Is there a way to write generic code to find out whether a slice contains specific element in Go?

Question

I want to know is there a generic way to write code to judge whether a slice contains an element, I find it will frequently useful since there is a lot of logic to fist judge whether specific elem is already in a slice and then decide what to do next. But there seemed not a built-in method for that(For God's sake, why?)

I try to use interface{} to do that like:

func sliceContains(slice []interface{}, elem interface{}) bool { for _, item := range slice { if item == elem { return true } } return false }

I thought interface{} is sort of like Object of Java, but apparently, I was wrong. Should I write this every time meet with a new struct of slice? Isn't there a generic way to do this?

Here's the modern way, should work since Go 1.18: pkg.go.dev/golang.org/x/exp/slices#Contains — Johan Walles
– Johan Walles, Commented Apr 21, 2023 at 14:02

Not_a_Golfer · Accepted Answer · 2015-03-03 10:26:10Z

You can do it with reflect, but it will be MUCH SLOWER than a non-generic equivalent function:

func Contains(slice, elem interface{}) bool { sv := reflect.ValueOf(slice) // Check that slice is actually a slice/array. // you might want to return an error here if sv.Kind() != reflect.Slice && sv.Kind() != reflect.Array { return false } // iterate the slice for i := 0; i < sv.Len(); i++ { // compare elem to the current slice element if elem == sv.Index(i).Interface() { return true } } // nothing found return false } func main(){ si := []int {3, 4, 5, 10, 11} ss := []string {"hello", "world", "foo", "bar"} fmt.Println(Contains(si, 3)) fmt.Println(Contains(si, 100)) fmt.Println(Contains(ss, "hello")) fmt.Println(Contains(ss, "baz")) }

How much slower? about x50-x60 slower: Benchmarking against a non generic function of the form:

func ContainsNonGeneic(slice []int, elem int) bool { for _, i := range slice { if i == elem { return true } } return false }

I'm getting:

Generic: N=100000, running time: 73.023214ms 730.23214 ns/op
Non Generic: N=100000, running time: 1.315262ms 13.15262 ns/op

Ainar-G · Accepted Answer · 2015-03-03 10:16:00Z

You can make it using the reflect package like that:

func In(s, e interface{}) bool { slice, elem := reflect.ValueOf(s), reflect.ValueOf(e) for i := 0; i < slice.Len(); i++ { if reflect.DeepEqual(slice.Index(i).Interface(), elem.Interface()) { return true } } return false }

Playground examples: http://play.golang.org/p/TQrmwIk6B4

Alternatively, you can:

define an interface and make your slices implement it
use maps instead of slices
just write a simple for loop

What way to choose depends on the problem you are solving.

425nesp · Accepted Answer · 2015-03-03 10:13:56Z

I'm not sure what your specific context is, but you'll probably want to use a map to check if something already exists.

package main import "fmt" type PublicClassObjectBuilderFactoryStructure struct { Tee string Hee string } func main() { // Empty structs occupy zero bytes. mymap := map[interface{}]struct{}{} one := PublicClassObjectBuilderFactoryStructure{Tee: "hi", Hee: "hey"} two := PublicClassObjectBuilderFactoryStructure{Tee: "hola", Hee: "oye"} three := PublicClassObjectBuilderFactoryStructure{Tee: "hi", Hee: "again"} mymap[one] = struct{}{} mymap[two] = struct{}{} // The underscore is ignoring the value, which is an empty struct. if _, exists := mymap[one]; exists { fmt.Println("one exists") } if _, exists := mymap[two]; exists { fmt.Println("two exists") } if _, exists := mymap[three]; exists { fmt.Println("three exists") } }

Another advantage of using maps instead of a slice is that there is a built-in delete function for maps. https://play.golang.org/p/dmSyyryyS8

Rick-777 · Accepted Answer · 2015-03-04 13:44:02Z

If you want a rather different solution, you might try the code-generator approach offered by tools such as Gen. Gen writes source code for each concrete class you want to hold in a slice, so it supports type-safe slices that let you search for the first match of an element.

(Gen also offers a few other kinds of collection and allows you to write your own.)

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