Creating Slices

Go offers slice literals, make, and slicing an existing array or slice. Each approach produces a slice value with a length and capacity.

Use a literal for known contents, such as []string{"red", "blue"}. Use make([]int, length, capacity) when you need zero-initialized elements and optional spare capacity. Use array[start:end] to form a view into an existing array.

The zero value of a slice is nil, and a nil slice can be appended to. An empty non-nil slice also has length zero, but comparisons to nil can distinguish them in code.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func main() {
	literal := []int{4, 5}
	allocated := make([]int, 2, 5)
	numbers := [4]int{10, 20, 30, 40}
	view := numbers[1:3]
	fmt.Println("Literal:", literal)
	fmt.Println("Allocated:", allocated)
	fmt.Println("View:", view)
	fmt.Println("Capacity:", cap(allocated))
}

Output

Literal: [4 5]
Allocated: [0 0]
View: [20 30]
Capacity: 5

The literal starts with 4 and 5. The make call produces two visible zero elements and capacity five. Slicing numbers from index 1 up to, but not including, index 3 creates [20 30] without copying those elements.

• make([]int, 2, 5) sets length 2, not length 5.
• A slice made from an array shares that array’s underlying elements.

Create a nil []int variable, append 7 to it, and compare it with an empty slice produced by make([]int, 0).

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Creating Slices in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.