update book code

eBook/exercises/chapter_10/days.go

@@ -0,0 +1,36 @@
+package main
+
+import "fmt"
+
+type Day int
+
+const (
+    MO Day = iota
+    TU
+    WE
+    TH
+    FR
+    SA
+    SU
+)
+
+var dayName = []string{"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"}
+
+func (day Day) String() string {
+	return dayName[day]
+}
+
+func main() {
+		var th Day = 3
+		fmt.Printf("The 3rd day is: %s\n", th)
+		// If index > 6: panic: runtime error: index out of range
+		// but use the enumerated type to work with valid values:
+		var day = SU;
+		fmt.Println(day);  // prints Sunday
+		fmt.Println(0, MO, 1, TU)
+}
+/* Output:
+The 3rd day is: Thursday
+Sunday
+0 Monday 1 Tuesday
+*/