Separate odd and even indexes in a sequence array

Question

A for in loop won't work because type int is does not conform to protocol Sequence.

I have 3 arrays:

1 master array, where the array is stored

1 odd array, empty at the beginning

1 even array, empty at the beginning

The meaning is that all the odd indexes of the master array will be stored at the odd array, and the same for the even array.

    while i < masterA.count {
        evenA.append(masterA[i])
        if i > 0{
        oddA.append(masterA[i - 1])
        }
        i += 2
    }

However this does not work good enough. Anyone has a better idea?

Answer 1

Here is another possible solution:

let evenA = stride(from: 0, to: masterA.count, by: 2).map { masterA[$0] }
let oddA = stride(from: 1, to: masterA.count, by: 2).map { masterA[$0] }

The elements are "picked" directly from the even/odd positions in the source array.

Performance comparison:

My simple, not very sophisticated ad-hoc benchmarking code:

import Swift

let N = 10_000_000
let RUNS = 50

let masterA = (0..<N).map { $0 }

var times = (0.0, 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, to: masterA.count, by: 2).map { masterA[$0] }
        let oddA = stride(from: 1, to: masterA.count, 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))

Results: (On a MacBook, running in Release mode)

#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
10,000,000  0.3025      0.1332    0.0909      0.1250