How to prepend int to slice
ArraysGoSlicePrependArrays Problem Overview
I am fairly new to Go and thus my question might seem a bit naive.
I have a slice which I created using
var x []int;
for i := 2; i < 10; i += 2 {
x = append(x, i);
}
I want to prepend an integer to this slice, something like
x = append(2, x)
but obviously it won't work since append needs a slice as the first argument.
I have tried this but it only works for strings and it's not working in my case.
Arrays Solutions
Solution 1 - Arrays
Use a slice composite literal: []int{1}, For example:
package main
import (
"fmt"
)
func main() {
var x []int
for i := 2; i < 10; i += 2 {
x = append(x, i)
}
fmt.Println(x)
x = append([]int{1}, x...)
fmt.Println(x)
}
Playground: https://play.golang.org/p/Yc87gO7gJlD
Output:
[2 4 6 8]
[1 2 4 6 8]
However, this more efficient version may make fewer allocations, An allocation is only necessary when there is no spare slice capacity.
package main
import (
"fmt"
)
func main() {
var x []int
for i := 2; i < 10; i += 2 {
x = append(x, i)
}
fmt.Println(x)
x = append(x, 0)
copy(x[1:], x)
x[0] = 1
fmt.Println(x)
}
Playground: https://play.golang.org/p/fswXul_YfvD
Output:
[2 4 6 8]
[1 2 4 6 8]
Good code must be readable. In Go, we often hide implementaion details inside a function. Go compilers are optimizing compilers, small, simple functions (like prependInt) are inlined.
package main
import (
"fmt"
)
func prependInt(x []int, y int) []int {
x = append(x, 0)
copy(x[1:], x)
x[0] = y
return x
}
func main() {
var x []int
for i := 2; i < 10; i += 2 {
x = append(x, i)
}
fmt.Println(len(x), cap(x), x)
x = prependInt(x, 1)
fmt.Println(len(x), cap(x), x)
}
Playground: https://play.golang.org/p/wl6gvoXraKH
Output:
4 4 [2 4 6 8]
5 8 [1 2 4 6 8]
See Go SliceTricks.
Solution 2 - Arrays
The current version is go1.14.11
Prepend without a for loop:
package main
import "fmt"
func main () {
data := []int{1, 2, 3, 4}
fmt.Println(data)
data = append([]int{99}, data...)
fmt.Println(data)
}
Example taken form: https://codingair.wordpress.com/2014/07/18/go-appendprepend-item-into-slice/
Works with integers: https://play.golang.org/p/gaLhB5_d1Iu
Solution 3 - Arrays
Another answer uses append and copy, but you can do it with just append,
in one or two lines:
package main
import "fmt"
func main() {
{
a := []int{20, 30}
a = append(append(a, 10), a...)[len(a):]
fmt.Println(a)
}
{
a := []int{20, 30}
a = append(make([]int, 1), a...)
a[0] = 10
fmt.Println(a)
}
{ // if starting with empty slice, use: a := make([]int, 0, 1)
a := []int{20, 30}
a = append(a[:1], a...)
a[0] = 10
fmt.Println(a)
}
}
Or as another option, you can use linked list:
package main
import (
"container/list"
"fmt"
)
func main() {
a := list.New()
a.PushBack(20)
a.PushBack(30)
a.PushFront(10)
for n := a.Front(); n != nil; n = n.Next() {
fmt.Println(n.Value)
}
}
Solution 4 - Arrays
I know the Question is only for int but thought I'd add this to the discussion:
https://play.golang.com/p/PNax2a1rL3q
import (
"golang.org/x/exp/constraints"
)
type Number interface {
constraints.Signed | constraints.Unsigned | constraints.Float
}
func prepend[T Number](list []T, elems ...T) []T {
results := make([]T, len(list)+len(elems))
results = append(results, 0)
copy(results[len(elems):], list)
copy(results[:len(elems)], elems)
return results
}
References: