rango sobre la interfaz{} que almacena un segmento

Given the scenario where you have a function which accepts t interface{}. If it is determined that the t is a slice, how do I range over that slice?

func main() {
    data := []string{"one","two","three"}
    test(data)
    moredata := []int{1,2,3}
    test(data)
}

func test(t interface{}) {
    switch reflect.TypeOf(t).Kind() {
    case reflect.Slice:
        // how do I iterate here?
        for _,value := range t {
            fmt.Println(value)
        }
    }
}

Go Playground Example: http://play.golang.org/p/DNldAlNShB

preguntado el 24 de diciembre de 12 a las 21:12

Why not have the function take a []interface{} instead? Are you trying to handle multiple complex types? -

Yes, it's for a templating system so interface{} could be a map, struct, slice, or array. In the real code there are many more case statements, but I removed them from the post to make the problem more concise. -

Jeremy, a []string is not a subtype of []interface{}, so you can't call a func([]interface{}) function with a []string or []int, etc. It would be nice to have a feature in Go to have a type meaning "slice of something", where you can then iterate over the elements as interface{}, but unfortunately you need reflection for that. -

@JeremyWall You can't use []interface{} to act as slice. You can refer aquí. -

5 Respuestas

Bueno, yo solía reflect.ValueOf and then if it is a slice you can call Len() y Index() sobre el valor para obtener el len of the slice and element at an index. I don't think you will be able to use the range operate to do this.

package main

import "fmt"
import "reflect"

func main() {
    data := []string{"one","two","three"}
    test(data)
    moredata := []int{1,2,3}
    test(moredata)
} 

func test(t interface{}) {
    switch reflect.TypeOf(t).Kind() {
    case reflect.Slice:
        s := reflect.ValueOf(t)

        for i := 0; i < s.Len(); i++ {
            fmt.Println(s.Index(i))
        }
    }
}

Go Playground Example: http://play.golang.org/p/gQhCTiwPAq

Respondido 03 ago 16, 14:08

Works great, thank you. The only thing to add would be that s.Index(i) devuelve un reflect.Value so in my case I needed s.Index(i).Interface() to reference the actual value. - Nucleón

What would you do if you had a pointer to a slice in interface{}? p.ej moredata := &[]int{1,2,3} - ryan paredes

Answering my question: If you have a pointer to a slice instead of a slice, you'll need to use Elem() to get the underlying value. e.g. reflect.TypeOf(reflect.ValueOf(t).Elem().Interface()).Kind() - ryan paredes

what if I want to get value of interface field from slice of interface without any struct. I have tried this solution but it is limited to get interface reference only from slice not the actual values of fields. - Amandeep Kaur

woh many thanks, that trick saved me a lot of time (and headache!) - Nicolás Garnier

You don't need to use reflection if you know which types to expect. You can use a interruptor de tipo, Así:

package main

import "fmt"

func main() {
    loop([]string{"one", "two", "three"})
    loop([]int{1, 2, 3})
}

func loop(t interface{}) {
    switch t := t.(type) {
    case []string:
        for _, value := range t {
            fmt.Println(value)
        }
    case []int:
        for _, value := range t {
            fmt.Println(value)
        }
    }
}

Check out the code on the playground.

respondido 01 nov., 18:09

This will not work over an array though, only over a slice - user1028741

@user1028741 No, the code works for every type including arrays + You can always take a slice from an array array[:]. - Inanc Gumus

But the type of an array will be with the actual capacity of it. [3]int is not of the same type as [2]int or []int. Therefore, if the type of 't' is actually an array the relevant case will not apply. - user1028741

What I mean is your example will not work on arrays. I changed it here: play.golang.org/p/DAsg_0aXz-r - user1028741

You can't easily make it work, if you don't know the type of your parameter in the first place. In order to use your trick (array[:]), you'll have to use reflection to decide that it is an array, then cast to array - then generate a slice of it. That's why I said it works on a slice, not on an array. Clearly with enough effort you can produce a slice out of the array... - user1028741

there is one exception from the way interface{} behaves, @Jeremy Wall gave already pointer. if the passed data is defined as []interface{} initially.

package main

import (
    "fmt"
)

type interfaceSliceType []interface{}

var interfaceAsSlice interfaceSliceType

func main() {
    loop(append(interfaceAsSlice, 1, 2, 3))
    loop(append(interfaceAsSlice, "1", "2", "3"))
    // or
    loop([]interface{}{[]string{"1"}, []string{"2"}, []string{"3"}})
    fmt.Println("------------------")


    // and of course one such slice can hold any type
    loop(interfaceSliceType{"string", 999, map[int]string{3: "three"}})
}

func loop(slice []interface{}) {
    for _, elem := range slice {
        switch elemTyped := elem.(type) {
        case int:
            fmt.Println("int:", elemTyped)
        case string:
            fmt.Println("string:", elemTyped)
        case []string:
            fmt.Println("[]string:", elemTyped)
        case interface{}:
            fmt.Println("map:", elemTyped)
        }
    }
}

salida:

int: 1
int: 2
int: 3
string: 1
string: 2
string: 3
[]string: [1]
[]string: [2]
[]string: [3]
------------------
string: string
int: 999
map: map[3:three]

pruébela

Respondido 07 Abr '19, 22:04

Expanding on the answer provided by masebase, you could generalize the iteration on an interface{} slice with a function like this:

func forEachValue(ifaceSlice interface{}, f func(i int, val interface{})) {
    v := reflect.ValueOf(ifaceSlice)
    if v.Kind() == reflect.Ptr {
        v = v.Elem()
    }
    if v.Kind() != reflect.Slice {
        panic(fmt.Errorf("forEachValue: expected slice type, found %q", v.Kind().String()))
    }

    for i := 0; i < v.Len(); i++ {
        val := v.Index(i).Interface()
        f(i, val)
    }
}

Entonces, lo usas así:

func main() {
    data := []string{"one","two","three"}
    test(data)
    moredata := []int{1,2,3}
    test(data)
}

func test(sliceIface interface{}) {
    forEachValue(sliceIface, func(i int, value interface{}) {
      fmt.Println(value)
    }
}

Respondido el 09 de enero de 21 a las 06:01

a common in function

import (
    "fmt"
    "reflect"
)

func In(item interface{}, list interface{}) (ret bool, err error) {
    defer func() {
        if r := recover(); r != nil {
            ret, err = false, fmt.Errorf("%v", r)
        }
    }()
    itemValue, listValue := reflect.ValueOf(item), reflect.ValueOf(list)
    for i := 0; i < listValue.Len(); i++ {
        if listValue.Index(i).Interface() == itemValue.Interface() {
            return true, nil
        }
    }
    return false, nil
}

not so graceful but works well

fmt.Println(In(1, []int(2, 1, 3)))

respondido 12 mar '21, 04:03

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