median function in statistic

src/Phpml/Math/Statistic/Mean.php

@@ -4,15 +4,42 @@ declare (strict_types = 1);
 
 namespace Phpml\Math\Statistic;
 
+use Phpml\Exception\InvalidArgumentException;
+
 class Mean
 {
     /**
-     * @param array $a
+     * @param array $numbers
      *
      * @return float
      */
-    public static function arithmetic(array $a)
+    public static function arithmetic(array $numbers)
     {
-        return array_sum($a) / count($a);
+        return array_sum($numbers) / count($numbers);
     }
+
+    /**
+     * @param array $numbers
+     *
+     * @return float|mixed
+     *
+     * @throws InvalidArgumentException
+     */
+    public static function median(array $numbers) {
+        $count = count($numbers);
+        if (0 == $count) {
+            throw InvalidArgumentException::arrayCantBeEmpty();
+        }
+
+        $middleIndex = floor($count / 2);
+        sort($numbers, SORT_NUMERIC);
+        $median = $numbers[$middleIndex];
+
+        if (0 == $count % 2) {
+            $median = ($median + $numbers[$middleIndex - 1]) / 2;
+        }
+
+        return $median;
+    }
+
 }

src/Phpml/Preprocessing/Imputer/Strategy/MeanStrategy.php

@@ -9,6 +9,11 @@ use Phpml\Math\Statistic\Mean;
 
 class MeanStrategy implements Strategy
 {
+    /**
+     * @param array $currentAxis
+     * 
+     * @return float
+     */
     public function replaceValue(array $currentAxis)
     {
         return Mean::arithmetic($currentAxis);

tests/Phpml/Math/Statistic/MeanTest.php

@@ -15,4 +15,27 @@ class MeanTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals(41.16, Mean::arithmetic([43, 21, 25, 42, 57, 59]), '', $delta);
         $this->assertEquals(1.7, Mean::arithmetic([0.5, 0.5, 1.5, 2.5, 3.5]), '', $delta);
     }
+
+    /**
+     * @expectedException \Phpml\Exception\InvalidArgumentException
+     */
+    public function testThrowExceptionOnEmptyArrayMedian()
+    {
+        Mean::median([]);
+    }
+
+    public function testMedianOnOddLengthArray()
+    {
+        $numbers = [5, 2, 6, 1, 3];
+
+        $this->assertEquals(3, Mean::median($numbers));
+    }
+
+    public function testMedianOnEvenLengthArray()
+    {
+        $numbers = [5, 2, 6, 1, 3, 4];
+
+        $this->assertEquals(3.5, Mean::median($numbers));
+    }
+
 }