Go filter and map
last modified August 24, 2023
Go filter/map tutorial shows how to create and use filter and map functions in Golang.
A filter function processes a collection and produces a new collection containing exactly those elements for which the given predicate returns true.
A map function applies a given function to each element of a collection, returning the results in a new collection.
A predicate is a single-argument function which returns a boolean value.
$ go version go version go1.18.1 linux/amd64
We use Go version 1.18.
Go filter example
In the next example, we apply the filter function on a slice of strings.
package main
import (
"fmt"
"strings"
)
func filter(data []string, f func(string) bool) []string {
fltd := make([]string, 0)
for _, e := range data {
if f(e) {
fltd = append(fltd, e)
}
}
return fltd
}
func main() {
words := []string{"war", "water", "cup", "tree", "storm"}
p := "w"
res := filter(words, func(s string) bool {
return strings.HasPrefix(s, p)
})
fmt.Println(res)
}
We have a slice of words. With the help of the filter function, we
filter out all words that start with 'w' and put them into a new slice.
func filter(data []string, f func(string) bool) []string {
fltd := make([]string, 0)
for _, e := range data {
if f(e) {
fltd = append(fltd, e)
}
}
return fltd
}
The filter function takes a slice of words and the predicate as
parameters. The predicate is a function that is applied on every slice element.
$ go run main.go [war water]
In the next example, we filter a slice of struct types.
package main
import "fmt"
type User struct {
name string
occupation string
country string
}
func filter(data []User, f func(User) bool) []User {
fltd := make([]User, 0)
for _, user := range data {
if f(user) {
fltd = append(fltd, user)
}
}
return fltd
}
func main() {
users := []User{
{"John Doe", "gardener", "USA"},
{"Roger Roe", "driver", "UK"},
{"Paul Smith", "programmer", "Canada"},
{"Lucia Mala", "teacher", "Slovakia"},
{"Patrick Connor", "shopkeeper", "USA"},
{"Tim Welson", "programmer", "Canada"},
{"Tomas Smutny", "programmer", "Slovakia"},
}
country := "Slovakia"
res := filter(users, func(u User) bool {
return u.country == country
})
fmt.Println(res)
}
The example filters all users that live in Slovakia.
$ go run main.go
[{Lucia Mala teacher Slovakia} {Tomas Smutny programmer Slovakia}]
Go filter generic example
In the following example, we create a generic version of a filter function.
package main
import (
"fmt"
"strings"
)
func filter[T any](data []T, f func(T) bool) []T {
fltd := make([]T, 0, len(data))
for _, e := range data {
if f(e) {
fltd = append(fltd, e)
}
}
return fltd
}
func main() {
words := []string{"war", "cup", "water", "tree", "storm"}
res := filter(words, func(s string) bool {
return strings.HasPrefix(s, "w")
})
fmt.Println(res)
vals := []int{-1, 0, 2, 5, -9, 3, 4, 7}
res2 := filter(vals, func(e int) bool {
return e > 0
})
fmt.Println(res2)
}
We have a generic version of the filter function. We use it to filter strings and integers.
$ go run main.go [war water] [2 5 3 4 7]
Go map example
In the following example, we transform a slice of integers. Note that we cannot name the function map, since it is a Go keyword for creating dictionaries/maps.
package main
import (
"fmt"
)
func map2(data []int, f func(int) int) []int {
mapped := make([]int, len(data))
for i, e := range data {
mapped[i] = f(e)
}
return mapped
}
func main() {
vals := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
res := map2(vals, func(i int) int {
return i * i
})
fmt.Println(res)
}
We have a slice of integers. We square all elements of the slice.
$ go run main.go [1 4 9 16 25 36 49 64 81 100]
In the next example, we apply a map function on a slice of strings.
package main
import (
"fmt"
"strings"
)
func map2(data []string, f func(string) string) []string {
mapped := make([]string, len(data))
for i, e := range data {
mapped[i] = f(e)
}
return mapped
}
func main() {
words := []string{"war", "water", "cup", "tree", "storm"}
res := map2(words, strings.ToUpper)
fmt.Println(res)
}
The example transforms a slice of strings into uppercase.
$ go run main.go [WAR WATER CUP TREE STORM]
Go map generic example
In the following example, we have a generic version of the map function.
package main
import (
"fmt"
"strconv"
"strings"
)
func map2[T, U any](data []T, f func(T) U) []U {
res := make([]U, 0, len(data))
for _, e := range data {
res = append(res, f(e))
}
return res
}
func main() {
words := []string{"war", "cup", "water", "tree", "storm"}
result := map2(words, func(s string) string {
return strings.ToUpper(s)
})
fmt.Println(result)
fmt.Println("-------------------")
numbers := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
squares := map2(numbers, func(n int) int {
return n * n
})
fmt.Println(squares)
fmt.Println("-------------------")
as_strings := map2(numbers, func(n int) string {
return strconv.Itoa(n)
})
fmt.Printf("%q", as_strings)
}
With the map function, we process integers and strings.
$ go run main.go [WAR CUP WATER TREE STORM] ------------------- [1 4 9 16 25 36 49 64 81 100] ------------------- ["1" "2" "3" "4" "5" "6" "7" "8" "9" "10"]
In this article we have worked with filter and map functions in Golang.
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