Contains method for a slice

Is there anything similar to a slice.contains(object) method in Go without having to do a search through each element in a slice?

Mostafa has already pointed out that such a method is trivial to write, and mkb gave you a hint to use the binary search from the sort package. But if you are going to do a lot of such contains checks, you might also consider using a map instead.

It's trivial to check if a specific map key exists by using the value, ok := yourmap[key] idiom. Since you aren't interested in the value, you might also create a map[string]struct{} for example. Using an empty struct{} here has the advantage that it doesn't require any additional space and Go's internal map type is optimized for that kind of values. Therefore, map[string] struct{} is a popular choice for sets in the Go world.

No, such method does not exist, but is trivial to write:

func contains(s []int, e int) bool {
    for _, a := range s {
	    if a == e {
		    return true
	    }
    }
    return false
}

You can use a map if that lookup is an important part of your code, but maps have cost too.

As of Go 1.21, you can use the stdlib slices package which was promoted from the experimental package previously mentioned.

import 	"slices"

things := []string{"foo", "bar", "baz"}
slices.Contains(things, "foo") // true

Original Answer:

Starting with Go 1.18, you can use the slices package – specifically the generic Contains function:
https://pkg.go.dev/golang.org/x/exp/slices#Contains.

go get golang.org/x/exp/slices

import 	"golang.org/x/exp/slices"

things := []string{"foo", "bar", "baz"}
slices.Contains(things, "foo") // true

Note that since this is outside the stdlib as an experimental package, it is not bound to the Go 1 Compatibility Promise™ and may change before being formally added to the stdlib.

With Go 1.18+ we could use generics.

func Contains[T comparable](s []T, e T) bool {
	for _, v := range s {
		if v == e {
			return true
		}
	}
	return false
}

The sort package provides the building blocks if your slice is sorted or you are willing to sort it.

input := []string{"bird", "apple", "ocean", "fork", "anchor"}
sort.Strings(input)

fmt.Println(contains(input, "apple")) // true
fmt.Println(contains(input, "grow"))  // false

...

func contains(s []string, searchterm string) bool {
	i := sort.SearchStrings(s, searchterm)
	return i < len(s) && s[i] == searchterm
}

SearchString promises to return the index to insert x if x is not present (it could be len(a)), so a check of that reveals whether the string is contained the sorted slice.

Instead of using a slice, map may be a better solution.

simple example:

package main

import "fmt"


func contains(slice []string, item string) bool {
	set := make(map[string]struct{}, len(slice))
	for _, s := range slice {
		set
展开收缩
 = struct{}{}
	}
	
	_, ok := set[item] 
	return ok
}

func main() {

	s := []string{"a", "b"}
	s1 := "a"
	fmt.Println(contains(s, s1))
	
}

http://play.golang.org/p/CEG6cu4JTf

If the slice is sorted, there is a binary search implemented in the sort package.

func Contain(target interface{}, list interface{}) (bool, int) {
	if reflect.TypeOf(list).Kind() == reflect.Slice || reflect.TypeOf(list).Kind() == reflect.Array {
		listvalue := reflect.ValueOf(list)
		for i := 0; i < listvalue.Len(); i++ {
			if target == listvalue.Index(i).Interface() {
				return true, i
			}
		}
	}
	if reflect.TypeOf(target).Kind() == reflect.String && reflect.TypeOf(list).Kind() == reflect.String {
		return strings.Contains(list.(string), target.(string)), strings.Index(list.(string), target.(string))
	}
	return false, -1
}

I think map[x]bool is more useful than map[x]struct{}.

Indexing the map for an item that isn't present will return false. so instead of _, ok := m[X], you can just say m[X].

This makes it easy to nest inclusion tests in expressions.

You can use the reflect package to iterate over an interface whose concrete type is a slice:

func HasElem(s interface{}, elem interface{}) bool {
    arrV := reflect.ValueOf(s)

	if arrV.Kind() == reflect.Slice {
    	for i := 0; i < arrV.Len(); i++ {
	
	    	// XXX - panics if slice element points to an unexported struct field
		    // see https://golang.org/pkg/reflect/#Value.Interface
			if arrV.Index(i).Interface() == elem {
    			return true
	    	}
	    }
	}

    return false
}

https://play.golang.org/p/jL5UD7yCNq

Not sure generics are needed here. You just need a contract for your desired behavior. Doing the following is no more than what you would have to do in other languages if you wanted your own objects to behave themselves in collections, by overriding Equals() and GetHashCode() for instance.

type Identifiable interface{
	GetIdentity() string
}

func IsIdentical(this Identifiable, that Identifiable) bool{
	return (&this == &that) || (this.GetIdentity() == that.GetIdentity())
}

func contains(s []Identifiable, e Identifiable) bool {
	for _, a := range s {
		if IsIdentical(a,e) {
			return true
		}
	}
	return false
}

If it is not feasable to use a map for finding items based on a key, you can consider the goderive tool. Goderive generates a type specific implementation of a contains method, making your code both readable and efficient.

Example;

type Foo struct {
    Field1 string
    Field2 int
} 

func Test(m Foo) bool {
     var allItems []Foo
     return deriveContainsFoo(allItems, m)
}

To generate the deriveContainsFoo method:

• Install goderive with go get -u github.com/awalterschulze/goderive
• Run goderive ./... in your workspace folder

This method will be generated for deriveContains:

func deriveContainsFoo(list []Foo, item Foo) bool {
	for _, v := range list {
		if v == item {
			return true
		}
	}
	return false
}

Goderive has support for quite some other useful helper methods to apply a functional programming style in go.

The go style:

func Contains(n int, match func(i int) bool) bool {
	for i := 0; i < n; i++ {
		if match(i) {
			return true
		}
	}
	return false
}


s := []string{"a", "b", "c", "o"}
// test if s contains "o"
ok := Contains(len(s), func(i int) bool {
	return s[i] == "o"
})

Complementing @Adolfo's answer (and other answers here), the Contains function won't be something experimental anymore. Starting with GO v1.21, which will be released during August 2023, the aforementioned slice package will be included into the core library (besides some other interesting packages like map and cmp), making it possible to run a linear search over a slice of elements (O(N) time complexity).

Additionally you will have some other interesting variations for searching an element (or elements) within a slice, like the new ContainsFunc, which reports whether at least one element e of s satisfies f(e). You can check this at the example below, which is taken from the actual docs:

package main

import (
	"fmt"
	"slices"
)

func main() {
	numbers := []int{0, 42, -10, 8}
	hasNegative := slices.ContainsFunc(numbers, func(n int) bool {
		return n < 0
	})
	fmt.Println("Has a negative:", hasNegative)
	hasOdd := slices.ContainsFunc(numbers, func(n int) bool {
		return n%2 != 0
	})
	fmt.Println("Has an odd number:", hasOdd)
}

Also, to keep in mind, if you are working with a sorted slice and you want to reduce the time complexity when searching for an element (O(logN)), you will also be able to use the BinarySearch and the BinarySearchFunc functions, which will also come with this new package.

Finally, if you want to make the search constant in time (O(1)), I would go for the approach suggested by @tux21b in the voted answer by using maps.

If you have a byte slice, you can use bytes package:

package main
import "bytes"

func contains(b []byte, sub byte) bool {
   return bytes.Contains(b, []byte{sub})
}

func main() {
   b := contains([]byte{10, 11, 12, 13, 14}, 13)
   println(b)
}

Or suffixarray package:

package main
import "index/suffixarray"

func contains(b []byte, sub byte) bool {
   return suffixarray.New(b).Lookup([]byte{sub}, 1) != nil
}

func main() {
   b := contains([]byte{10, 11, 12, 13, 14}, 13)
   println(b)
}

If you have an int slice, you can use intsets package:

package main
import "golang.org/x/tools/container/intsets"

func main() {
   var s intsets.Sparse
   for n := 10; n < 20; n++ {
      s.Insert(n)
   }
   b := s.Has(16)
   println(b)
}

• https://golang.org/pkg/bytes
• https://golang.org/pkg/index/suffixarray
• https://pkg.go.dev/golang.org/x/tools/container/intsets

func contains(slice []string, item string) bool {
	for _, s := range slice {
		if s == item {
			return true
		}
	}
	return false
}

I created the following Contains function using reflect package.
This function can be used for various types like int32 or struct etc.

// Contains returns true if an element is present in a slice
func Contains(list interface{}, elem interface{}) bool {
	listV := reflect.ValueOf(list)

	if listV.Kind() == reflect.Slice {
		for i := 0; i < listV.Len(); i++ {
			item := listV.Index(i).Interface()

			target := reflect.ValueOf(elem).Convert(reflect.TypeOf(item)).Interface()
			if ok := reflect.DeepEqual(item, target); ok {
				return true
			}
		}
	}
	return false
}

Usage of contains function is below

// slice of int32
containsInt32 := Contains([]int32{1, 2, 3, 4, 5}, 3)
fmt.Println("contains int32:", containsInt32)

// slice of float64
containsFloat64 := Contains([]float64{1.1, 2.2, 3.3, 4.4, 5.5}, 4.4)
fmt.Println("contains float64:", containsFloat64)


// slice of struct
type item struct {
    ID   string
    Name string
}
list := []item{
    item{
        ID:   "1",
        Name: "test1",
    },
    item{
        ID:   "2",
        Name: "test2",
    },
    item{
        ID:   "3",
        Name: "test3",
    },
}
target := item{
	ID:   "2",
	Name: "test2",
}
containsStruct := Contains(list, target)
fmt.Println("contains struct:", containsStruct)

// Output:
// contains int32: true
// contains float64: true
// contains struct: true

Please see here for more details:
https://github.com/glassonion1/xgo/blob/main/contains.go

There are several packages that can help, but this one seems promising:

https://github.com/wesovilabs/koazee

var numbers = []int{1, 5, 4, 3, 2, 7, 1, 8, 2, 3}
contains, _ := stream.Contains(7)
fmt.Printf("stream.Contains(7): %v\n", contains)

It might be considered a bit 'hacky' but depending the size and contents of the slice, you can join the slice together and do a string search.

For example you have a slice containing single word values (e.g. "yes", "no", "maybe"). These results are appended to a slice. If you want to check if this slice contains any "maybe" results, you may use

exSlice := ["yes", "no", "yes", "maybe"]
if strings.Contains(strings.Join(exSlice, ","), "maybe") {
  fmt.Println("We have a maybe!")
}

How suitable this is really depends on the size of the slice and length of its members. There may be performance or suitability issues for large slices or long values, but for smaller slices of finite size and simple values it is a valid one-liner to achieve the desired result.