Expanding variables and fmt.Printf() issue

[yves baumes]

Hello,

considering this code

```

package main

import (

"fmt"

)

func main() {

host := []string{"127", "0", "0", "1"}

fmt.Printf("%v.%v.%v.%v\n", host[0], host[1], host[2], host[3])

fmt.Printf("%v.%v.%v.%v\n", host[0:4]...)

}

```

The first Printf works and prints the host value.

While the second one does not compile : "./args.go:11:34: cannot use host[0:4] (type []string) as type []interface {} in argument to fmt.Printf"

If I use append() instead of Printf(), this expanding of the host variables just works out. Is this a compiler bug in the case of the fmt.Printf() ?

Regards

Yves.

[Jan Mercl]

On Mon, Jun 29, 2020 at 9:22 AM yves baumes <yba...@gmail.com> wrote:

> If I use append() instead of Printf(), this expanding of the host variables just works out. Is this a compiler bug in the case of the fmt.Printf() ?

No bug. That's how variable arguments and assignability rules are
specified to work.

[Volker Dobler]

 

If I use append() instead of Printf(), this expanding of the host variables just works out. Is this a compiler bug in the case of the fmt.Printf() ?

I doubt that

     append("%v.%v.%v.%v\n", host[0:4]...)

does compile.

V.

[Marvin Renich]

* yves baumes <yba...@gmail.com> [200629 03:22]:

From the language spec at
https://golang.org/ref/spec#Passing_arguments_to_..._parameters:

If the final argument is assignable to a slice type []T, it is
passed unchanged as the value for a ...T parameter if the argument
is followed by .... In this case no new slice is created.

The final argument to Printf is []interface{}, while you are trying to
pass []string. In order for this to work, []string would have to be
assignable to []interface{}, which it isn't.

This is related to a FAQ at
https://golang.org/doc/faq#convert_slice_of_interface.

If, after reading the above links and the definition of assignability at
https://golang.org/ref/spec#Assignability, you are still unclear why it
is not compiling, ask on the list for more help.

...Marvin

[Marvin Renich]

[pedantic correction]

* Marvin Renich <mr...@renich.org> [200629 14:10]:

> The final argument to Printf is []interface{}, while you are trying to

^
...interface{}

...Marvin