How to remove duplicates strings or int from Slice in Go

Sort and then compact, e.g. like this:

package main

import (
    "fmt"

    "slices"
)

func main() {
    list := []string{"c", "a", "b", "c", "b", "b"}
    slices.Sort(list)
    fmt.Println(slices.Compact(list)) // [a b c]
}

Adding this answer which worked for me, does require/include sorting, however.

func removeDuplicateStrings(s []string) []string {
    if len(s) < 1 {
        return s
    }

    sort.Strings(s)
    prev := 1
    for curr := 1; curr < len(s); curr++ {
        if s[curr-1] != s[curr] {
            s[prev] = s[curr]
            prev++
        }
    }

    return s[:prev]
}

For fun, I tried using generics! (Go 1.18+ only)

type SliceType interface {
    ~string | ~int | ~float64 // add more *comparable* types as needed
}

func removeDuplicates[T SliceType](s []T) []T {
    if len(s) < 1 {
        return s
    }

    // sort
    sort.SliceStable(s, func(i, j int) bool {
        return s[i] < s[j]
    })

    prev := 1
    for curr := 1; curr < len(s); curr++ {
        if s[curr-1] != s[curr] {
            s[prev] = s[curr]
            prev++
        }
    }

    return s[:prev]
}

Go Playground Link with tests: https://go.dev/play/p/bw1PP1osJJQ

You can do in-place replacement guided with a map:

processed := map[string]struct{}{}
w := 0
for _, s := range cities {
    if _, exists := processed[s]; !exists {
        // If this city has not been seen yet, add it to the list
        processed[s] = struct{}{}
        cities[w] = s
        w++
    }
}
cities = cities[:w]

reduce memory usage:

package main

import (
    "fmt"
    "reflect"
)

type void struct{}

func main() {

    digits := [6]string{"one", "two", "three", "four", "five", "five"}

    set := make(map[string]void, 6)

    for _, element := range digits {
        set[element] = void{}
    }

    fmt.Println(reflect.ValueOf(set).MapKeys())
}

p.s. playground

try: https://github.com/samber/lo#uniq

names := lo.Uniq[string]([]string{"Samuel", "John", "Samuel"})
// []string{"Samuel", "John"}

Go 1.20 adds the comparable generic type, which makes it possible to expand on Riyaz' answer like so:

func dedupeSlice[T comparable](sliceList []T) []T {
    dedupeMap := make(map[T]struct{})
    list := []T{}

    for _, slice := range sliceList {
        if _, exists := dedupeMap[slice]; !exists {
            dedupeMap[slice] = struct{}{}
            list = append(list, slice)
        }
    }

    return list
}

This allows you to feed in slices of any comparable type in the language, including strings, integers, floats, booleans, pointers, and more.

It can also be done with a set-like map:

ddpStrings := []string{}
m := map[string]struct{}{}

for _, s := range strings {
    if _, ok := m[scopeStr]; ok {
        continue
    }
    ddpStrings = append(ddpStrings, s)
    m[s] = struct{}{}
}

Simple to understand.

func RemoveDuplicate(array []string) []string {
    m := make(map[string]string)
    for _, x := range array {
        m[x] = x
    }
    var ClearedArr []string
    for x, _ := range m {
        ClearedArr = append(ClearedArr, x)
    }
    return ClearedArr
}

Updated answer based on Jasper's original answer now that slices is part of the standard library:

package main

import (
    "fmt"
    "slices"
)

func main() {
    list := []string{"a", "b", "c", "a", "a", "c", "d", "d"} //[a b c a a c d d]
    slices.Sort(list) //[a a a b c c d d]
    list = slices.Compact(list) //[a b c d]

    fmt.Printf("Compacted List: %v\n", list)
}

Playground: https://go.dev/play/p/n6ozDU8RFgN

If you want to don't waste memory allocating another array for copy the values, you can remove in place the value, as following:

package main

import "fmt"

var studentsCities = []string{"Mumbai", "Delhi", "Ahmedabad", "Mumbai", "Bangalore", "Delhi", "Kolkata", "Pune"}

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

func main() {
    fmt.Printf("Cities before remove: %+v\n", studentsCities)
    for i := 0; i < len(studentsCities); i++ {
        if contains(studentsCities[i+1:], studentsCities[i]) {
            studentsCities = remove(studentsCities, i)
            i--
        }
    }
    fmt.Printf("Cities after remove: %+v\n", studentsCities)
}
func remove(slice []string, s int) []string {
    return append(slice[:s], slice[s+1:]...)
}

Result:

Cities before remove: [Mumbai Delhi Ahmedabad Mumbai Bangalore Delhi Kolkata Pune]
Cities after remove: [Ahmedabad Mumbai Bangalore Delhi Kolkata Pune]

func UniqueNonEmptyElementsOf(s []string) []string {
    unique := make(map[string]bool, len(s))
    var us []string
    for _, elem := range s {
        if len(elem) != 0 {
            if !unique[elem] {
                us = append(us, elem)
                unique[elem] = true
            }
        }
    }

    return us
}

send the duplicated splice to the above function, this will return the splice with unique elements.

func main() {
    studentsCities := []string{"Mumbai", "Delhi", "Ahmedabad", "Mumbai", "Bangalore", "Delhi", "Kolkata", "Pune"}

    uniqueStudentsCities := UniqueNonEmptyElementsOf(studentsCities)
    
    fmt.Println(uniqueStudentsCities)
}

Based on Riyaz's solution, you can use generics since Go 1.18

func removeDuplicate[T string | int](tSlice []T) []T {
    allKeys := make(map[T]bool)
    list := []T{}
    for _, item := range tSlice {
        if _, value := allKeys[item]; !value {
            allKeys[item] = true
            list = append(list, item)
        }
    }
    return list
}

Generics minimizes code duplication.

Go Playground link : https://go.dev/play/p/Y3fEtHJpP7Q

So far @snassr has given the best answer as it is the most optimized way in terms of memory (no extra memory) and runtime (nlogn). But one thing I want to emphasis here is if we want to delete any index/element of an array we should loop from end to start as it reduces complexity. If we loop from start to end then if we delete nth index then we will accidentally miss the nth element (which was n+1th before deleting nth element) as in the next iteration we will get the n+1th element.

Example Code

func Dedup(strs []string) {
    sort.Strings(strs)
    for i := len(strs) - 1; i > 0; i-- {
        if strs[i] == strs[i-1] {
            strs = append(strs[:i], strs[i+1:]...)
        }
    }
}

Here is a little bit simpler solution than the accepted answer

func RemoveDuplicates[T comparable](slice []T) []T {
    for i := 0; i < len(slice); i++ {
        for j := len(slice) - 1; j > i; j-- {
            if slice[i] == slice[j] {
                slice = append(slice[:j], slice[j+1:]...)
            }
        }
    }
    return slice
}

Modern, generic solution to remove duplicates without reordering (stable):

func RemoveDuplicates[T comparable](s []T) []T {
    alreadySeen := make(map[T]struct{}, len(s))
    return slices.DeleteFunc(s, func(val T) bool {
        _, duplicate := alreadySeen[val]
        alreadySeen[val] = struct{}{}
        return duplicate
    })
}

It works on all comparable types and uses slices.DeleteFunc for memory efficiency.

Here's a mapless index based slice's duplicate "remover"/trimmer. It use a sort method.

The n value is always 1 value lower than the total of non duplicate elements that's because this methods compare the current (consecutive/single) elements with the next (consecutive/single) elements and there is no matches after the lasts so you have to pad it to include the last.

Note that this snippet doesn't empty the duplicate elements into a nil value. However since the n+1 integer start at the duplicated item's indexes, you can loop from said integer and nil the rest of the elements.

sort.Strings(strs)
for n, i := 0, 0; ; {
    if strs[n] != strs[i] {
        if i-n > 1 {
            strs[n+1] = strs[i]
        }
        n++
    }
    i++
    if i == len(strs) {
        if n != i {
            strs = strs[:n+1]
        }
        break
    }
}
fmt.Println(strs)