for循环中的for不起作用,因为int类型不符合协议序列.
我有3个数组：
1个主数组,存储数组
1个奇数阵,开头为空
1个偶数组,开头为空
意思是主阵列的所有奇数索引将存储在奇数阵列中,而偶数阵列则相同.
while i < mastera.count {
evenA.append(mastera[i])
if i > 0{
oddA.append(mastera[i - 1])
}
i += 2
}
然而,这不够好.谁有更好的主意？
这是另一种可能的解决方案
let evenA = stride(from: 0,to: mastera.count,by: 2).map { mastera[$0] }
let oddA = stride(from: 1,by: 2).map { mastera[$0] }
元素直接从源阵列中的偶数/奇数位置“拾取”.
表现比较：
我的简单,不是非常复杂的特殊基准测试代码：
import Swift
let N = 10_000_000
let RUNS = 50
let mastera = (0..<N).map { $0 }
var times = (0.0,0.0,0.0)
for _ in 1...RUNS {
// filter+map (dfri)
do {
let start = Date()
let evenA = mastera.enumerated().filter { $0.0 % 2 == 0 }.map{ $0.1 }
let oddA = mastera.enumerated().filter { $0.0 % 2 != 0 }.map{ $0.1 }
let time = Date().timeIntervalSince(start)
times.0 += time
}
// flatMap (dfri)
do {
let start = Date()
let evenA = mastera.enumerated().flatMap { $0 % 2 == 0 ? $1 : nil }
let oddA = mastera.enumerated().flatMap { $0 % 2 != 0 ? $1 : nil }
let time = Date().timeIntervalSince(start)
times.1 += time
}
// stride+map (me)
do {
let start = Date()
let evenA = stride(from: 0,by: 2).map { mastera[$0] }
let oddA = stride(from: 1,by: 2).map { mastera[$0] }
let time = Date().timeIntervalSince(start)
times.2 += time
}
// loop (Keiwan)
do {
let start = Date()
var evenA = [Int]()
var oddA = [Int]()
for (index,element) in mastera.enumerated() {
if index % 2 == 0 {
evenA.append(element)
} else {
oddA.append(element)
}
}
let time = Date().timeIntervalSince(start)
times.3 += time
}
}
print(N,RUNS)
print(times.0/Double(RUNS),times.1/Double(RUNS),times.2/Double(RUNS),times.3/Double(RUNS))
结果:(在MacBook上,在发布模式下运行)
#elements filter+map flatMap stride+map loop 10,000 0.0001 0.00008 0.00004 0.00004 100,000 0.0016 0.0008 0.0004 0.0004 1,000,000 0.0295 0.0136 0.0090 0.0091 10,000 0.3025 0.1332 0.0909 0.1250