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])
}
}
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})
Is there any efficient way to get intersection of two slices in Go?
I want to avoid nested for loop like solution
slice1 := []string{"foo", "bar","hello"}
slice2 := []string{"foo", "bar"}
intersection(slice1, slice2)
=> ["foo", "bar"]
order of string does not matter
How do I get the intersection between two arrays as a new array?
Simple Intersection: Compare each element in A to each in B (O(n^2))
Hash Intersection: Put them into a hash table (O(n))
Sorted Intersection: Sort A and do an optimized intersection (O(n*log(n)))
All of which are implemented here
https://github.com/juliangruber/go-intersect
simple, generic and mutiple slices ! (Go 1.18)
Time Complexity : may be linear
func interSection[T constraints.Ordered](pS ...[]T) []T {
hash := make(map[T]*int) // value, counter
result := make([]T, 0)
for _, slice := range pS {
duplicationHash := make(map[T]bool) // duplication checking for individual slice
for _, value := range slice {
if _, isDup := duplicationHash[value]; !isDup { // is not duplicated in slice
if counter := hash[value]; counter != nil { // is found in hash counter map
if *counter++; *counter >= len(pS) { // is found in every slice
result = append(result, value)
}
} else { // not found in hash counter map
i := 1
hash[value] = &i
}
duplicationHash[value] = true
}
}
}
return result
}
func main() {
slice1 := []string{"foo", "bar", "hello"}
slice2 := []string{"foo", "bar"}
fmt.Println(interSection(slice1, slice2))
// [foo bar]
ints1 := []int{1, 2, 3, 9, 8}
ints2 := []int{10, 4, 2, 4, 8, 9} // have duplicated values
ints3 := []int{2, 4, 8, 1}
fmt.Println(interSection(ints1, ints2, ints3))
// [2 8]
}
playground : https://go.dev/play/p/lE79D0kOznZ
It's a best method for intersection two slice. Time complexity is too low.
Time Complexity : O(m+n)
m = length of first slice.
n = length of second slice.
func intersection(s1, s2 []string) (inter []string) {
hash := make(map[string]bool)
for _, e := range s1 {
hash[e] = true
}
for _, e := range s2 {
// If elements present in the hashmap then append intersection list.
if hash[e] {
inter = append(inter, e)
}
}
//Remove dups from slice.
inter = removeDups(inter)
return
}
//Remove dups from slice.
func removeDups(elements []string)(nodups []string) {
encountered := make(map[string]bool)
for _, element := range elements {
if !encountered[element] {
nodups = append(nodups, element)
encountered[element] = true
}
}
return
}
if there exists no blank in your []string, maybe you need this simple code:
func filter(src []string) (res []string) {
for _, s := range src {
newStr := strings.Join(res, " ")
if !strings.Contains(newStr, s) {
res = append(res, s)
}
}
return
}
func intersections(section1, section2 []string) (intersection []string) {
str1 := strings.Join(filter(section1), " ")
for _, s := range filter(section2) {
if strings.Contains(str1, s) {
intersection = append(intersection, s)
}
}
return
}
Try it
https://go.dev/play/p/eGGcyIlZD6y
first := []string{"one", "two", "three", "four"}
second := []string{"two", "four"}
result := intersection(first, second) // or intersection(second, first)
func intersection(first, second []string) []string {
out := []string{}
bucket := map[string]bool{}
for _, i := range first {
for _, j := range second {
if i == j && !bucket[i] {
out = append(out, i)
bucket[i] = true
}
}
}
return out
}
https://github.com/viant/toolbox/blob/a46fd679bbc5d07294b1d1b646aeacd44e2c7d50/collections.go#L869-L920
Another O(m+n) Time Complexity solution that uses a hashmap.
It has two differences compared to the other solutions discussed here.
Passing the target slice as a parameter instead of new slice returned
Faster to use for commonly used types like string/int instead of reflection for all
Yes there are a few different ways to go about it.. Here's an example that can be optimized.
package main
import "fmt"
func intersection(a []string, b []string) (inter []string) {
// interacting on the smallest list first can potentailly be faster...but not by much, worse case is the same
low, high := a, b
if len(a) > len(b) {
low = b
high = a
}
done := false
for i, l := range low {
for j, h := range high {
// get future index values
f1 := i + 1
f2 := j + 1
if l == h {
inter = append(inter, h)
if f1 < len(low) && f2 < len(high) {
// if the future values aren't the same then that's the end of the intersection
if low[f1] != high[f2] {
done = true
}
}
// we don't want to interate on the entire list everytime, so remove the parts we already looped on will make it faster each pass
high = high[:j+copy(high[j:], high[j+1:])]
break
}
}
// nothing in the future so we are done
if done {
break
}
}
return
}
func main() {
slice1 := []string{"foo", "bar", "hello", "bar"}
slice2 := []string{"foo", "bar"}
fmt.Printf("%+v\n", intersection(slice1, slice2))
}
Now the intersection method defined above will only operate on slices of strings, like your example.. You can in theory create a definition that looks like this func intersection(a []interface, b []interface) (inter []interface), however you would be relying on reflection and type casting so that you can compare, which will add latency and make your code harder to read. It's probably easier to maintain and read to write a separate function for each type you care about.
func intersectionString(a []string, b []string) (inter []string),
func intersectionInt(a []int, b []int) (inter []int),
func intersectionFloat64(a []Float64, b []Float64) (inter []Float64), ..ect
You can then create your own package and reuse once you settle how you want to implement it.
package intersection
func String(a []string, b []string) (inter []string)
func Int(a []int, b []int) (inter []int)
func Float64(a []Float64, b []Float64) (inter []Float64)
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.
This question already has answers here:
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
}