How to find out element position in slice?

Question

How does one determine the position of an element present in slice?

I need something like the following:

type intSlice []int

func (slice intSlice) pos(value int) int {
    for p, v := range slice {
        if (v == value) {
            return p
        }
    }
    return -1
}

Answer 1

Sorry, there's no generic library function to do this. Go doesn't have a straight forward way of writing a function that can operate on any slice.

Your function works, although it would be a little better if you wrote it using range.

If you happen to have a byte slice, there is bytes.IndexByte.

Answer 2

You can create generic function in idiomatic go way:

func SliceIndex(limit int, predicate func(i int) bool) int {
    for i := 0; i < limit; i++ {
        if predicate(i) {
            return i
        }
    }
    return -1
}

And usage:

xs := []int{2, 4, 6, 8}
ys := []string{"C", "B", "K", "A"}
fmt.Println(
    SliceIndex(len(xs), func(i int) bool { return xs[i] == 5 }),
    SliceIndex(len(xs), func(i int) bool { return xs[i] == 6 }),
    SliceIndex(len(ys), func(i int) bool { return ys[i] == "Z" }),
    SliceIndex(len(ys), func(i int) bool { return ys[i] == "A" }))

Answer 3

You could write a function;

func indexOf(element string, data []string) (int) {
   for k, v := range data {
       if element == v {
           return k
       }
   }
   return -1    //not found.
}

This returns the index of a character/string if it matches the element. If its not found, returns a -1.

Answer 4

Go supports generics as of version 1.18, which allows you to create a function like yours as follows:

func IndexOf[T comparable](collection []T, el T) int {
    for i, x := range collection {
        if x == el {
            return i
        }
    }
    return -1
}

If you want to be able to call IndexOf on your collection you can alternatively use @mh-cbon's technique from the comments.

Answer 5

There is no library function for that. You have to code by your own.

Answer 6

Since Go 1.18 you can also use the experimental generic slices package from https://pkg.go.dev/golang.org/x/exp/slices like this:

package main

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

func main() {
    s := []int{1,2,3,4,5}
    wanted := 3
    idx := slices.Index(s, wanted)
    fmt.Printf("the index of %v is %v", wanted, idx)
}

It will return -1, if wanted is not in the slice. Test it at the playground.

This is my preferred way, since this might become part of the standard library someday.

Answer 7

You can just iterate of the slice and check if an element matches with your element of choice.

func index(slice []string, item string) int {
    for i := range slice {
        if slice[i] == item {
            return i
        }
    }
    return -1
}

Answer 8

Use slices.Index in Go 1.21 and later.

haystack := []string{"foo", "bar", "quux"}
fmt.Println(slices.Index(haystack, "bar")) // prints 1
fmt.Println(slices.Index(haystack, "rsc")) // prints -1

Answer 9

Another option is to sort the slice using the sort package, then search for the thing you are looking for:

package main

import (
    "sort"
    "log"
    )

var ints = [...]int{74, 59, 238, -784, 9845, 959, 905, 0, 0, 42, 7586, -5467984, 7586}

func main() {
        data := ints
        a := sort.IntSlice(data[0:])
        sort.Sort(a)
        pos := sort.SearchInts(a, -784)
        log.Println("Sorted: ", a)
        log.Println("Found at index ", pos)
}

prints

2009/11/10 23:00:00 Sorted:  [-5467984 -784 0 0 42 59 74 238 905 959 7586 7586 9845]
2009/11/10 23:00:00 Found at index  1

This works for the basic types and you can always implement the sort interface for your own type if you need to work on a slice of other things. See http://golang.org/pkg/sort

Depends on what you are doing though.

Answer 10

I had the same issue few months ago and I solved in two ways:

First method:

func Find(slice interface{}, f func(value interface{}) bool) int {
    s := reflect.ValueOf(slice)
    if s.Kind() == reflect.Slice {
        for index := 0; index < s.Len(); index++ {
            if f(s.Index(index).Interface()) {
                return index
            }
        }
    }
    return -1
}

Use example:

type UserInfo struct {
    UserId          int
}

func main() {
    var (
        destinationList []UserInfo
        userId      int = 123
    )
    
    destinationList = append(destinationList, UserInfo { 
        UserId          : 23,
    }) 
    destinationList = append(destinationList, UserInfo { 
        UserId          : 12,
    }) 
    
    idx := Find(destinationList, func(value interface{}) bool {
        return value.(UserInfo).UserId == userId
    })
    
    if idx < 0 {
        fmt.Println("not found")
    } else {
        fmt.Println(idx)    
    }
}

Second method with less computational cost:

func Search(length int, f func(index int) bool) int {
    for index := 0; index < length; index++ {
        if f(index) {
            return index
        }
    }
    return -1
}

Use example:

type UserInfo struct {
    UserId          int
}

func main() {
    var (
        destinationList []UserInfo
        userId      int = 123
    )
    
    destinationList = append(destinationList, UserInfo { 
        UserId          : 23,
    }) 
    destinationList = append(destinationList, UserInfo { 
        UserId          : 123,
    }) 
    
    idx := Search(len(destinationList), func(index int) bool {
        return destinationList[index].UserId == userId
    })
    
    if  idx < 0 {
        fmt.Println("not found")
    } else {
        fmt.Println(idx)    
    }
}

Answer 11

Another option if your slice is sorted is to use SearchInts(a []int, x int) int which returns the element index if it's found or the index the element should be inserted at in case it is not present.

s := []int{3,2,1}
sort.Ints(s)
fmt.Println(sort.SearchInts(s, 1)) // 0
fmt.Println(sort.SearchInts(s, 4)) // 3

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