update book code

eBook/exercises/chapter_10/rectangle.go

@@ -0,0 +1,28 @@
+// rectangle.go
+package main
+
+import "fmt"
+
+type Rectangle struct {
+    length, width int
+}
+
+func (r *Rectangle) Area() int {
+    return r.length * r.width
+}
+
+func (r *Rectangle) Perimeter() int {
+    return 2* (r.length + r.width)
+}
+
+func main() {
+    r1 := Rectangle{4, 3}
+    fmt.Println("Rectangle is: ", r1)
+    fmt.Println("Rectangle area is: ", r1.Area())
+    fmt.Println("Rectangle perimeter is: ", r1.Perimeter())
+}
+/* Output:
+Rectangle is:  {4 3}
+Rectangle area is:  12
+Rectangle perimeter is:  14
+*/