Convert interface{} into []string in Golang

Question

I am new to GO language and I am trying to get a value from a function which returns two strings, that is [str1, str2]

 ret, err := fun1(....)

And function's prototype is

func fun1(...) (interface{}, error)

I print ret, it is

[[57 57 56 56] [97 100 98 49 57 55 102 98 49 101 57 48 52 98 102 56 55 102 52 56 57 57 55 54 49 55 101 57 49 100 98 49]]

which looks like the type [][]uint8, so, I tried this way

    v, _ := ret.([][]uint8)
    str1, _ := strconv.Atoi(string(v[0]))
    str2, _ := strconv.Atoi(string(v[1]))

it failed, v is []

And I also tried:

   v, _ := ret.([]string)

nothing changed

How to solve this? Any help will be appreciated :)

PS: I debug and find the first string is "9988"

Do not disregard errors by assigning them to underscores. Check them and print them. They will most probably provide you a solution. — Grzegorz Żur, Nov 25 '15 at 14:07
Does `v := ret.([]string)` panic? If yes, what does the panic message say? — Ainar-G, Nov 25 '15 at 14:07
go types are not covariant. You need to iterate through the `[][]byte` and convert each slice to a string. — JimB, Nov 25 '15 at 14:17
Read the Go FAQ: https://golang.org/doc/faq. Do the Tour of Go: https://tour.golang.org/welcome/1. Do not use `interface {}`. Really. Don't. — Volker, Nov 25 '15 at 14:29
@simon_xia: you need to read the API documentation to find out what to expect for return values. It's definitely not returning `[]string` or `[][]byte`. — JimB, Nov 25 '15 at 14:43

Charlie Tumahai · Accepted Answer · 2022-06-24T19:27:51.000

The value ret is an []interface{} containing []byte elements. Use the following code to convert to a slice of strings:

// Use type assertion to get []interface from ret
sitf, ok := ret.([]interface{})
if !ok { /* handle unexpected type here */ }

// Create the result slice.
s := make([]string, len(sitf))

// For each element …
for i := range sitf {
    // Use type assertion to get []byte from interface{}
    sbyte, ok := sitf[i].([]byte)
    if !ok { /* handle unexpected type here */ }

    // Convert the []byte to a string and set in result
    s[i] = string(sbyte)
}

I know from your other questions that you are asking about Redigo. You can use the redis.Strings helper function instead of the code above:

s, err := redis.Strings(fun1(....))

The variabile s is a []string.

See the documentation about reply helpers for more information.

well that would have been helpful information in the question. — JimB, Nov 25 '15 at 15:38
It really helps, that's just what @JimB says, ref: https://github.com/garyburd/redigo/blob/master/redis/reply.go#L250 — simon_xia, Nov 25 '15 at 15:52

score 0 · Answer 2 · answered Nov 25 '15 at 14:29

0

You have a slice of slices, you need to cast each slice into what you want. Let's say they're bytes and you want to print each string in the slice, then:

ret, err := fun1(....)
if err != nil {
     panic(err)
}
for idx, val := range ret.([][]byte) {
     fmt.Println("At idx", idx, "string:", string(val))
}

Example with the data above: https://play.golang.org/p/VsZhSaGmQk

answered Nov 25 '15 at 14:29

Matt

41
3

I got a panic here: interface conversion: interface is []interface {}, not [][]uint8 – simon_xia Nov 25 '15 at 14:34
Why is your function that is supposed to be returning two strings (as you said) not returning two strings in the first place? Surely that's a better place to start. – Ralph King Nov 25 '15 at 14:48

Convert interface{} into []string in Golang

2 Answers2