Intro to Go Lab 1: Imperative Programming

If you're stuck look at examples on Go by Example

Setting up Go

Before starting this lab, make sure that your machine has Go correctly installed and your favourite editor/IDE properly configured. Go is supported by Linux, macOS and Windows.

If you're using the 2.11 lab machines we wrote a quick setup guide for configuring Go and VS Code.

You can find all setup guides here.

Using the lab sheet

There are two ways to use the lab sheet, you can either:

• create a new repo from this template - this is the recommended way
• download a zip file

Each question is rated to help you balance your work:

• 🔴⚪⚪⚪⚪ - Easy, strictly necessary.
• 🔴🔴⚪⚪⚪ - Medium, still necessary.
• 🔴🔴🔴⚪⚪ - Hard, necessary if you're aiming for higher marks.
• 🔴🔴🔴🔴⚪ - Hard, useful for coursework extensions.
• 🔴🔴🔴🔴🔴 - Hard, beyond what you need for any part of the coursework.

Question 1 - Hello World 🔴⚪⚪⚪⚪

Below is a complete 'Hello World' program written in Go:

package main

import "fmt"

func main() {
    fmt.Println("Hello World")
}

Question 1a

Type the above program into a new file hello.go (don't just copy and paste). To run your code you can either use go run hello.go or go build hello.go followed by ./hello. Verify that Hello World is printed in both cases.

Question 1b

Modify hello.go so that it uses a for loop to print Hello World 20 times.

Question 2 - Quiz 🔴⚪⚪⚪⚪

Open quiz.go. It's a skeleton for a quiz program. Write a main() using the provided helper functions so that your program asks the 6 questions from quiz-questions.csv and prints out the final score at the end.

Hint 1

Use s := score(0) to initialise your score variable.

Hint 2

Use a for-range loop to ask all the questions.

Question 3 - Arrays vs Slices 🔴🔴⚪⚪⚪

Open sequences.go.

Question 3a

Implement mapSlice and mapArray using for-range loops.

They are the same as Haskell's map. For example mapping addOne onto [5, 10, 15] should return [6, 11, 16].

Question 3b

In main():

• Create a slice intsSlice with values [1, 2, 3].

• Map addOne onto this slice.

• Print intsSlice.

• Create an array intsArray of length 3 with values [1, 2, 3].

• Map addOne onto this array.

• Print intsArray.

Explain the result. Modify the skeleton to solve any issues that you may have observed.

Hint

How are slices different from arrays? What exactly are they?

Question 3c

Change the definitions of intsArray and intsSlice so that they both contain values [1, 2, 3, 4, 5]. Without modifying mapSlice or mapArray run your new program. Explain the result.

Question 3d

Slices support a “slice” operator with the syntax slice[lowIndex:highIndex]. It allows you to cut out a portion of your slice. For example:

// Given: intsSlice  = [2, 3, 4, 5, 6]
newSlice := intsSlice[1:3]  
// newSlice = [3, 4]

Define newSlice as shown above, map square onto newSlice and print both newSlice and the original intSlice. Explain the result.

Question 3e

The function double should append a slice onto itself. For example, given [5, 6, 7] it should return [5, 6, 7, 5, 6, 7]. In main, try doubling and then printing your intsSlice. Explain the result. Modify the skeleton to solve any issues that you may have observed.

Question 3f

Given the differences that you found between arrays and slices:

• Explain how arrays and slices are passed to functions.
• Explain how append() works.
• Give use cases for arrays and slices.

Question 4 - Game of Life 🔴🔴⚪⚪⚪

Time to test what you've learnt!

Open gol.go. This is a skeleton for a program that can run a serial game of life simulation.

What is Game of Life?

The British mathematician John Horton Conway devised a cellular automaton named ‘The Game of Life’. The game resides on a 2-valued 2D matrix, i.e. a binary image, where the cells can either be ‘alive’ (pixel value 255 - white) or ‘dead’ (pixel value 0 - black). The game evolution is determined by its initial state and requires no further input. Every cell interacts with its eight neighbour pixels: cells that are horizontally, vertically, or diagonally adjacent. At each matrix update in time the following transitions may occur to create the next evolution of the domain:

• any live cell with fewer than two live neighbours dies
• any live cell with two or three live neighbours is unaffected
• any live cell with more than three live neighbours dies
• any dead cell with exactly three live neighbours becomes alive

Consider the image to be on a closed domain (pixels on the top row are connected to pixels at the bottom row, pixels on the right are connected to pixels on the left and vice versa). A user can only interact with the Game of Life by creating an initial configuration and observing how it evolves. Note that evolving such complex, deterministic systems is an important application of scientific computing, often making use of parallel architectures and concurrent programs running on large computing farms.

Question 4a

Complete the calculateNextState(p golParams, world [][]byte) [][]byte function. This function takes the current state of the world and completes one evolution of the world. It then returns the result.

Question 4b

Complete the calculateAliveCells(p golParams, world [][] byte) []cell function. This function takes the world as input and returns the (x, y) coordinates of all the cells that are alive.

Make sure that all your tests pass! (go test .)

Once all your tests pass you can use go run . and see if you get the hidden message when you open output.pgm.

Question 5 - Concurrent Hello World 🔴⚪⚪⚪⚪

A goroutine is a lightweight thread of execution. Modify your hello.go so that it uses a for loop to start 5 goroutines and print Hello from goroutine i where i is the number of the goroutine.

Example output:

$ go run hello.go

Hello from goroutine 2
Hello from goroutine 3
Hello from goroutine 4
Hello from goroutine 0
Hello from goroutine 1

Hint 1 - How do I start a new goroutine?

Starting a goroutine is easy, just say:

go someFunc()

Hint 2 - Why does my program exit without printing anything?

You may notice that your program exits without printing anything. For now you can fix this by placing this after your for loop:

time.Sleep(1 * time.Second)

Soon you'll see how to fix this problem with channels.