Go Slices and more
Go Slices and more
A slice literal is like an array literal without the length.
This is an array literal:
[3]bool{true, true, false}
And this creates the same array as above, then builds a slice that references it:
[]bool{true, true, false}
pradeep:~$cat slice-literal.go
package main
import "fmt"
func main() {
q := []int{2, 3, 5, 7, 11, 13}
fmt.Println(q)
r := []bool{true, false, true, true, false, true}
fmt.Println(r)
s := []struct {
i int
b bool
}{
{2, true},
{3, false},
{5, true},
{7, true},
{11, false},
{13, true},
}
fmt.Println(s)
}
pradeep:~$go run slice-literal.go
[2 3 5 7 11 13]
[true false true true false true]
[{2 true} {3 false} {5 true} {7 true} {11 false} {13 true}]
Slice Defaults
When slicing, you may 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.
For the array
var a [10]int
these slice expressions are equivalent:
a[0:10]
a[:10]
a[0:]
a[:]
pradeep:~$cat slice-bounds.go
package main
import "fmt"
func main() {
s := []int{2, 3, 5, 7, 11, 13}
s = s[1:4]
fmt.Println(s)
s = s[:2]
fmt.Println(s)
s = s[1:]
fmt.Println(s)
}
pradeep:~$go run slice-bounds.go
[3 5 7]
[3 5]
[5]
Let us make a small change to the low index, and try again
pradeep:~$cat slice-bounds.go
package main
import "fmt"
func main() {
s := []int{2, 3, 5, 7, 11, 13}
s = s[2:5]
fmt.Println(s)
s = s[:2]
fmt.Println(s)
s = s[1:]
fmt.Println(s)
}
pradeep:~$go run slice-bounds.go
[5 7 11]
[5 7]
[7]
Slice Length and Capacity
A slice has both a length and a capacity.
The length of a slice is the number of elements it contains.
The capacity of a slice is the number of elements in the underlying array, counting from the first element in the slice.
The length and capacity of a slice s can be obtained using the expressions len(s) and cap(s).
You can extend a slice’s length by re-slicing it, provided it has sufficient capacity. Try changing one of the slice operations in the example program to extend it beyond its capacity and see what happens.
pradeep:~$cat slice-len-cap.go
package main
import "fmt"
func main() {
s := []int{2, 3, 5, 7, 11, 13}
printSlice(s)
// Slice the slice to give it zero length.
s = s[:0]
printSlice(s)
// Extend its length.
s = s[:4]
printSlice(s)
// Drop its first two values.
s = s[2:]
printSlice(s)
}
func printSlice(s []int) {
fmt.Printf("len=%d cap=%d %v\n", len(s), cap(s), s)
}
pradeep:~$go run slice-len-cap.go
len=6 cap=6 [2 3 5 7 11 13]
len=0 cap=6 []
len=4 cap=6 [2 3 5 7]
len=2 cap=4 [5 7]
Nil Slices
The zero value of a slice is nil.
A nil slice has a length and capacity of 0 and has no underlying array.
pradeep:~$cat nil-slices.go
package main
import "fmt"
func main() {
var s []int
fmt.Println(s, len(s), cap(s))
if s == nil {
fmt.Println("nil!")
}
}
pradeep:~$go run nil-slices.go
[] 0 0
nil!
Creating a slice with make
Slices can be created with the built-in make function; this is how you create dynamically-sized arrays.
The make function allocates a zeroed array and returns a slice that refers to that array:
a := make([]int, 5) // len(a)=5
To specify a capacity, pass a third argument to make:
b := make([]int, 0, 5) // len(b)=0, cap(b)=5
b = b[:cap(b)] // len(b)=5, cap(b)=5
b = b[1:] // len(b)=4, cap(b)=4
pradeep:~$cat making-slices.go
package main
import "fmt"
func main() {
a := make([]int, 5)
printSlice("a", a)
b := make([]int, 0, 5)
printSlice("b", b)
c := b[:2]
printSlice("c", c)
d := c[2:5]
printSlice("d", d)
}
func printSlice(s string, x []int) {
fmt.Printf("%s len=%d cap=%d %v\n",
s, len(x), cap(x), x)
}
pradeep:~$go run making-slices.go
a len=5 cap=5 [0 0 0 0 0]
b len=0 cap=5 []
c len=2 cap=5 [0 0]
d len=3 cap=3 [0 0 0]
Slices of slices
Slices can contain any type, including other slices.
pradeep:~$cat slices-of-slices.go
package main
import (
"fmt"
"strings"
)
func main() {
// Create a tic-tac-toe board.
board := [][]string{
[]string{"_", "_", "_"},
[]string{"_", "_", "_"},
[]string{"_", "_", "_"},
}
// The players take turns.
board[0][0] = "X"
board[2][2] = "O"
board[1][2] = "X"
board[1][0] = "O"
board[0][2] = "X"
for i := 0; i < len(board); i++ {
fmt.Printf("%s\n", strings.Join(board[i], " "))
}
}
pradeep:~$go run slices-of-slices.go
X _ X
O _ X
_ _ O
Appending to a Slice
It is common to append new elements to a slice, and so Go provides a built-in append function. The documentation of the built-in package describes append.
func append(s []T, vs ...T) []T
The first parameter s of append 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 of s 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.
(To learn more about slices, read the Slices: usage and internals article.)
pradeep:~$cat append.go
package main
import "fmt"
func main() {
var s []int
printSlice(s)
// append works on nil slices.
s = append(s, 0)
printSlice(s)
// The slice grows as needed.
s = append(s, 1)
printSlice(s)
// We can add more than one element at a time.
s = append(s, 2, 3, 4)
printSlice(s)
}
func printSlice(s []int) {
fmt.Printf("len=%d cap=%d %v\n", len(s), cap(s), s)
}
pradeep:~$go run append.go
len=0 cap=0 []
len=1 cap=1 [0]
len=2 cap=2 [0 1]
len=5 cap=6 [0 1 2 3 4]
Range
The range form of the for loop iterates over a slice or map.
When ranging over a slice, two values are returned for each iteration. The first is the index, and the second is a copy of the element at that index.
pradeep:~$cat range.go
package main
import "fmt"
var pow = []int{1, 2, 4, 8, 16, 32, 64, 128}
func main() {
for i, v := range pow {
fmt.Printf("2**%d = %d\n", i, v)
}
}
pradeep:~$go run range.go
2**0 = 1
2**1 = 2
2**2 = 4
2**3 = 8
2**4 = 16
2**5 = 32
2**6 = 64
2**7 = 128
You can skip the index or value by assigning to _.
for i, _ := range pow
for _, value := range pow
If you only want the index, you can omit the second variable.
for i := range pow
pradeep:~$cat range-another.go
package main
import "fmt"
func main() {
pow := make([]int, 10)
for i := range pow {
pow[i] = 1 << uint(i) // == 2**i
}
for _, value := range pow {
fmt.Printf("%d\n", value)
}
}
pradeep:~$go run range-another.go
1
2
4
8
16
32
64
128
256
512
