The append function in Go is used to add elements to slices, and the result must be reassigned to the slice because the underlying array may change; 1. Use append(slice, elem) to add a single or multiple elements; 2. Use append(slice, otherslice...) to merge two slices, which need to be expanded with...; 3. Append can be safely used for nil slices; 4. When the capacity is insufficient, append will automatically expand and may reassign the underlying array; 5. Append is suitable for any type of slice, such as merging rune slices to build strings.
In Go, the append function is used to add elements to slices. It's one of the most commonly used built-in functions when working with dynamic arrays (slices). Here's a clear breakdown with examples to show how append works.
? Basic Usage of append
The append function takes a slice and one or more values, then returns a new slice containing all the original elements plus the new ones.
package main
import "fmt"
func main() {
// Start with a slice
nums := []int{1, 2, 3}
// Append a single element
nums = append(nums, 4)
fmt.Println(nums) // Output: [1 2 3 4]
// Append multiple elements
nums = append(nums, 5, 6, 7)
fmt.Println(nums) // Output: [1 2 3 4 5 6 7]
}? Note:
appendmay modify the underlying array if there's enough capacity. Otherwise, it allocates a new, larger underlying array.
? Appending a Slice to Another Slice
To append one slice to another, you need to unpack the second slice using the ... operator.
package main
import "fmt"
func main() {
a := []int{1, 2}
b := []int{3, 4, 5}
// Append slice b to slice a
a = append(a, b...) // The '...' spreads the elements of b
fmt.Println(a) // Output: [1 2 3 4 5]
}? Without
..., you'll get a compile-time error because Go expects individual elements, not a slice.
? Common Pattern: Building a Slice Dynamically
You often start with an empty or nil slice and build it up using append .
package main
import "fmt"
func main() {
var fruits []string // nil slice
fruits = append(fruits, "apple")
fruits = append(fruits, "banana")
fruits = append(fruits, "cherry")
fmt.Println(fruits) // Output: [apple banana cherry]
} This works safely even if the slice is nil — append handles it correctly.
? Capacity and Reallocation
Go slices have both length and capacity. When a slice runs out of capacity, append automatically allocates a bigger underlying array.
package main
import "fmt"
func main() {
s := make([]int, 2, 5) // len=2, cap=5
fmt.Printf("len=%d cap=%d value=%v\n", len(s), cap(s), s) // len=2 cap=5 value=[0 0]
s = append(s, 3)
fmt.Printf("len=%d cap=%d value=%v\n", len(s), cap(s), s) // len=3 cap=5 value=[0 0 3]
s = append(s, 4, 5, 6)
fmt.Printf("len=%d cap=%d value=%v\n", len(s), cap(s), s) // len=6 cap=10 (likely) value=[0 0 3 4 5 6]
}? The growth strategy is not guaranteed but typically increases capacity by around 100% when full.
? Appending to Strings or Other Types
append works with slices of any type — not just integers.
chars := []rune{'h', 'e', 'l'}
chars = append(chars, 'l', 'o')
result := string(chars)
fmt.Println(result) // Output: helloThis is useful when building strings from runes safely (eg, handling Unicode).
Key Takeaways
- Use
append(slice, elem)to add one or more elements. - Use
append(slice, otherslice...)to merge two slices. -
appendis safe onnilslices. - Always assign the result:
slice = append(slice, ...)— because the underlying array might change. -
appendcan trigger reallocation when capacity is exceeded.
That's it. append is simple but powerful — just remember to use ... when adding another slice, and always reassign the result.
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