Kotlin function which counts standard deviation

Question

Here is the code. The code should counts the standard deviation for double array, but also for Int arrays.

fun calculateSD(numArray: DoubleArray): Double {
var sum = 0.0
var standardDeviation = 0.0

for (num in numArray) {
    sum += num
}

val mean = sum / numArray.size

for (num in numArray) {
    standardDeviation += Math.pow(num - mean, 2.0)
}

val divider = numArray.size - 1

return Math.sqrt(standardDeviation / divider )
}

Answer 1

You may want to be more idiomatic when working with collections, for instance:

import kotlin.math.pow
import kotlin.math.sqrt

fun sd(data: DoubleArray): Double {
    val mean = data.average()
    return data
        .fold(0.0, { accumulator, next -> accumulator + (next - mean).pow(2.0) })
        .let { sqrt(it / data.size )
    }
}

BTW, I don't think that data.size - 1 is correct

Answer 2

Performance

Something to consider (in general) is whether Math.pow(x, 2.0) is going to be optimized by the interpreter/compiler into x * x or not. The former can be slower if, say, it is computed as Math.exp(Math.log(x) * 2.0).