How to search for an element in a golang slice
GoStructSliceGo Problem Overview
I have a slice of structs.
type Config struct {
Key string
Value string
}
// I form a slice of the above struct
var myconfig []Config
// unmarshal a response body into the above slice
if err := json.Unmarshal(respbody, &myconfig); err != nil {
panic(err)
}
fmt.Println(config)
Here is the output of this:
[{key1 test} {web/key1 test2}]
How can I search this array to get the element where key="key1"?
Go Solutions
Solution 1 - Go
Starting with Go 1.18 which adds generics support, there's a golang.org/x/exp/slices package which contains a generic "find" function named slices.IndexFunc():
> func IndexFunc[E any](s []E, f func(E) bool) int > IndexFunc returns the first index i satisfying f(s[i]), or -1 if none do.
Using that:
idx := slices.IndexFunc(myconfig, func(c Config) bool { return c.Key == "key1" })
Try it on the Go Playground.
Prior to Go 1.18 and for a faster alternative, read on:
With a simple for loop:
for _, v := range myconfig {
if v.Key == "key1" {
// Found!
}
}
Note that since element type of the slice is a struct (not a pointer), this may be inefficient if the struct type is "big" as the loop will copy each visited element into the loop variable.
It would be faster to use a range loop just on the index, this avoids copying the elements:
for i := range myconfig {
if myconfig[i].Key == "key1" {
// Found!
}
}
Notes:
It depends on your case whether multiple configs may exist with the same key, but if not, you should break out of the loop if a match is found (to avoid searching for others).
for i := range myconfig {
if myconfig[i].Key == "key1" {
// Found!
break
}
}
Also if this is a frequent operation, you should consider building a map from it which you can simply index, e.g.
// Build a config map:
confMap := map[string]string{}
for _, v := range myconfig {
confMap[v.Key] = v.Value
}
// And then to find values by key:
if v, ok := confMap["key1"]; ok {
// Found
}
Solution 2 - Go
You can use sort.Slice() plus sort.Search()
type Person struct {
Name string
}
func main() {
crowd := []Person{{"Zoey"}, {"Anna"}, {"Benni"}, {"Chris"}}
sort.Slice(crowd, func(i, j int) bool {
return crowd[i].Name <= crowd[j].Name
})
needle := "Benni"
idx := sort.Search(len(crowd), func(i int) bool {
return string(crowd[i].Name) >= needle
})
if idx < len(crowd) && crowd[idx].Name == needle {
fmt.Println("Found:", idx, crowd[idx])
} else {
fmt.Println("Found noting: ", idx)
}
}
Solution 3 - Go
You can save the struct into a map by matching the struct Key and Value components to their fictive key and value parts on the map:
mapConfig := map[string]string{}
for _, v := range myconfig {
mapConfig[v.Key] = v.Value
}
Then using the golang comma ok idiom you can test for the key presence:
if v, ok := mapConfig["key1"]; ok {
fmt.Printf("%s exists", v)
}
Solution 4 - Go
There is no library function for that. You have to code by your own.
for _, value := range myconfig {
if value.Key == "key1" {
// logic
}
}
Working code: https://play.golang.org/p/IJIhYWROP_
package main
import (
"encoding/json"
"fmt"
)
func main() {
type Config struct {
Key string
Value string
}
var respbody = []byte(`[
{"Key":"Key1", "Value":"Value1"},
{"Key":"Key2", "Value":"Value2"}
]`)
var myconfig []Config
err := json.Unmarshal(respbody, &myconfig)
if err != nil {
fmt.Println("error:", err)
}
fmt.Printf("%+v\n", myconfig)
for _, v := range myconfig {
if v.Key == "Key1" {
fmt.Println("Value: ", v.Value)
}
}
}
Solution 5 - Go
As other guys commented before you can write your own procedure with anonymous function to solve this issue.
I used two ways to solve it:
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
}
Uses 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
}
Uses 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)
}
}
Solution 6 - Go
Here is a simple function based on @Tarion's answer.
func findProgram (programs []Program, id uint) (Program, error) {
sort.Slice(programs, func(i, j int) bool {
return programs[i].ID <= programs[j].ID
})
idx := sort.Search(len(programs), func(i int) bool {
return programs[i].ID >= id
})
if idx < len(programs) && programs[idx].ID == id {
return programs[idx], nil
} else {
return Program{}, fmt.Errorf("program not found")
}
}
Solution 7 - Go
If anyone is coming from Java or C# like me this is what I ended up doing:
type Person struct {
Name string
Age int
}
// create slice of people
var people []Person = []Person{
{"Tono", 33},
{"Regina", 25},
{"Bob", 40},
}
// find person where its Name equals to Bob <------------------
bob := FirstOrDefault(people, func(p *Person) bool { return p.Name == "Bob" })
if bob != nil {
fmt.Printf("Found bob: %v \n", *bob)
} else {
fmt.Println("could not find bob")
}
peopleOlderThan30 := Where(people, func(p *Person) bool { return p.Age > 30 })
fmt.Println("People older than 30 are:")
for _, element := range peopleOlderThan30 {
fmt.Println(*element)
}
I was able to run that code with the help of these functions:
func FirstOrDefault[T any](slice []T, filter func(*T) bool) (element *T) {
for i := 0; i < len(slice); i++ {
if filter(&slice[i]) {
return &slice[i]
}
}
return nil
}
func Where[T any](slice []T, filter func(*T) bool) []*T {
var ret []*T = make([]*T, 0)
for i := 0; i < len(slice); i++ {
if filter(&slice[i]) {
ret = append(ret, &slice[i])
}
}
return ret
}