update book code

eBook/examples/chapter_10/method_string.go

@@ -0,0 +1,27 @@
+package main
+
+import (
+	"fmt"
+	"strconv"
+)
+
+type TwoInts struct {
+	a int
+	b int
+}
+
+func main() {
+	two1 := new(TwoInts)
+	two1.a = 12
+	two1.b = 10
+	fmt.Printf("two1 is: %v\n", two1)
+	fmt.Println("two1 is:", two1)
+	fmt.Printf("two1 is: %T\n", two1)
+	fmt.Printf("two1 is: %#v\n", two1)
+}
+
+func (tn *TwoInts) String() string {
+	return "(" + strconv.Itoa(tn.a) + " / " + strconv.Itoa(tn.b) + ")"
+}
+
+