in_array() in Go

Question

This function's objective is very simple. It takes array and checks if val is a value inside of this array. It returns whether val exists and which is the index inside the array that contains val.

Could this be coded in a better way? How would I implement this to work with any kind of array (the code below works only with arrays of strings)?

package main

import "fmt"

func in_array(val string, array []string) (exists bool, index int) {
    exists = false
    index = -1;

    for i, v := range array {
        if val == v {
            index = i
            exists = true
            return
        }   
    }

    return
}

func main() {
    names := []string{ "Mary", "Anna", "Beth", "Johnny", "Beth" }

    fmt.Println( in_array("Jack", names) )
}

Go probably provides a similar function, but this is for study purposes only.

Answer 1

Rather than tie yourself to only one type (string), you could use the reflect package as well as interfaces to make it somewhat type indifferent. The following is my reworking of your code:

package main

import "fmt"
import "reflect"

func in_array(val interface{}, array interface{}) (exists bool, index int) {
    exists = false
    index = -1

    switch reflect.TypeOf(array).Kind() {
    case reflect.Slice:
        s := reflect.ValueOf(array)

        for i := 0; i < s.Len(); i++ {
            if reflect.DeepEqual(val, s.Index(i).Interface()) == true {
                index = i
                exists = true
                return
            }
        }
    }

    return
}

Note that we now import the reflect package. We also changed the types of val and array to interface{} so that we may pass any type in. We then use the reflect.Typeof() to glean the reflection reflect.Type of the value in the array interface{}. We then glean the type with Kind(), and use a case to fall into our inner code if its a slice (can add more cases to extend this).

In our inner code, we get the value of the array argument, and store it in s. We then iterate over the length of s, and compare val to s at the index i declared as an interface with Interface() and check for truthiness. If its true, we exit with a true and the index.

Running the main function with both a slice of strings and a slice of integers, as follows, works:

func main() {
    names := []string{"Mary", "Anna", "Beth", "Johnny", "Beth"}
    fmt.Println(in_array("Anna", names))
    fmt.Println(in_array("Jon", names))

    ints := []int{1, 4, 3, 2, 6}
    fmt.Println(in_array(3, ints))
    fmt.Println(in_array(95, ints))
}

The above example gets us:

true 1
false -1
true 2
false -1

EDIT: June 2021 - refactored the above code as follows:

func inArray(val interface{}, array interface{}) (index int) {
    values := reflect.ValueOf(array)

    if reflect.TypeOf(array).Kind() == reflect.Slice || values.Len() > 0 {
        for i := 0; i < values.Len(); i++ {
            if reflect.DeepEqual(val, values.Index(i).Interface()) {
                return i
            }
        }
    }

    return -1
}

with the runner function:

func main() {
    name := "Mary Anna"
    fmt.Println(inArray("Anna", name))

    var empty []string
    fmt.Println(inArray("Anna", empty))

    names := []string{"Mary", "Anna", "Beth", "Johnny", "Beth"}
    fmt.Println(inArray("Anna", names))
    fmt.Println(inArray("Jon", names))

    ints := []int{1, 4, 3, 2, 6}
    fmt.Println(inArray(3, ints))
    fmt.Println(inArray(95, ints))
}

Answer 2

Using reflection for that simple task is a slow and an inefficient way of doing it, your first approach was fine, you could trim it a little, for example:

func in_array(val string, array []string) (ok bool, i int) {
    for i = range array {
        if ok = array[i] == val; ok {
            return
        }
    }
    return
}

Generic version using interfaces:

func in_array(v interface{}, in interface{}) (ok bool, i int) {
    val := reflect.Indirect(reflect.ValueOf(in))
    switch val.Kind() {
    case reflect.Slice, reflect.Array:
        for ; i < val.Len(); i++ {
            if ok = v == val.Index(i).Interface(); ok {
                return
            }
        }
    }
    return
}

playground

Answer 3

Now with go 1.18 and generics you can have a type-safe version of this:

// Index returns the index of the first occurrence of v in s, or -1 if not present.
func Index[E comparable](s []E, v E) int {
    for i, vs := range s {
        if v == vs {
            return i
        }
    }
    return -1
}

Couple of notes here:

• You can simplify the interface by returning -1 instead of (ok, val) tuple. This will also make sure the code that uses the function is error-prone and does not use the value w/o checking.
• In your case you may actually want to consider using a set (map[string]struct{}) to get a O(1) runtime.
• nit: naming-wise you do not want to have underscores in the function.

Please note that golang.org/x/exp/slices has this exact function among with lots of useful generic code.