Swift 3 Array 包含 vs Set 包含

Question

只是想知道哪个会更快？
AFAIKSet使用，hash因此在 Set 中查找元素应该更快。但是在相当多的项目中，我已经看到了array contains它可以contains在Set.

Answer 1

给定一个非常快速的测试，set 的操作看起来确实更快.contains(。

import Foundation

let iterations = 1000000
let array = [ "cat", "dog", "fish", "gerbil", "hamster", "octopus" ]
let set = Set(array)
let bestCase = "cat"
let worstCase = "apple" // Note: Not in the collection.

print("For \(iterations) iterations:")

var start = Date()
for _ in 1...iterations {
    _ = array.contains(worstCase)
}
print("Array took \(-start.timeIntervalSinceNow)s in the worst case")

start = Date()
for _ in 1...iterations {
    _ = set.contains(worstCase) // Note: Not in the collection.
}
print("Set   took \(-start.timeIntervalSinceNow)s in the worst case")

start = Date()
for _ in 1...iterations {
    _ = array.contains(bestCase)
}
print("Array took \(-start.timeIntervalSinceNow)s in the best case")

start = Date()
for _ in 1...iterations {
    _ = set.contains(bestCase)
}

print("Set   took \(-start.timeIntervalSinceNow)s in the best case")

这输出：

For 1000000 iterations:
Array took 1.67272698879242s in the worst case
Set   took 0.307300984859467s in the worst case
Array took 0.412128031253815s in the best case
Set   took 0.216085016727448s in the best case

在 2015 年年中使用 swift 4.0.2 的 macbook pro 上。更长的阵列确实会影响最坏的情况。对于 24 个字符串的数组（上面相同的 6 个字符串重复了四次），数组最坏情况上升到 5.9s；其他人大致保持不变。

笔记：

• 此测试未考虑将 an 转换Array为 a的成本Set。
• 我不得不将它提高到一百万次迭代才能在一秒内获得一个值。
• 你失去了订货和使用时存储的值的多个副本的能力Set在代替Array。

开发人员可能会使用合法的理由，Array即使Set这一项操作可能更快。