Activity 1.12

The Crossing

Key Concepts

Nothing New Today

One Decision, Many Pictures

Testing Every Branch

What You Are Building

Your signal from session 3, now reacting to something.

No New Ideas

Motion, comparisons, elseif, and — all of it from the last five sessions.

Putting five sessions together is the work.

Decide Once, Draw Three Times

local lamp = "green"

if ballX > signalX - 100 and ballX < signalX + 100 then
    lamp = "red"
elseif ballX > signalX - 250 and speedX > 0 then
    lamp = "yellow"
end

Decide Once, Draw Three Times

local lamp = "green"

if ballX > signalX - 100 and ballX < signalX + 100 then
    lamp = "red"
elseif ballX > signalX - 250 and speedX > 0 then
    lamp = "yellow"
end

One chain works out which lamp is lit. Then three separate decisions each ask am I the lit one?

Deciding and drawing are different jobs. Doing them separately is what keeps this readable.

Each Lamp Asks the Same Question

if lamp == "red" then
    love.graphics.setColor(0.9, 0.2, 0.2)
else
    love.graphics.setColor(0.25, 0.1, 0.1)
end
love.graphics.circle("fill", signalX, signalY, 30)

Bright if it is the lit one, nearly black if it is not. All three lamps are always drawn — only their colors change.

Comparing Text

if lamp == "red" then

== works on text too, and the quotes matter. lamp holds the word red, not a color.

A misspelling here is silent: no error, and one lamp that never lights.

Test Every Branch

• Ball far away, moving toward the signal
• Ball approaching, inside 250
• Ball directly under it
• Ball past it, moving away
• Ball far away, moving away

Five situations, three lamps. One of them is easy to miss.

Today's Objectives

• Combine motion, chains and compound conditions in one program
• Separate deciding from drawing
• Compare text with ==
• Test that every branch can happen

Key Terms

State

What is currently true about the program — here, which lamp is lit.

Branch coverage

Checking that every branch of your decisions can actually run.

String comparison

Using == on text instead of numbers.

'F' → Fullscreen

Build

You are combining two programs you have already written: the bouncing ball, and the traffic signal from session 3. The signal now responds to where the ball is.

Task 1: Plan the Rules on Paper

Before you open VS Code. On your sheet, write the three rules in plain English — when is it red, when is it yellow, when is it green?

Then mark on a line from 0 to 800 where the signal sits and where each rule takes over. Two of the rules need to know which direction the ball is going. Say which two, and why.

Task 2: The Ball and the Signal, Not Talking

Copy template-game to 1-12-crossing. Start with the variables:

1
local ballX = 100
2
local ballY = 470
3
local radius = 25
4
local speedX = 220
5

6
local signalX = 600
7
local signalY = 120
8
local gap = 90

Then the movement, which is last session's fixed version:

1
function love.update(dt)
2
    ballX = ballX + speedX * dt
3

4
    if ballX > 800 - radius and speedX > 0 then
5
        speedX = -speedX
6
    end
7

8
    if ballX < radius and speedX < 0 then
9
        speedX = -speedX
10
    end
11
end

Draw the signal body and three lamps in fixed colors, plus the ball. Run it and get that working before any lamp changes. A bouncing ball and a signal that ignores it.

Task 3: Work Out Which Lamp

At the top of love.draw, decide before you draw anything:

1
local lamp = "green"
2

3
if ballX > signalX - 100 and ballX < signalX + 100 then
4
    lamp = "red"
5
elseif ballX > signalX - 250 and speedX > 0 then
6
    lamp = "yellow"
7
end

Then give each lamp its own if / else, bright or dim:

1
function love.draw()
2
    local lamp = "green"
3

4
    if ballX > signalX - 100 and ballX < signalX + 100 then
5
        lamp = "red"
6
    elseif ballX > signalX - 250 and speedX > 0 then
7
        lamp = "yellow"
8
    end
9

10
    love.graphics.setColor(0.35, 0.35, 0.35)
11
    love.graphics.rectangle("fill", signalX - 50, signalY - 60, 100, 300)
12

13
    if lamp == "red" then
14
        love.graphics.setColor(0.9, 0.2, 0.2)
15
    else
16
        love.graphics.setColor(0.25, 0.1, 0.1)
17
    end
18
    love.graphics.circle("fill", signalX, signalY, 30)
19

20
    if lamp == "yellow" then
21
        love.graphics.setColor(0.9, 0.8, 0.2)
22
    else
23
        love.graphics.setColor(0.25, 0.22, 0.1)
24
    end
25
    love.graphics.circle("fill", signalX, signalY + gap, 30)
26

27
    if lamp == "green" then
28
        love.graphics.setColor(0.2, 0.8, 0.3)
29
    else
30
        love.graphics.setColor(0.1, 0.25, 0.12)
31
    end
32
    love.graphics.circle("fill", signalX, signalY + gap + gap, 30)
33

34
    love.graphics.setColor(1, 1, 1)
35
    love.graphics.circle("fill", ballX, ballY, radius)
36
end

Task 4: Test Every Branch

Watch a full round trip and record which lamp is lit in each of the five situations from the slides. One of them behaves in a way you might not expect.

Compare rows 2 and 5. The ball is in the same place both times and the signal does something different. Write down what the program is using to tell them apart.

Then look hard at row 4. The ball has already gone past the signal and is heading away from it, and the lamp is still yellow. The rules do exactly what they say; they just do not say what a person would mean. Write down a rule that would fix it.

Task 5: Make One Rule Your Own

Change one of the three rules and test it again. Some options:

• Make the red zone narrower, so red is a shorter moment
• Make yellow work in both directions
• Add a fourth state for when the ball is stopped — set speedX to 0 to test it

Whatever you change, write the new rule on your sheet in English first and test all five situations again afterward.

Challenge (Optional): Break It on Purpose

Misspell one of the lamp names — change lamp = "yellow" to lamp = "yelow" and leave the comparison alone.

No error. Nothing crashes. Describe exactly what you see, and explain why this kind of mistake is harder to find than a misspelled variable name.

Challenge (Optional): Two Balls

Add a second ball with its own position and speed. The signal should go red if either ball is under it, and yellow if either is approaching.

The conditions get long. Work out on paper what "either ball is under it" means as a single condition before you type it.

Checkpoint

• The three rules were written in English on the sheet first
• 1-12-crossing has a bouncing ball and a signal that responds to it
• All five test situations are recorded
• One rule has been changed and retested
• The lamp is decided once, above the drawing, rather than three times inside it

Reflection

Answer the following questions before submitting your work.

1. The lamp is worked out once at the top of love.draw and then used three times. Describe what the program would look like if each lamp worked out the rules for itself, and say which version you would rather change later.
2. Two of your test rows put the ball in the same place and got different lamps. Explain what the program is using to tell those two situations apart, and why a position alone is not enough.
3. Misspelling a lamp name produced no error at all, while misspelling a variable name in session 3 produced one immediately. Explain the difference, and say what that means for how carefully you have to test text comparisons.

Submit

Submit the required files to the appropriate dropbox.

Activity Complete