Go Flags

Learning a new programming language is always exciting! Go is elegant and easy to learn. I simply executed the install staps and dove in the Hello World and the Modules Tutorial mentioned at the bottom of the page. The Go By Example is also being a rich resource.

My initial interest in Go was motivated by finding a good WebSocket programming API and I learned the standard lib is powerful and easy to use. This post shows how the flag package provides a clean syntax to parse command-line arguments.

Flag Calculator

A very basic calculator is implemented by the following code, where the arguments are passed to the program using some command-line arguments.

package main

import (
  "fmt"
  "flag"
)

func main() {
  x := flag.Int("x", 1234, "Arg 1")
  y := flag.Int("y", 8765, "Arg 2")
  o := flag.String("o", "+", "One of +, -, *, /")

  flag.Parse()
  a, b, c := *x, *y, *o

  fmt.Println("x:", a)
  fmt.Println("y:", b)
  fmt.Println("o:", c)
  fmt.Println("tail:", flag.Args())

  calculate := func(c string, a, b int) (int, string) {
    switch c {
    case "+":
      return a + b, ""
    case "-":
      return a - b, ""
    case "*":
      return a * b, ""
    case "/":
      return a / b, ""
    default:
      return 0, fmt.Sprintf("[%v]", c)
    }
  }
  output, err := calculate(c, a, b)

  if err == "" {
    msg := fmt.Sprintf("%d %v %d = %d", a, c, b, output)
    fmt.Println(msg)
  } else {
    fmt.Println("Invalid Operand:", err)
  }
}

Lets first run the program with --help flag:

/app# go run . --help
Usage of /tmp/go-build621157370/b001/exe/app:
  -o string
    	One of +, -, *, / (default "+")
  -x int
    	Arg 1 (default 1234)
  -y int
    	Arg 2 (default 8765)
exit status 2

In the absence of arguments, the default values are applied:

go run .
x:  1234
y:  8765
o:  +
tail: []
9999

In the presence of arguments, the calculations uses the flag operands:

# go run . -x 20 -y 10 -o "+" Anything Else
x: 20
y: 10
o: +
tail: [Anything Else]
20 + 10 = 30

# go run . -x 20 -y 10 -o "-" Anything Else
x: 20
y: 10
o: -
tail: [Anything Else]
20 - 10 = 10

# go run . -x 20 -y 10 -o "*" Anything Else
x: 20
y: 10
o: *
tail: [Anything Else]
20 * 10 = 200

# go run . -x 20 -y 10 -o "/" Anything Else
x: 20
y: 10
o: /
tail: [Anything Else]
20 / 10 = 2

# go run . -x 20 -y 10 -o ":" Anything Else
x: 20
y: 10
o: :
tail: [Anything Else]
Invalid Operand: [:]

Flag Calculator

About Fábio Miranda