Go slice
last modified April 11, 2024
In this article we show how to work with slices in Golang.
An array is a collection of elements of a single data type. An array holds a fixed number of elements, and it cannot grow or shrink. Elements of an array are accessed through indexes.
A slice is a dynamically-sized, flexible view into the elements of an array. A slice can grow and shrink within the bounds of the underlying array. A slice does not store any data, it just describes a section of the array.
Go declare slice
var s []T
We declare a slice having type T. The slice is declared just
like an array except that we do not specify any size in the brackets [].
$ go version go version go1.22.2 linux/amd64
We use Go version 1.22.2.
Slice literal
A slice can be created with a slice literal.
package main
import "fmt"
func main() {
var s1 = []int{2, 5, 6, 7, 8}
s2 := []int{3, 5, 1, 2, 8}
fmt.Println("s1:", s1)
fmt.Println("s2:", s2)
}
In the code example, we create two slices.
var s1 = []int{2, 5, 6, 7, 8}
The expression on the right-hand side is a slice literal.
s2 := []int{3, 5, 1, 2, 8}
Here we have a short-hand equivalent.
$ go run main.go s1: [2 5 6 7 8] s2: [3 5 1 2 8]
The make function
We can use the make built-in function to create new slices in Go.
func make([]T, len, cap) []T
The make function takes a type, a length, and an optional capacity.
It allocates an underlying array with size equal to the given capacity, and
returns a slice that refers to that array.
package main
import "fmt"
func main() {
vals := make([]int, 5)
fmt.Println("vals: ", vals)
vals[0] = 1
vals[1] = 2
vals[2] = 3
vals[3] = 4
vals[4] = 5
fmt.Println("vals: ", vals)
}
We create a slice of integer having size 5 with the make function.
Initially, the elements of the slice are all zeros. We then assign new values to
the slice elements.
$ go run main.go vals: [0 0 0 0 0] vals: [1 2 3 4 5]
Slice length and capacity
The len function returns the number of elements in the slice. The
cap function returns the capacity of the slice. The capacity of a
slice is the number of elements in the underlying array, counting from the first
element in the slice.
package main
import "fmt"
func main() {
vals := make([]int, 5, 10)
n := len(vals)
c := cap(vals)
fmt.Printf("The size is: %d\n", n)
fmt.Printf("The capacity is: %d\n", c)
vals2 := vals[0:4]
n2 := len(vals2)
c2 := cap(vals2)
fmt.Printf("The size is: %d\n", n2)
fmt.Printf("The capacity is: %d\n", c2)
}
In the code example, we print the size and the capacity of two slices.
$ go run main.go The size is: 6 The capacity is: 6 The size is: 4 The capacity is: 6
Slicing an array or slice
We can create a slice by slicing an existing array or slice.
To form a slice, we specify a low bound and a high bound:
a[low:high]. This selects a half-open range which includes the first
element, but excludes the last.
We can omit the high or low bounds to use their defaults instead. The default is zero for the low bound and the length of the slice for the high bound.
package main
import "fmt"
func main() {
vals := [...]int{1, 2, 3, 4, 5, 6, 7}
s1 := vals[1:4]
fmt.Printf("s1: %v, cap: %d\n", s1, cap(s1))
s2 := vals[5:7]
fmt.Printf("s2: %v, cap: %d\n", s2, cap(s2))
s3 := vals[:4]
fmt.Printf("s3: %v, cap: %d\n", s3, cap(s3))
s4 := vals[2:]
fmt.Printf("s4: %v, cap: %d\n", s4, cap(s4))
s5 := vals[:]
fmt.Printf("s5: %v, cap: %d\n", s5, cap(s5))
}
We create slices from an array of integers.
vals := [...]int{1, 2, 3, 4, 5, 6, 7}
An array of integers is created. With the ... operator, Go calculates
the size of the array for us.
s2 := vals[5:7]
fmt.Printf("s2: %v, cap: %d\n", s2, cap(s2))
We create a slice from the vals array. The resulting slice
contains elements starting from index 5 up to index 7; the upper bound is
non-inclusive.
$ go run main.go s1: [2 3 4], cap: 6 s2: [6 7], cap: 2 s3: [1 2 3 4], cap: 7 s4: [3 4 5 6 7], cap: 5 s5: [1 2 3 4 5 6 7], cap: 7
Slice iteration
With for loops, we can iterate over slice elements in Go.
package main
import "fmt"
func main() {
words := []string{"falcon", "bold", "bear", "sky", "cloud", "ocean"}
for idx, word := range words {
fmt.Println(idx, word)
}
}
In the code example, we iterate over a slice of words with for/range
statements.
$ go run main.go 0 falcon 1 bold 2 bear 3 sky 4 cloud 5 ocean
In the following example, we iterate a slice with a classic for loop.
package main
import "fmt"
func main() {
words := []string{"falcon", "bold", "bear", "sky", "cloud", "ocean"}
for i := 0; i < len(words); i++ {
fmt.Println(words[i])
}
}
We iterate over a slice of words with for statement.
$ go run main.go falcon bold bear sky cloud ocean
The append function
The built-in append function appends new elements to the slice.
func append(s []T, vs ...T) []T
The first parameter is a slice of type T, and the rest are
T values to append to the slice.
The resulting value of append is a slice containing all the elements of the original slice plus the provided values. If the backing array is too small to fit all the given values, a bigger array will be allocated. The returned slice will point to the newly allocated array.
package main
import "fmt"
func main() {
vals := make([]int, 3)
fmt.Printf("slice: %v; len: %d; cap: %d \n", vals, len(vals), cap(vals))
fmt.Println("---------------------------")
vals = append(vals, 1)
vals = append(vals, 2)
vals = append(vals, 3)
vals = append(vals, 4, 5, 6)
fmt.Printf("slice: %v; len: %d; cap: %d \n", vals, len(vals), cap(vals))
}
In the code example, we append new elements to a slice, which already has three elements.
vals := make([]int, 3)
First, we create a slice having three elements initiated to 0.
vals = append(vals, 1) vals = append(vals, 2) vals = append(vals, 3) vals = append(vals, 4, 5, 6)
We append six values to the slice. Multiple elements can be appended in one go.
$ go run main.go slice: [0 0 0]; len: 3; cap: 3 --------------------------- slice: [0 0 0 1 2 3 4 5 6]; len: 9; cap: 12
Under the hood, Go enlarged the underlying array to include all the new elements.
package main
import (
"fmt"
"strings"
)
func main() {
words := []string{}
words = append(words, "an")
words = append(words, "old")
words = append(words, "falcon")
res := strings.Join(words, " ")
fmt.Println(res)
}
We start with an empty string slice. We append three words to the slice. Then
we join the words with strings.Join function, while inserting a
space between the words.
$ go run main.go an old falcon
The copy function
The built-in copy function copies a slice.
func copy(dst, src []T) int
The function returns the number of elements copied.
package main
import "fmt"
func main() {
vals := []int{1, 2, 3, 4, 5}
vals2 := make([]int, len(vals))
n := copy(vals2, vals)
fmt.Printf("%d elements copied\n", n)
fmt.Println("vals:", vals)
fmt.Println("vals2:", vals2)
}
In the code example, we copy a slice of integers.
$ go run main.go 5 elements copied vals: [1 2 3 4 5] vals2: [1 2 3 4 5]
Removing element
There is no built-in function to remove items from a slice. We can do the
deletion with the append function.
package main
import (
"fmt"
)
func main() {
words := []string{"falcon", "bold", "bear", "sky", "cloud", "ocean"}
fmt.Println(words)
words = append(words[1:2], words[2:]...)
fmt.Println(words)
words = append(words[:2], words[4:]...)
fmt.Println(words)
}
In the code example, we delete an element and then two elements from the slice.
words = append(words[1:2], words[2:]...)
This removes the first element from the slice. We accomplish the removal by appending two slices omitting the one to be deleted.
$ go run main.go [falcon bold bear sky cloud ocean] [bold bear sky cloud ocean] [bold bear ocean]
Concatenatings slices
The slices.Concat method creates a new slice concatenating the
passed in slices.
package main
import (
"fmt"
"slices"
"strings"
)
func main() {
s1 := []string{"an old"}
s2 := []string{"falcon"}
s3 := []string{"in the sky"}
msg := slices.Concat(s1, s2, s3)
fmt.Println(msg)
fmt.Println(strings.Join(msg, " "))
}
The example concatenates three slices. It prints the contents of the new slice
and then joins all the elements of the slice with strings.Join.
$ go run main.go [an old falcon in the sky] an old falcon in the sky
Unique elements
In the next example, we generate a slice with unique elements.
package main
import "fmt"
func uniq(vals []int) []int {
uvals := []int{}
seen := make(map[int]bool)
for _, val := range vals {
if _, in := seen[val]; !in {
seen[val] = true
uvals = append(uvals, val)
}
}
return uvals
}
func main() {
vals := []int{1, 2, 2, 3, 4, 4, 5, 6, 7, 8, 8, 8, 9, 9}
uvals := uniq(vals)
fmt.Printf("Original slice: %v\n", vals)
fmt.Printf("Unique slice: %v\n", uvals)
}
We have a slice with duplicate elements. We create a new slice that contains only unique elements.
seen := make(map[int]bool)
To accomplish this task, we create a map that stores a boolean true for all values that we encounter in the slice.
for _, val := range vals {
if _, in := seen[val]; !in {
seen[val] = true
uvals = append(uvals, val)
}
}
We go over the elements of the slice with possible duplicates. If it is not
present in the seen map, we store it there and append it to the
new uvals slice. Otherwise, we skip the if block.
$ go run main.go Original slice: [1 2 2 3 4 4 5 6 7 8 8 8 9 9] Unique slice: [1 2 3 4 5 6 7 8 9]
Sorting slice elements
Go contains the sort package to sort slices.
package main
import (
"fmt"
"sort"
)
func main() {
words := []string{"falcon", "bold", "bear", "sky", "cloud", "ocean"}
vals := []int{4, 2, 1, 5, 6, 8, 0, -3}
sort.Strings(words)
sort.Ints(vals)
fmt.Println(words)
fmt.Println(vals)
}
In the code example, we sort a slice of words and integers.
$ go run main.go [bear bold cloud falcon ocean sky] [-3 0 1 2 4 5 6 8]
Slice initial value
The default zero value of a slice is nil. The nil
slice has a length and capacity of 0 and has no underlying array.
When we create a slice with the make function, all elements
are initialized to 0.
package main
import "fmt"
func main() {
var vals []int
if vals == nil {
fmt.Printf("slice is nil\n")
}
fmt.Printf("slice: %v; len: %d; cap: %d \n", vals, len(vals), cap(vals))
fmt.Println("---------------------------")
var vals2 = make([]int, 5)
fmt.Printf("slice: %v; len: %d; cap: %d \n", vals2, len(vals2), cap(vals2))
}
We create two slices with a default zero value and make-function initialized elements.
$ go run main.go slice is nil slice: []; len: 0; cap: 0 --------------------------- slice: [0 0 0 0 0]; len: 5; cap: 5
Slice is a reference type
A slice is a reference type in Go. This means that when we assign a reference to a new variable or pass a slice to a function, the reference to the slice is copied.
package main
import "fmt"
func main() {
vals := []int{ 1, 2, 3, 4, 5, 6 }
vals2 := vals
vals2[0] = 11
vals2[1] = 22
fmt.Println(vals)
fmt.Println(vals2)
}
In the code example, we define a slice and assign the slice to a new variable. The changes made through the second variable are reflected in the original slice.
$ go run main.go [11 22 3 4 5 6] [11 22 3 4 5 6]
The original slice is also modified.
Slice of slices
Go slice can also contain other slices.
package main
import "fmt"
func main() {
words := [][]string{
{"sky", "ocean"},
{"red", "blue"},
{"C#", "Go"},
}
fmt.Printf("slice: %v; len: %d; cap: %d \n", words, len(words), cap(words))
}
The example creates a slice of slices.
$ go run main.go slice: [[sky ocean] [red blue] [C# Go]]; len: 3; cap: 3
Filtering a slice of structs
In the next example, we filter a slice of Go structures.
package main
import "fmt"
type User struct {
name string
occupation string
country string
}
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"},
}
var programmers []User
for _, user := range users {
if isProgrammer(user) {
programmers = append(programmers, user)
}
}
fmt.Println("Programmers:")
for _, u := range programmers {
fmt.Println(u)
}
}
func isProgrammer(user User) bool {
return user.occupation == "programmer"
}
In the code example, we define a slice of users. We create a new slice that contains only programmers.
type User struct {
name string
occupation string
country string
}
The User struct has three fields.
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"},
}
This is the original slice of User structures.
var programmers []User
The filtered users/programmers are stored in the programmers
slice.
for _, user := range users {
if isProgrammer(user) {
programmers = append(programmers, user)
}
}
We go over the users slice and add a current user to the
programmers slice only if the user satisfies the
isProgrammer predicate.
func isProgrammer(user User) bool {
return user.occupation == "programmer"
}
The IsProgrammer predicate returns true for all users whose
occupation field equals to "programmer".
$ go run main.go
Programmers:
{Paul Smith programmer Canada}
{Tim Welson programmer Canada}
{Tomas Smutny programmer Slovakia}
Syntax sugar
Go has a simplified syntax for passing a slice to another function.
package main
import "fmt"
func showUsers(users ...string) {
for _, e := range users {
fmt.Println(e)
}
}
func main() {
showUsers("sky", "tomorrow", "bored", "falcon")
}
The users ...string is a parameter declaration for a slice of
strings. We can then pass the elements separated by comma:
"sky", "tomorrow", "bored", "falcon".
Source
The Go Programming Language Specification
In this article we have worked with slice in Golang.
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