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Cannot convert []string to []interface {}
(7 answers)
Closed 7 months ago.
I just want to write some code like this:
func (w Writer) WriteVString(strs []string) (int, error) {
return writeV(func(index int, str interface{}) (int, error) {
return w.WriteString(str.(string))
}, strs) // it doesn't work
}
func (w Writer) WriteV(bs [][]byte) (int, error) {
return writeV(func(index int, b interface{}) (int, error) {
return w.Write(b.([]byte))
}, []interface{}{bs...}) // it also can't be compiled
}
type writeFunc func(int, interface{}) (int, error)
func writeV(fn writeFunc, slice []interface{}) (n int, err error) {
var m int
for index, s := range slice {
if m, err = fn(index, s); err != nil {
break
}
n += m
)
return
}
I thought interface{} can represent any type, so []interface can also represent any []type before, now I know I'm wrong, []type is a whole type, can't be considered as []interface{}.
So, can anyone help me how to make this code work, or any other solution?
PS: I know that []byte or string can be converted to one another, but it's not actually my intention, may be there is another type rather than []byte and string.
now I know I'm wrong, []type is a whole type, can't be considered as []interface{}.
Yes, and that is because interface{} is its own type (and not an "alias" for any other type).
As I mention in "what is the meaning of interface{} in golang?" (if v is a interface{} variable):
Beginner gophers are led to believe that “v is of any type”, but that is wrong.
v is not of any type; it is of interface{} type.
The FAQ mentions
they do not have the same representation in memory.
It is necessary to copy the elements individually to the destination slice.
This example converts a slice of int to a slice of interface{}:
t := []int{1, 2, 3, 4}
s := make([]interface{}, len(t))
for i, v := range t {
s[i] = v
}
Tom L propose this example (in the comments):
package main
import "fmt"
func main() {
x := []string{"a", "b", "c", "d"}
fmt.Printf("%T: %v\n", x, x)
//converting a []string to a []interface{}
y := make([]interface{}, len(x))
for i, v := range x {
y[i] = v
}
fmt.Printf("%T: %v\n", y, y)
//converting a []interface{} to a []string
z := make([]string, len(y))
for i, v := range y {
z[i] = fmt.Sprint(v)
}
fmt.Printf("%T: %v\n", z, z)
}
Create a utility function, like this
func ToGenericArray(arr ...interface{}) []interface{} {
return arr
}
And use it:
func yourfunc(arr []interface{}) {
....
}
yourfunc(ToGenericArray([...]string{"a", "b", "c"}))
IMPORTANT NOTICE: the following will not work
func yourfunc(arr []interface{}) {
....
}
arr:=[...]string{"a", "b", "c"}
yourfunc(ToGenericArray(arr))
With generics, useful with sql package
func toAnyList[T any](input []T) []any{
list := make([]any, len(input))
for i, v := range input {
list[i] = v
}
return list
}
Related
Suppose I wrote two functions like this:
func ToInterfaceSlice[T any](s []T) []interface{} {
res := make([]interface{}, len(s))
for i, v := range s {
res[i] = v
}
return res
}
func FromInterfaceSlice[T any](s []interface{}) (res []T, err error) {
res = make([]T, len(s))
for i, v := range s {
vt, ok := v.(T)
if !ok {
return nil, fmt.Errorf("%v (type=%T) doesn't fit the target type %T", v, v, res)
}
res[i] = vt
}
return
}
When I parse type from the input parameters, I can simply use
var m = []int{1, 2, 3}
fmt.Println(ToInterfaceSlice(m))
The compiler knows the T is int.
However when I try passing type from the return variables
var m []int
m, _ = FromInterfaceSlice([]interface{}{1, 2, 3})
fmt.Println(m)
The compiler gives error:
.\scratch.go:29:27: cannot infer T
I must explicitly pass the type in the function call:
var m []int
m, _ = FromInterfaceSlice[int]([]interface{}{1, 2, 3})
fmt.Println(m)
Is there anything hard to infer type parameters from return type when the receiver vars are not interface? Or just not implemented, even not to implement on purpose?
Update #1 after the comment
I do know a, b := GenericFunc() cannot refer the type of returned value. Currently Go does have "it depends" case whether requires the explicit instantiation or not from the user input.
type Set[T comparable] map[T]struct{}
func NewSet[T comparable](eles ...T) Set[T] {
s := make(Set[T])
for _, ele := range eles {
s[ele] = struct{}{}
}
return s
}
It's okay to use both t := NewSet(1, 2, 3) and t := NewSet[string](), but not var t NewSet[float64] = NewSet() now because of this
The current rules for type inference are explicit. How the return values are used is not taken into account:
Type inference is based on
a type parameter list
a substitution map M initialized with the known type arguments, if any
a (possibly empty) list of ordinary function arguments (in case of a function call only)
As of Go 1.18 might simply rewrite your function to accept an argument of the required type; this has also the benefit of not hiding allocations inside the function body:
func FromInterfaceSlice[T any](s []interface{}, dst []T) error {
if len(s) != len(dst) {
return errors.New("lengths don't match")
}
for i, v := range s {
vt, ok := v.(T)
if !ok {
return nil, fmt.Errorf("%v (type=%T) doesn't fit the target type %T", v, v, res)
}
dst[i] = vt
}
return nil
}
And pass in a destination slice with the required length:
func main() {
src := []interface{}{1, 2, 3}
m := make([]int, len(src))
_ = FromInterfaceSlice(src, m)
fmt.Println(m)
}
If you can't or don't want to determine the slice's length beforehand, you are left with explicit instantiation:
var m []int
m, _ = FromInterfaceSlice[int]([]interface{}{1, 2, 3})
// ^^^ explicit type argument
Also the type parameters are still not inferrable with := shorthand declaration:
// what is m???
m, err := FromInterfaceSlice([]interface{}{1, 2, 3})
I have a function, that takes an empty interface (any type, which im looking for 2 in specific) and then returning a slice of the selected type.
func testingInterface(temp interface{}) (interface{}, interface{}) {
var doc interface{}
array := make([]interface{}, 3)
switch x := temp.(type) {
case int:
doc = x
tempArray := make([]string, 3)
for i, v := range tempArray {
array[i] = string(v)
}
fmt.Printf("Int to string %T, %T ", doc, tempArray)
case string:
doc = x
tempArray := make([]int, 3)
for i, v := range tempArray {
array[i] = int(v)
}
fmt.Printf("String to int %T, %T ", doc, tempArray)
}
return array, doc
}
So what happens, it is that the doc variable indeed changes the type of it, but the slice when i return it, it stays as []interface{}
When i test an element individual, it changes the type but the whole array it is still an []interface{}
The tempArray in the question has the slice you want. Return it instead of copying the values to the []interface{} that you don't want.
Use this code:
func testingInterface(x interface{}) (interface{}, interface{}) {
var result interface{}
switch x.(type) {
case int:
result = make([]int, 3)
case string:
result = make([]string, 3)
}
return result, x
}
Suppose I would like to apply a function to every element in a list, and then put the resulting values in another list so I can immediately use them. In python, I would do something like this:
list = [1,2,3]
str = ', '.join(multiply(x, 2) for x in list)
In Go, I do something like this:
list := []int{1,2,3}
list2 := []int
for _,x := range list {
list2 := append(list2, multiply(x, 2))
}
str := strings.Join(list2, ", ")
Is it possible to do this in a shorter way?
I would do exactly as you did, with a few tweaks to fix typos
import (
"fmt"
"strconv"
"strings"
)
func main() {
list := []int{1,2,3}
var list2 []string
for _, x := range list {
list2 = append(list2, strconv.Itoa(x * 2)) // note the = instead of :=
}
str := strings.Join(list2, ", ")
fmt.Println(str)
}
This is an old question, but was the top hit in my Google search, and I found information that I believe will be helpful to the OP and anyone else who arrives here, looking for the same thing.
There is a shorter way, although you have to write the map function yourself.
In go, func is a type, which allows you to write a function that accepts as input the subject slice and a function, and which iterates over that slice, applying that function.
See the Map function near the bottom of this Go by Example page : https://gobyexample.com/collection-functions
I've included it here for reference:
func Map(vs []string, f func(string) string) []string {
vsm := make([]string, len(vs))
for i, v := range vs {
vsm[i] = f(v)
}
return vsm
}
You then call it like so:
fmt.Println(Map(strs, strings.ToUpper))
So, yes: The shorter way you are looking for exists, although it is not built into the language itself.
I've created a small utility package with Mapand Filter methods now that generics have been introduced in 1.18 :)
https://pkg.go.dev/github.com/sa-/slicefunk
Example usage
package main
import (
"fmt"
sf "github.com/sa-/slicefunk"
)
func main() {
original := []int{1, 2, 3, 4, 5}
newArray := sf.Map(original, func(item int) int { return item + 1 })
newArray = sf.Map(newArray, func(item int) int { return item * 3 })
newArray = sf.Filter(newArray, func(item int) bool { return item%2 == 0 })
fmt.Println(newArray)
}
With go1.18+ you can write a much cleaner generic Map function:
func Map[T, V any](ts []T, fn func(T) V) []V {
result := make([]V, len(ts))
for i, t := range ts {
result[i] = fn(t)
}
return result
}
Usage, e.g:
input := []int{4, 5, 3}
outputInts := Map(input, func(item int) int { return item + 1 })
outputStrings := Map(input, func(item int) string { return fmt.Sprintf("Item:%d", item) })
Found a way to define a generic map array function
func Map(t interface{}, f func(interface{}) interface{} ) []interface{} {
switch reflect.TypeOf(t).Kind() {
case reflect.Slice:
s := reflect.ValueOf(t)
arr := make([]interface{}, s.Len())
for i := 0; i < s.Len(); i++ {
arr[i] = f(s.Index(i).Interface())
}
return arr
}
return nil
}
origin := []int{4,5,3}
newArray := Map(origin, func(item interface{}) interface{} { return item.(int) + 1})
You can use lo's Map in order to quickly apply a function to all elements. For example, in order to multiply by 2 and convert to string, you can use:
l := lo.Map[int, string]([]int{1, 2, 3, 4}, func(x int, _ int) string { return strconv.Itoa(x * 2) })
Then you can convert back to a comma delimited string like so:
strings.Join(l, ",")
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
}
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.
I am programming in Go programming language.
Say there's a variable of type interface{} that contains an array of integers. How do I convert interface{} back to []int?
I have tried
interface_variable.([]int)
The error I got is:
panic: interface conversion: interface is []interface {}, not []int
It's a []interface{} not just one interface{}, you have to loop through it and convert it:
the 2022 answer
https://go.dev/play/p/yeihkfIZ90U
func ConvertSlice[E any](in []any) (out []E) {
out = make([]E, 0, len(in))
for _, v := range in {
out = append(out, v.(E))
}
return
}
the pre-go1.18 answer
http://play.golang.org/p/R441h4fVMw
func main() {
a := []interface{}{1, 2, 3, 4, 5}
b := make([]int, len(a))
for i := range a {
b[i] = a[i].(int)
}
fmt.Println(a, b)
}
As others have said, you should iterate the slice and convert the objects one by one.
Is better to use a type switch inside the range in order to avoid panics:
a := []interface{}{1, 2, 3, 4, 5}
b := make([]int, len(a))
for i, value := range a {
switch typedValue := value.(type) {
case int:
b[i] = typedValue
break
default:
fmt.Println("Not an int: ", value)
}
}
fmt.Println(a, b)
http://play.golang.org/p/Kbs3rbu2Rw
Func return value is interface{} but real return value is []interface{}, so try this instead:
func main() {
values := returnValue.([]interface{})
for i := range values {
fmt.Println(values[i])
}
}