I'm creating a utility package for my project.
Many of my string slices need a function to
1. remove duplicates
2. remove empty strings
I know 1 way to do this:1. Add a function for each case which accepts a string slice and returns a string slice
func removeEmpty(s []string) []string {
i := 0 // i points to next available pos
for _, v := range s {
if v != "" {
s[i] = v
i++
}
}
return s[:i]
}
func dedup(s []string) []string {
i := 0 // i points to next available pos
mp := map[string]bool{}
for _, v := range s {
if _, ok := mp[v]; !ok {
s[i] = v
mp[v] = true
i++
}
}
return s[:i]
}
when I apply these 2 functions to my slice, I can do:
mySlice := string[]{}
mySlice = dedup(removeEmpty(mySlice))
I want to make it somthing like:
mySlice = mySlice.dedup().removeEmpty()
or
mySlice.dedup().removeEmpty()
Maybe I can add custom method to slice[] ?
I tried writing it
func (s []string) removeEmpty() {
}
I'm getting complie error.
You can't define methods on []string, but you can define your own type based on []string and define methods on that:
type myStringSlice []string
func (s myStringSlice) removeEmpty() myStringSlice {
i := 0 // i points to next available pos
for _, v := range s {
if v != "" {
s[i] = v
i++
}
}
return s[:i]
}
func (s myStringSlice) dedup() myStringSlice {
i := 0 // i points to next available pos
mp := map[string]bool{}
for _, v := range s {
if _, ok := mp[v]; !ok {
s[i] = v
mp[v] = true
i++
}
}
return s[:i]
}
See https://play.golang.org/p/u1z_N3c_wPP.
As mentioned in the documentation:
You can only declare a method with a receiver whose type is defined in the same package as the method. You cannot declare a method with a receiver whose type is defined in another package.
So you have to declare a type on []string and then declare the method on your own type.
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 want to take data from DB and write to excel
let's say I have a struct like:
type user struct {
ID int64
Name string
Age int
}
I can get a pointer to slice of user type form DB &[]user{}
but I want to convert that slice to a 2D slice of string [][]string{}
and here's my code try to do such job:
func toStrings(slice interface{}) [][]string {
switch reflect.TypeOf(slice).Elem().Kind() {
case reflect.Slice:
ret := [][]string{}
val := reflect.ValueOf(slice).Elem()
for i := 0; i < val.Len(); i++ {
tempSlice := []string{}
tempV := reflect.ValueOf(val.Index(i))
for j := 0; j < tempV.NumField(); j++ {
tempSlice = append(tempSlice, tempV.Field(j).String())
}
ret = append(ret, tempSlice)
}
return ret
}
return nil
}
But from the code above all I get is a slice like [<*reflect.rtype Value> <unsafe.Pointer Value> <reflect.flag Value>]
where I do it wrong?
my codes in golang playground
sorry, I found where I do it wrong, I got tempV wrong
func toStrings(slice interface{}) [][]string {
switch reflect.TypeOf(slice).Elem().Kind() {
case reflect.Slice:
ret := [][]string{}
val := reflect.ValueOf(slice).Elem()
for i := 0; i < val.Len(); i++ {
tempSlice := []string{}
// tempV should be:
tempV := val.Index(i)
// instead of reflect.ValueOf(val.Index(i))
for j := 0; j < tempV.NumField(); j++ {
tempSlice = append(tempSlice, tempV.Field(j).String())
}
ret = append(ret, tempSlice)
}
return ret
}
return nil
}
There are two problems in the code in the question. The first problem is the slice element is doubled wrapped by a a reflect.Value in the expression reflect.Value(val.Index(i)). Fix by removing the extra call to reflect.Value. The second problem is that the reflect.Value String method does not convert the underlying value to its string representation. Use fmt.Sprint (or one of its friends) to do that.
Try this:
func toStrings(slice interface{}) [][]string {
// Get reflect value for slice. Use Indirect to
// handle slice argument and pointer to slice
// argument.
v := reflect.Indirect(reflect.ValueOf(slice))
if v.Kind() != reflect.Slice {
return nil
}
var result [][]string
// For each element...
for i := 0; i < v.Len(); i++ {
// Get reflect value for slice element (a struct). Use
// Indirect to handle slice of struct and slice of
// pointer to struct.
e := reflect.Indirect(v.Index(i))
if e.Kind() != reflect.Struct {
return nil
}
// Convert fields to string and append.
var element []string
for i := 0; i < e.NumField(); i++ {
// Use fmt.Sprint to convert arbitrary Go value
// to a string.
element = append(element, fmt.Sprint(e.Field(i).Interface()))
}
result = append(result, element)
}
return result
}
Run it on the playground.
Maybe I have a simple way to resolve the problem, golang playground here
I used encoding/json to convert to json data, then convert it to map[string]interface{}.
func toStrings2(slice interface{}) [][]string {
jsonData, _ := json.Marshal(slice)
var out []map[string]interface{}
_ = json.Unmarshal(jsonData, &out)
var fields []string
if len(out) > 0 {
for k := range out[0] {
fields = append(fields, k)
}
}
var ret [][]string
for _, row := range out {
var r []string
for _, k := range fields {
r = append(r, fmt.Sprint(row[k]))
}
ret = append(ret, r)
}
return ret
}
Notice:
With the help of #CeriseLimón, I known that the code in this answer can't handle large values for User.ID.
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
}
I have the following code to double the slice.
func doubleSlice(s []int) []int {
t := make([]int, len(s), (cap(s) + 1) * 2 )
for i := range s {
t[i] = s[i]
}
return t
}
I want to make the func to double any type of slice. And I need to know the element type first.
func showInterfaceItem(s interface{}) interface{} {
if reflect.TypeOf(s).Kind() != reflect.Slice {
fmt.Println("The interface is not a slice.")
return
}
var t interface{}
newLen := reflect.ValueOf(s).Len()
newCap := (cap(reflect.ValueOf(s).Cap()) + 1) * 2
t = make([]reflect.TypeOf(s), newLen, newCap)
return t
}
The reflect.TypeOf(s) return the type of interface{}, not the type of element. How can I get the element type of slice interface?
You can use reflect.TypeOf(s).Elem()
to get the type of element of slice.
package main
import (
"fmt"
"reflect"
)
func doubleSlice(s interface{}) interface{} {
if reflect.TypeOf(s).Kind() != reflect.Slice {
fmt.Println("The interface is not a slice.")
return nil
}
v := reflect.ValueOf(s)
newLen := v.Len()
newCap := (v.Cap() + 1) * 2
typ := reflect.TypeOf(s).Elem()
t := reflect.MakeSlice(reflect.SliceOf(typ), newLen, newCap)
reflect.Copy(t, v)
return t.Interface()
}
func main() {
xs := doubleSlice([]string{"foo", "bar"}).([]string)
fmt.Println("data =", xs, "len =", len(xs), "cap =", cap(xs))
ys := doubleSlice([]int{3, 1, 4}).([]int)
fmt.Println("data =", ys, "len =", len(ys), "cap =", cap(ys))
}
The output will be:
data = [foo bar] len = 2 cap = 6
data = [3 1 4] len = 3 cap = 8
Check it in: Go Playground
This is doable in golang and takes me whole day to discover the pattern.
Firstly, we want to get a pointer of slice to make gorm happy, which is has type "*[]Obj". To achieve that in golang, we can create a make wrapper like so:
func makeWrapper(cap uint) interface{} {
arr:= make([]Sth, 0, cap)
return &arr
}
Notice that, we can't directly reference the maked value, which might be the book keeping data need to have a stack space to store.
//Not working example
func makeWrapper(cap uint) interface{} {
return &(make([]Sth, 0, cap))
}
And as the answer before, the reflect.MakeSlice(reflect.SliceOf(typ), 0, capacity).Interface() returns interface{[]Sth}. (the typ here is refer to reflect.TypeOf(Sth{}), which equiv to typ == reflect.TypeOf(v))
Thus we need to create a return object of *[]Sth and the value inside is a slice []Sth with capacity. After understanding the objective, we can have this code:
package main
import (
"reflect"
)
type Sth struct {
a, b string
}
func main() {
af:= createSlice(Sth{})
arr := makeWrapper(10).(*[]Sth)
println(reflect.TypeOf(arr).String())
// equiv to makeWrapper, but we do it via reflection
arr = af(10).(*[]Sth)
println(reflect.TypeOf(arr).String())
}
func makeWrapper(cap uint) interface{} {
arr:= make([]Sth, 0, cap)
return &arr
}
func createSlice(v interface{}) func(int) interface{} {
var typ reflect.Type
if reflect.ValueOf(v).Kind() == reflect.Ptr {
typ = reflect.ValueOf(v).Elem().Type()
} else if reflect.ValueOf(v).Kind() == reflect.Struct {
typ = reflect.TypeOf(v)
} else {
panic("only support instance of struct or pointer of that instance")
}
return func(capacity int) interface{}{
// create the outer object saves our slice
outerObj:=reflect.New(reflect.SliceOf(typ))
// create the slice and save it to return
outerObj.Elem().Set(reflect.MakeSlice(reflect.SliceOf(typ), 0, capacity))
// retrive the interface of outer object
return outerObj.Interface()
}
}
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.