How to Implement Digital Root

Below is an implementation of Digital Root in Go.

package main

import (
    "fmt"
    "os"
    "strconv"
)

func digitalRoot(n int) int {
    if n < 10 {
        return n
    }
    sum := 0
    for n > 0 {
        sum += n % 10
        n /= 10
    }
    return digitalRoot(sum)
}

func main() {
    n, e := strconv.Atoi(os.Args[1])
    if e != nil {
        fmt.Println("Invalid number:", os.Args[1])
        os.Exit(1)
    }
    fmt.Println(digitalRoot(n))
}