update book code

eBook/examples/chapter_10/method3.go

@@ -0,0 +1,24 @@
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+type Point struct {
+	x, y float64
+}
+
+func (p *Point) Abs() float64 {
+	return math.Sqrt(p.x*p.x + p.y*p.y)
+}
+
+type NamedPoint struct {
+	Point
+	name string
+}
+
+func main() {
+	n := &NamedPoint{Point{3, 4}, "Pythagoras"}
+	fmt.Println(n.Abs()) // prints 5
+}