数组中的最大值

Question

Sorry guys, I'm new here and I'm learning iOS developing from scratch.

I know that in order to find the largest value in an array of Int, we can use the propertie ".max()". But I need to do this using a for-in loop. Would someone please help me? I know it's so easy, but I can't find it and can't find out how to do it on my own as well. Thanks.

Answer 1

Well the complexity of array.max() of Swift is O(N) just like the for-in loop.

There are two ways to for-in in Swift.

First solution (for-in here like for each value)

let arr = [1, 4, 3]
var max = Int.min

// get each value
for val in arr {
    if (max < val) {
        max = val
    }
}

Second solution (for-in here is for each index)

let arr = [1, 4, 3]
var max = Int.min

// get each index
for i in 0..<arr.count {
    if (max < arr[i]) {
        max = arr[i]
    }
}

Two ways have the same output. Feel free when choosing which to use in your further code.

Answer 2

If your array is empty, then returning Int.min as the maximum value is not correct.

Returning an optional is more correct:

var max: Int? = nil

for val in arr {
    guard let maxSoFar = max else {
        max = val
        continue
    }

    if val > maxSoFar {
        max = val
    }
}

Though you might prefer to write it as an extension to Collection, like:

extension Collection where Element: Comparable {
    func mMax() -> Element? {
        var max: Element? = nil

        // get each value
        for val in self {
            guard let maxSoFar = max else {
                max = val
                continue
            }

            if val > maxSoFar {
                max = val
            }
        }

        return max
    }

}

[1, 2, 3].mMax() // 3

([] as [Int]).mMax() // nil

["a", "c", "b"].mMax() // "c"

Or perhaps more generally, so it's not tied to the '>' function, like:

extension Collection {
    func mMax(by compare: (Element, Element) -> Bool) -> Element? {
        var max: Element? = nil

        // get each value
        for val in self {
            guard let maxSoFar = max else {
                max = val
                continue
            }

            if compare(val, maxSoFar) {
                max = val
            }
        }

        return max
    }

}

[1, 2, 3].mMax(by: >) // 3

([] as [Int]).mMax(by: >) // nil


let longestString = ["aaa", "a", "aaaaa"].mMax(by: { $0.count > $1.count })  /// "aaaaa"