He(Heva) Wu's Blog - Pass Array or ArraySlice into Function

Array & ArraySlice

Array is Swift ordered, random-access collection.

ArraySlice is a slice of an Array, ContiguousArray or ArraySlice instance.

We could simply get a ArraySlice element by Array’s subsequence. ex:

let absences = [0, 2, 0, 4, 0, 3, 1, 0]
let midpoint = absences.count / 2

let firstHalf = absences[..<midpoint]
let secondHalf = absences[midpoint...]

Test Measurement

We could simply test its behavior by ⬇️

let points = (0..<100_000).map { CGPoint(x: $0 % 100, y: $0 % 100) }

func testArrSlices() {
    measure {
        _ = points[...]
    }
}

func testArrCopy() {
    measure {
        _ = points
    }
}

Result:

Test Case '-[__lldb_expr_13.MyTestCase testArrCopy]' started.
<unknown>:0: Test Case '-[__lldb_expr_13.MyTestCase testArrCopy]' measured [Time, seconds] average: 0.000, relative standard deviation: 262.929%, values: [0.004014, 0.000099, 0.000048, 0.000041, 0.000041, 0.000038, 0.000038, 0.000084, 0.000066, 0.000046], performanceMetricID:com.apple.XCTPerformanceMetric_WallClockTime, baselineName: "", baselineAverage: , maxPercentRegression: 10.000%, maxPercentRelativeStandardDeviation: 10.000%, maxRegression: 0.100, maxStandardDeviation: 0.100
Test Case '-[__lldb_expr_13.MyTestCase testArrCopy]' passed (0.499 seconds).

Test Case '-[__lldb_expr_13.MyTestCase testArrSlices]' started.
<unknown>:0: Test Case '-[__lldb_expr_13.MyTestCase testArrSlices]' measured [Time, seconds] average: 0.000, relative standard deviation: 51.664%, values: [0.000201, 0.000072, 0.000061, 0.000062, 0.000065, 0.000065, 0.000072, 0.000066, 0.000056, 0.000075], performanceMetricID:com.apple.XCTPerformanceMetric_WallClockTime, baselineName: "", baselineAverage: , maxPercentRegression: 10.000%, maxPercentRelativeStandardDeviation: 10.000%, maxRegression: 0.100, maxStandardDeviation: 0.100
Test Case '-[__lldb_expr_13.MyTestCase testArrSlices]' passed (0.315 seconds).

We could find under 10 reference tests, copy array using 0.499s where ArraySlice using 0.315s. This is only at the copy part. If you pass this element into function, you will find ArraySlice processing speed is faster than copy array into the function.

Conclusion

Use ArraySlice when we wanna recursively processing array element

Note: But as doc said:

To safely reference the starting and ending indices of a slice, always use the startIndex and endIndex properties instead of specific values.

ArraySlice startIndex will change according to current slice indices, so we should always use its startIndex & endIndex to retrieve the first & last element in the slice.

Reference

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