golang - how to get element from the interface{} type of slice?

Question

I want to write a function that can convert slice([]int, []string, []bool, []int64, []float64) to string.

[]string{a,b,c} -> a,b,c []int{1,2,3} -> 1,2,3

There is my code:

func sliceToString(itr interface{}) string { switch itr.(type) { case []string: return strings.Join(itr.([]string), ",") case []int: s := []string{} for _, v := range itr.([]int) { s = append(s, fmt.Sprintf("%v", v)) } return strings.Join(s, ",") case []int64: s := []string{} for _, v := range itr.([]int64) { s = append(s, fmt.Sprintf("%v", v)) } return strings.Join(s, ",") case []float64: s := []string{} for _, v := range itr.([]float64) { s = append(s, fmt.Sprintf("%v", v)) } return strings.Join(s, ",") case []bool: s := []string{} for _, v := range itr.([]bool) { s = append(s, fmt.Sprintf("%v", v)) } return strings.Join(s, ",") } return "" }

But it's a little complicated, if i can convert interface{}(type is slice) to []interface{} or get element , it's getting more simple.

 func sliceToString(itr interface{}) string { s := []string{} // convert interface{} to []interface{} or get elements // els := ... for _,v:= range els{ s = append(s, fmt.Sprintf("%v", v)) } return s }

HectorJ · Accepted Answer · 2015-10-10 03:57:00Z

You can't do that, because a slice of int, string or anything can't be directly casted to a slice of interfaces. (see that question for more explanation on this).

To do the conversion, you need to cast each item of the slice as an interface{} separately. And you can't access the items without casting to a slice first, but for that you need to know the slice's type (so we're back to square one).

One way to shorten your syntax is to take in a slice of interfaces as argument, and let the caller do the conversion (because the caller knows the slice's type). Here is an example : https://play.golang.org/p/6yLYk1OM25

package main import ( "fmt" "strings" ) func main() { mySlice := []int{1, 2, 3} interfaceSlice := make([]interface{}, len(mySlice)) for index := range mySlice { interfaceSlice[index] = mySlice[index] } fmt.Println(sliceToString(interfaceSlice)) } func sliceToString(values []interface{}) string { s := make([]string, len(values)) // Pre-allocate the right size for index := range values { s[index] = fmt.Sprintf("%v", values[index]) } return strings.Join(s, ",") }

This will work with any slice as mySlice, but on the way you lose a lot of convenience for the caller.

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