implement sample median

Author: jbytecode

src/main/scala/statistics.scala

@@ -40,3 +40,11 @@ object Statistics:
     val xty = Matrix.mul(x.transpose, Matrix.makeRowMatrix(y))
     val beta = Matrix.mul(Matrix.inverse(xtx), xty)
     beta(0)
+
+  def median(x: Vec): Double = 
+    val L = x.length
+    val sorted = x.sorted
+    L match 
+      case n if n % 2 == 0 => (sorted(n / 2 - 1) + sorted(n / 2)) / 2.0
+      case _ => sorted(L / 2)
+

src/test/scala/teststatistics.scala

@@ -70,5 +70,16 @@ class TestStatistics extends munit.FunSuite {
     val expected = Array(1.0, 2.0)
     A.assert(Matrix.elementwise_equal(beta, expected, 1e-6))
   }
+  test("Median"){
+    val x = Array(1.0, 2.0, 3.0, 4.0, 5.0)
+    val median = Statistics.median(x)
+    val expected = 3.0
+    A.assertEqualsDouble(median, expected, 1e-6)
+
+    val y = Array(1.0, 2.0, 3.0, 4.0, 5.0, 6.0).reverse
+    val median2 = Statistics.median(y)
+    val expected2 = 3.5
+    A.assertEqualsDouble(median2, expected2, 1e-6)
+  }
 
 }