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

• 200_success: There are two kinds of standard deviation. The sample standard deviation involves dividing by N - 1.
• David Soroko: True: en.wikipedia.org/wiki/Bessel%27s_correction . Do you think that this is the case here ?
• 200_success: You could complain about unclear naming or lack of documentation for this function, but I'd hesitate to call it "wrong".
• Groostav: data.sumOf { (next - mean).pow(2) } instead of aggregate is simpler, but requires kotlin 1.4.
• David Soroko: Did you mean "instead of fold" ?

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).