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
 }

score 1 · Answer 1 · answered Oct 10 '15 at 03:57

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.

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

1 Answers1