Go Basics

Go Slices: append, len and cap

The list type you will use most in Go: grow it with append, cut it with [a:b], and watch out for shared memory.

Beginner
3 min read
1 runnable example

What you'll learn

append
Slicing with [a:b]
make, len and cap

Run code →

numbers[1:4]

The slice covers indexes 1, 2 and 3.

A growable view of an array

A slice is a view of an array that can grow. It is the list type you will use most in Go. append adds items and returns the updated slice.

numbers[1:4] takes the items at positions 1, 2 and 3. That new slice shares memory with numbers, so changing middle[0] also changes numbers[1]. make creates an empty slice with room for three items, and cap shows that room.

Key points

append returns the updated slice.
numbers[1:4] has the items at 1, 2 and 3.
A slice shares memory with the slice it was cut from.

Runnable Go example

Run code →

main.go

package main

import "fmt"

func main() {
	numbers := []int{3, 1, 4}
	numbers = append(numbers, 1, 5)
	fmt.Println(numbers, len(numbers))

	middle := numbers[1:4]
	fmt.Println(middle)

	middle[0] = 100 // a slice shares the array it was cut from
	fmt.Println(numbers)

	squares := make([]int, 0, 3)
	for i := 1; i <= 3; i++ {
		squares = append(squares, i*i)
	}
	fmt.Println(squares, len(squares), cap(squares))

	for index, value := range squares {
		fmt.Println(index, value)
	}
}

Get a Run button for this example Result of running the code

[3 1 4 1 5] 5
[1 4 1]
[3 100 4 1 5]
[1 4 9] 3 3
0 1
1 4
2 9

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

Finished this lesson?

Mark it as learned to track your progress.

Next: Maps →

Practice with a challenge

Use what this lesson covers to solve a real problem. Each challenge runs in your browser with instant tests and an AI review.

All challenges