Practical Ways to Combine Slices in Go
Combining slices is common when data comes from several sources or processing stages. In Go, the standard approach is append, optionally with preallocated capacity when the final size is known.
1. Combine Slices with append
package main
import "fmt"
func main() {
slice1 := []int{1, 2, 3}
slice2 := []int{4, 5, 6}
slice3 := []int{7, 8, 9}
combined := append(slice1, slice2...)
combined = append(combined, slice3...)
fmt.Println(combined)
}Output:
[1 2 3 4 5 6 7 8 9]The ... syntax passes the elements of the second slice to the variadic append function.
One important detail: append(slice1, ...) may reuse slice1’s backing array. If the original slices must remain isolated from future mutations, build the result in a separate allocation.
2. Preallocate Capacity
If you know the final length, preallocate capacity to reduce reallocations:
package main
import "fmt"
func main() {
slice1 := []int{1, 2, 3}
slice2 := []int{4, 5, 6}
slice3 := []int{7, 8, 9}
totalLen := len(slice1) + len(slice2) + len(slice3)
combined := make([]int, 0, totalLen)
combined = append(combined, slice1...)
combined = append(combined, slice2...)
combined = append(combined, slice3...)
fmt.Println(combined)
}This also guarantees that combined has its own backing storage from the start.
3. Combine a Dynamic Number of Slices
When the number of input slices varies, loop over them:
package main
import "fmt"
func main() {
slices := [][]int{
{1, 2, 3},
{4, 5, 6},
{7, 8, 9},
}
totalLen := 0
for _, s := range slices {
totalLen += len(s)
}
combined := make([]int, 0, totalLen)
for _, s := range slices {
combined = append(combined, s...)
}
fmt.Println(combined)
}Conclusion
Use append for simple concatenation, preallocate when you know the combined size, and loop when the number of source slices is dynamic. When aliasing matters, remember that slices may share an underlying array unless you deliberately allocate a separate result.
