update book code

eBook/examples/chapter_6/mut_recurs.go

@@ -0,0 +1,27 @@
+// mut_recurs.go
+package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	fmt.Printf("%d is even: is %t\n", 16, even(16)) // 16 is even: is true
+	fmt.Printf("%d is odd: is %t\n", 17, odd(17))	// 17 is odd: is true
+	fmt.Printf("%d is odd: is %t\n", 18, odd(18))	// 18 is odd: is false
+}
+
+func even(nr int) bool {
+    if nr == 0 {return true}
+    return odd(RevSign(nr)-1)
+}
+
+func odd(nr int) bool {
+    if nr == 0 {return false}
+    return even(RevSign(nr)-1)
+}
+
+func RevSign(nr int) int {
+	if nr < 0 {return -nr}
+	return nr
+}