Add diagonal test.

Author: mcordingley

tests/MatrixTest.php

@@ -227,6 +227,22 @@ public function testMultiplyScalar() {
         $this->assertEquals(4, $multiplied->get(2, 2));
     }
 
+    public function testDiagonal() {
+        $matrix = new Matrix([
+            [1, 2, 3],
+            [4, 5, 6],
+            [7, 8, 9]
+        ]);
+        
+        $diagonal = $matrix->diagonal();
+        
+        $this->assertEquals(1, $matrix->rows);
+        $this->assertEquals(3, $matrix->columns);
+        $this->assertEquals(1, $matix->get(0, 1));
+        $this->assertEquals(5, $matix->get(0, 1));
+        $this->assertEquals(9, $matix->get(0, 1));
+    }
+    
     public function testTrace() {
         $matrix = new Matrix([
             [1, 2, 3],
@@ -331,4 +347,4 @@ public function testToString() {
 
         $this->assertEquals('[[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]]', (string) $matrix);
     }
-}
+}