KTLN-694: Calculate Standard Deviation in Kotlin (#756)

* pom kotlin version

* addedd unit tests

* fixed unit tests

Author: nappy29

kotlin-math-2/pom.xml

@@ -12,4 +12,12 @@
         <artifactId>kotlin-modules</artifactId>
         <version>1.0.0-SNAPSHOT</version>
     </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-math3</artifactId>
+            <version>3.6.1</version>
+        </dependency>
+    </dependencies>
 </project>

kotlin-math-2/src/test/kotlin/com/baeldung/math/standardDeviation/StandardDeviationUnitTest.kt

@@ -0,0 +1,39 @@
+package com.baeldung.math.standardDeviation
+import org.apache.commons.math3.stat.descriptive.moment.StandardDeviation
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+import kotlin.math.pow
+
+class StandardDeviationUnitTest {
+
+    @Test
+    fun `standard deviation using the math package`() {
+        val dataset1 = doubleArrayOf(1.0, 2.0, 3.0, 4.0, 5.0, 6.0)
+        val dataset2 = doubleArrayOf(11.0, 14.0, 19.0, 23.0, 28.0, 30.0)
+
+        assertEquals(1.707825127659933, standardDeviationUsingMathPackage(dataset1))
+        assertEquals(6.914156170897181, standardDeviationUsingMathPackage(dataset2))
+    }
+
+    @Test
+    fun `standard deviation using the apache commons math library`() {
+        val dataset1 = doubleArrayOf(1.0, 2.0, 3.0, 4.0, 5.0, 6.0)
+        val dataset2 = doubleArrayOf(11.0, 14.0, 19.0, 23.0, 28.0, 30.0)
+
+        assertEquals(1.707825127659933, calculateStandardDeviationUsingApacheCommonsMath(dataset1))
+        assertEquals(6.914156170897181, calculateStandardDeviationUsingApacheCommonsMath(dataset2))
+    }
+
+
+    fun standardDeviationUsingMathPackage(dataset: DoubleArray): Double {
+        val mean = dataset.average()
+        val variance = dataset.map { (it - mean).pow(2) }.average()
+        return Math.sqrt(variance)
+    }
+
+    fun calculateStandardDeviationUsingApacheCommonsMath(dataset: DoubleArray): Double {
+        val sd = StandardDeviation(false)
+
+        return sd.evaluate(dataset)
+    }
+}