add: calculator example to x-tba

x-tba/foundations/calc/08-funcs/main.go

@@ -0,0 +1,67 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"strconv"
+)
+
+func main() {
+	// lesson: many - split this
+
+	if len(os.Args) != 4 {
+		fmt.Println("Usage: calc <number1> <operator> <number2>")
+		return
+	}
+
+	var (
+		a, b float64
+		err  error
+	)
+
+	if a, err = parse(os.Args[1]); err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	if b, err = parse(os.Args[3]); err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	op := os.Args[2]
+	res, err := calc(a, b, op)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	fmt.Printf("%v %s %v = %v\n", a, op, b, res)
+}
+
+func parse(snum string) (n float64, err error) {
+	n, err = strconv.ParseFloat(snum, 64)
+	if err != nil {
+		err = errors.New("Please provide a valid number")
+	}
+	return
+}
+
+func calc(a, b float64, op string) (res float64, err error) {
+	switch op {
+	case "+", "plus":
+		op, res = "+", a+b
+	case "-", "minus":
+		op, res = "-", a-b
+	case "*", "times":
+		op, res = "*", a*b
+	case "/", "div":
+		op, res = "/", a/b
+	case "%", "mod":
+		res = float64(int(a) % int(b))
+	default:
+		return 0, errors.New("Wrong operation: '" + op + "'")
+	}
+	return
+}