add chapter 4

chapter4/const/const.go

@@ -0,0 +1,18 @@
+package main
+
+import "fmt"
+
+// объявление константы с типом
+const TriangleSides int = 3
+
+// объявление константы без типа
+const SquareSides = 4
+
+/* константам переназначить значение невозможно
+const PentagonSides = 5
+PentagonSides = 6 // error
+*/
+
+func main() {
+	fmt.Println(TriangleSides, SquareSides)
+}

chapter4/const/dates/dates.go

@@ -0,0 +1,11 @@
+package dates
+
+const DaysInWeek = 7
+
+func WeeksToDays(weeks int) int {
+	return weeks * DaysInWeek
+}
+
+func DaysToWeek(days int) float64 {
+	return float64(days) / float64(DaysInWeek)
+}

chapter4/const/planner/main.go

@@ -0,0 +1,12 @@
+package main
+
+import (
+	"fmt"
+	"golang-book/head-first-go/chapter4/const/dates"
+)
+
+func main() {
+	days := 3
+	fmt.Println("Your appointment is", days, "days")
+	fmt.Println("with a follow-up in", days+dates.DaysInWeek, "days")
+}