Replace all variables in Sprintf with same variable

Question

Is it possible using fmt.Sprintf() to replace all variables in the formatted string with the same value?

Something like:

val := "foo"
s := fmt.Sprintf("%v in %v is %v", val)

which would return

"foo in foo is foo"

Answer 1

It's possible, but the format string must be modified, you must use explicit argument indicies:

Explicit argument indexes:

In Printf, Sprintf, and Fprintf, the default behavior is for each formatting verb to format successive arguments passed in the call. However, the notation [n] immediately before the verb indicates that the nth one-indexed argument is to be formatted instead. The same notation before a '*' for a width or precision selects the argument index holding the value. After processing a bracketed expression [n], subsequent verbs will use arguments n+1, n+2, etc. unless otherwise directed.

Your example:

val := "foo"
s := fmt.Sprintf("%[1]v in %[1]v is %[1]v", val)
fmt.Println(s)

Output (try it on the Go Playground):

foo in foo is foo

Of course the above example can simply be written in one line:

fmt.Printf("%[1]v in %[1]v is %[1]v", "foo")

Also as a minor simplification, the first explicit argument index may be omitted as it defaults to 1:

fmt.Printf("%v in %[1]v is %[1]v", "foo")

Answer 2

You could also use text/template:

package main

import (
   "strings"
   "text/template"
)

func format(s string, v interface{}) string {
   t, b := new(template.Template), new(strings.Builder)
   template.Must(t.Parse(s)).Execute(b, v)
   return b.String()
}

func main() {
   val := "foo"
   s := format("{{.}} in {{.}} is {{.}}", val)
   println(s)
}

https://pkg.go.dev/text/template