How to append a slice in Golang?

In Go, slices are dynamically-sized, flexible views into the elements of an array. Appending elements to a slice is a common operation that allows for the expansion of the slice as needed. The built-in append function is used to add elements to the end of a slice.In this article,we will learn How to Append a Slice in Golang.

Example

 package main  
import "fmt"

func main() {
    // Initial slice
    numbers := []int{1, 2, 3}
    fmt.Println("Original slice:", numbers)

    // Appending elements
    numbers = append(numbers, 4, 5, 6)
    fmt.Println("After appending:", numbers)
}

Syntax of append

The append function has the following syntax:

slice = append(slice, elements...)

Where:

• slice is the original slice to which elements are being added.
• elements are the values being appended. The ... operator is used to append multiple elements.

Appending Multiple Elements

You can append multiple elements to a slice at once using the append function.

Example

package main
import "fmt"

func main() {
    // Initial slice
    letters := []string{"a", "b"}
    fmt.Println("Original slice:", letters)

    // Appending multiple elements
    letters = append(letters, "c", "d")
    fmt.Println("After appending:", letters)
}

Original slice: [a b]
After appending: [a b c d]

Appending Another Slice

You are allowed to append one slice to another with the help of the ... operator. This allows for a seamless combination of slices.

Example

package main
import "fmt"

func main() {
    // Initial slices
    s1 := []int{1, 2, 3}
    s2 := []int{4, 5, 6}
    
    fmt.Println("S1:", s1)
    fmt.Println("S2:", s2)

    // Appending slice2 to slice1
    s1 = append(s1, s2...) // Note the ... operator
    fmt.Println("After appending S2 to S1:", s1)
}

S1: [1 2 3]
S2: [4 5 6]
After appending S2 to S1: [1 2 3 4 5 6]

Understanding Capacity and Growth

When appending to a slice, if the length of the slice exceeds its capacity, Go automatically allocates a new underlying array to accommodate the additional elements.

Example

package main
import "fmt"

func main() {
    // Initial slice
    numbers := make([]int, 0, 3) // Length: 0, Capacity: 3
    fmt.Println("Initial slice capacity:", cap(numbers))

    // Appending elements
    numbers = append(numbers, 1, 2, 3)
    fmt.Println("After initial append, capacity:", cap(numbers))

    // Appending more elements
    numbers = append(numbers, 4, 5)
    fmt.Println("After exceeding capacity, capacity:", cap(numbers))
}

Initial slice capacity: 3
After initial append, capacity: 3
After exceeding capacity, capacity: 6