Programonic

Before you start

Go variables and functions.

Grow a sequence with append and take a part of it.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

A slice is a window onto an underlying array and is the everyday sequence type in Go. []int{82, 91, 76} creates one, len reports its length, and append returns a slice with a value added at the end. You must assign the result, because append may allocate a new array.

scores[1:3] is a sub-slice that includes index 1 and stops before index 3. It shares memory with the original, so changing an element through one is visible through the other.

Code example

package main

import "fmt"

func main() {
  scores := []int{82, 91, 76}
  scores = append(scores, 95)
  fmt.Println(len(scores), scores[3])
  fmt.Println(scores[1:3])
}

Output

4 95
[91 76]

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

Show guided walkthrough

Guided walkthroughReady

1package main

3import "fmt"

5func main() {

6 scores := []int{82, 91, 76}

7 scores = append(scores, 95)

8 fmt.Println(len(scores), scores[3])

9 fmt.Println(scores[1:3])

10}

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Create a slice of three ints.	`scores = [82 91 76]`	—
Append and keep the returned slice.	`scores = [82 91 76 95]`	—
The length is 4 and index 3 holds 95.	`scores = [82 91 76 95]`	4 95
The window from index 1 up to, but not including, 3.	`scores = [82 91 76 95]`	4 95 [91 76]

Behind the scenes

How Go processes this code

The Go compiler builds an executable. Its runtime starts the main function in package main.

Cost and common mistakes

append is amortized O(1); slicing is O(1) because it copies no elements.

Calling append without assigning the result throws the new slice away.
Sub-slices share storage with the original; copy them if you need independence.

Try it yourself

What does scores[:2] print for the final slice?

Reveal answer and reasoning

It runs from the start up to index 2, so it prints [82 91].

Go Slices