How to ignore extra fields for fmt.Sprintf

Question

I have a Golang program that reads a string parameter from command line and passes it to the fmt.Sprintf function. Let's say tmp_str is the target string from command line.

package main

import "fmt"

func main() {
    tmp_str := "hello %s"
    str := fmt.Sprintf(tmp_str, "world")
    fmt.Println(str)
}

In some cases, the program will pass a completed string like "Hello Friends", instead of a string template.. The program would panic and return:

Hello Friends%!(EXTRA string=world)

So, how to ignore the extra fields for fmt.Sprintf?

Answer 1

Yes you can do it, by slicing the arguments you pass to the variadic Sprintf function:

func TruncatingSprintf(str string, args ...interface{}) (string, error) {
    n := strings.Count(str, "%s")
    if n > len(args) {
        return "", errors.New("Unexpected string:" + str)
    }
    return fmt.Sprintf(str, args[:n]...), nil
}

func main() {
    tmp_str := "hello %s %s %s"         // don't hesitate to add many %s here
    str, err := TruncatingSprintf(tmp_str, "world") // or many arguments here
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(str)
}

Demonstration 1

Demonstration 2 (a different version outputting even when there's more %s than arguments)

But you don't usually use dynamic formatted strings, this isn't secure and if you want to accept any string, you should also adapt this code to no choke on %%s. If you venture this far, then you should probably have a look at templates (which would let you use named strings, and thus the missing one wouldn't have to be the last one).