How to convert interface{} to string?

Question

I'm using docopt to parse command-line arguments. This works, and it results in a map, such as

map[<host>:www.google.de <port>:80 --help:false --version:false]

Now I would like to concatenate the host and the port value to a string with a colon in-between the two values. Basically, something such as:

host := arguments["<host>"] + ":" + arguments["<port>"]

Unfortunately, this doesn't work, as I get the error message:

invalid operation: arguments[""] + ":" (mismatched types interface {} and string)

So obviously I need to convert the value that I get from the map (which is just interface{}, so it can be anything) to a string. Now my question is, how do I do that?

golang.org/ref/spec#Type_assertions

twotwotwo
– twotwotwo

2014-11-25 22:00:46 +00:00
Commented Nov 25, 2014 at 22:00 — twotwotwo
– twotwotwo, Commented Nov 25, 2014 at 22:00
fmt.Sprint(data) // where data is interface{}

Eric
– Eric

2022-01-12 22:48:16 +00:00
Commented Jan 12, 2022 at 22:48 — Eric
– Eric, Commented Jan 12, 2022 at 22:48

Grzegorz Żur · Accepted Answer · 2018-11-22 19:37:23Z

You need to add type assertion .(string). It is necessary because the map is of type map[string]interface{}:

host := arguments["<host>"].(string) + ":" + arguments["<port>"].(string)

Latest version of Docopt returns Opts object that has methods for conversion:

host, err := arguments.String("<host>") port, err := arguments.String("<port>") host_port := host + ":" + port

Would like to add that this is called Type Assertion - golang.org/ref/spec#Type_assertions. I once had trouble figuring out this exact terminology.

Peter Stace · Accepted Answer · 2014-11-26 20:43:56Z

117

You don't need to use a type assertion, instead just use the %v format specifier with Sprintf:

hostAndPort := fmt.Sprintf("%v:%v", arguments["<host>"], arguments["<port>"])

answered Nov 26, 2014 at 20:43

Peter Stace

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2 Comments

FilBot3 Over a year ago

This was the only thing I could get to work for me when trying to type cast interface{}'s with as strings.

Felipe Valdes Over a year ago

For me, this didn't work. This ended up printing the byte ASCII charts instead of the string, like 201 become "[50,48,49]", so I ended up using fmt.Sprintf("%s", theThing) instead...

justitan · Accepted Answer · 2017-02-17 16:31:00Z

To expand on what Peter said: Since you are looking to go from interface{} to string, type assertion will lead to headaches since you need to account for multiple incoming types. You'll have to assert each type possible and verify it is that type before using it.

Using fmt.Sprintf (https://golang.org/pkg/fmt/#Sprintf) automatically handles the interface conversion. Since you know your desired output type is always a string, Sprintf will handle whatever type is behind the interface without a bunch of extra code on your behalf.

Zombo · Accepted Answer · 2021-07-14 20:21:28Z

You could also use text/template:

package main import ( "text/template" "strings" ) 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() { m := map[string]interface{}{"<host>": "www.google.de", "<port>": "80"} s := format(`{{index . "<host>"}}:{{index . "<port>"}}`, m) println(s == "www.google.de:80") }

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

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