True or false

A variable that holds true or false remembers whether a power-up is on, so an overlap can ask about it.

A mushroom in Mario, a power pellet in Pac-Man, a shield in a space shooter: each one is a variable that is true for a while.

A variable can hold true or false (in Logic). In this game shield starts false. Catching a yellow star sets it to true. When a rock hits, if shield asks whether it is true right now: true smashes the rock for 5 points, false costs a life.

Predict: Read the code and say what it will do before you run anything. A wrong guess is fine: it shows you what to look for.
Run: Run it and compare the result with your prediction.
Investigate: Trace how each line of code connects to what you see.
Modify: Change the code on purpose and check the result.
Make: Build something new using what you have learned. Expect a few tries; that is normal.

Explain it to me

A power-up has three parts · Sound on for the voiceover

A power-up has three parts:

a variable that remembers it: shield is true while it is on,
an event that changes it: catching a star sets it to true,
a rule that uses it: if shield, a rock costs no life.

It stays true until a block sets it back to false.

Show as code

let rock: Sprite = null
let star: Sprite = null
let shield = false
sprites.onOverlap(SpriteKind.Player, SpriteKind.Enemy, function (sprite, otherSprite) {
    otherSprite.destroy()
    if (shield) {
        info.changeScoreBy(5)
    } else {
        info.changeLifeBy(-1)
    }
})
sprites.onOverlap(SpriteKind.Player, SpriteKind.Food, function (sprite, otherSprite) {
    otherSprite.destroy()
    shield = true
    scene.setBackgroundColor(8)
})
scene.setBackgroundColor(15)
let ship = sprites.create(img`
    8 8 . . . . . .
    8 9 9 9 9 . . .
    8 9 1 1 9 9 9 9
    8 9 9 9 9 . . .
    8 8 . . . . . .
    `, SpriteKind.Player)
ship.setPosition(20, 60)
controller.moveSprite(ship, 0, 100)
ship.setStayInScreen(true)
info.setLife(3)
game.onUpdateInterval(1000, function () {
    rock = sprites.create(img`
        . e e e .
        e e d e e
        e d e e e
        . e e e .
        `, SpriteKind.Enemy)
    rock.setPosition(160, randint(10, 110))
    rock.vx = -50
    info.changeScoreBy(1)
})
game.onUpdateInterval(5000, function () {
    star = sprites.create(img`
        . . 5 . .
        5 5 5 5 5
        . 5 5 5 .
        . 5 . 5 .
        `, SpriteKind.Food)
    star.setPosition(160, randint(10, 110))
    star.vx = -30
})

The example, as blocks

What it makes (a moment from the game)

PredictRead the code and say what it will do before you run anything. A wrong guess is fine: it shows you what to look for.

Predict

Show as code

lesson.ts

let rock: Sprite = null
let star: Sprite = null
let shield = false
sprites.onOverlap(SpriteKind.Player, SpriteKind.Enemy, function (sprite, otherSprite) {
    otherSprite.destroy()
    if (shield) {
        info.changeScoreBy(5)
    } else {
        info.changeLifeBy(-1)
    }
})
sprites.onOverlap(SpriteKind.Player, SpriteKind.Food, function (sprite, otherSprite) {
    otherSprite.destroy()
    shield = true
    scene.setBackgroundColor(8)
})
scene.setBackgroundColor(15)
let ship = sprites.create(img`
    8 8 . . . . . .
    8 9 9 9 9 . . .
    8 9 1 1 9 9 9 9
    8 9 9 9 9 . . .
    8 8 . . . . . .
    `, SpriteKind.Player)
ship.setPosition(20, 60)
controller.moveSprite(ship, 0, 100)
ship.setStayInScreen(true)
info.setLife(3)
game.onUpdateInterval(1000, function () {
    rock = sprites.create(img`
        . e e e .
        e e d e e
        e d e e e
        . e e e .
        `, SpriteKind.Enemy)
    rock.setPosition(160, randint(10, 110))
    rock.vx = -50
    info.changeScoreBy(1)
})
game.onUpdateInterval(5000, function () {
    star = sprites.create(img`
        . . 5 . .
        5 5 5 5 5
        . 5 5 5 .
        . 5 . 5 .
        `, SpriteKind.Food)
    star.setPosition(160, randint(10, 110))
    star.vx = -30
})

Read the two overlap events. The background turns dark blue while the shield is on.

You catch one star. How long does the shield last?5 secondsUntil the next rock hits youFor the rest of the gameOne frame

RunRun it and compare the result with your prediction.

Run

Catch a star (one floats past every 5 seconds), then fly into the rocks. Is this power-up fair?

Show as code

lesson.ts

let rock: Sprite = null
let star: Sprite = null
let shield = false
sprites.onOverlap(SpriteKind.Player, SpriteKind.Enemy, function (sprite, otherSprite) {
    otherSprite.destroy()
    if (shield) {
        info.changeScoreBy(5)
    } else {
        info.changeLifeBy(-1)
    }
})
sprites.onOverlap(SpriteKind.Player, SpriteKind.Food, function (sprite, otherSprite) {
    otherSprite.destroy()
    shield = true
    scene.setBackgroundColor(8)
})
scene.setBackgroundColor(15)
let ship = sprites.create(img`
    8 8 . . . . . .
    8 9 9 9 9 . . .
    8 9 1 1 9 9 9 9
    8 9 9 9 9 . . .
    8 8 . . . . . .
    `, SpriteKind.Player)
ship.setPosition(20, 60)
controller.moveSprite(ship, 0, 100)
ship.setStayInScreen(true)
info.setLife(3)
game.onUpdateInterval(1000, function () {
    rock = sprites.create(img`
        . e e e .
        e e d e e
        e d e e e
        . e e e .
        `, SpriteKind.Enemy)
    rock.setPosition(160, randint(10, 110))
    rock.vx = -50
    info.changeScoreBy(1)
})
game.onUpdateInterval(5000, function () {
    star = sprites.create(img`
        . . 5 . .
        5 5 5 5 5
        . 5 5 5 .
        . 5 . 5 .
        `, SpriteKind.Food)
    star.setPosition(160, randint(10, 110))
    star.vx = -30
})

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It takes a few seconds to load. Your work in this step is kept.

I caught a star and saw how long the shield lasted.

InvestigateTrace how each line of code connects to what you see.

Investigate

Show as code

lesson.ts

let rock: Sprite = null
let star: Sprite = null
let shield = false
sprites.onOverlap(SpriteKind.Player, SpriteKind.Enemy, function (sprite, otherSprite) {
    otherSprite.destroy()
    if (shield) {
        info.changeScoreBy(5)
    } else {
        info.changeLifeBy(-1)
    }
})
sprites.onOverlap(SpriteKind.Player, SpriteKind.Food, function (sprite, otherSprite) {
    otherSprite.destroy()
    shield = true
    scene.setBackgroundColor(8)
})
scene.setBackgroundColor(15)
let ship = sprites.create(img`
    8 8 . . . . . .
    8 9 9 9 9 . . .
    8 9 1 1 9 9 9 9
    8 9 9 9 9 . . .
    8 8 . . . . . .
    `, SpriteKind.Player)
ship.setPosition(20, 60)
controller.moveSprite(ship, 0, 100)
ship.setStayInScreen(true)
info.setLife(3)
game.onUpdateInterval(1000, function () {
    rock = sprites.create(img`
        . e e e .
        e e d e e
        e d e e e
        . e e e .
        `, SpriteKind.Enemy)
    rock.setPosition(160, randint(10, 110))
    rock.vx = -50
    info.changeScoreBy(1)
})
game.onUpdateInterval(5000, function () {
    star = sprites.create(img`
        . . 5 . .
        5 5 5 5 5
        . 5 5 5 .
        . 5 . 5 .
        `, SpriteKind.Food)
    star.setPosition(160, randint(10, 110))
    star.vx = -30
})

Trace a rock hitting the ship.

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It takes a few seconds to load. Your work in this step is kept.

Fill in what happens when a rock hits the ship.

shield	lives before	lives after	score change
false	3
true	3
false	1

Which needs true or false, and which needs a number?

Is the shield on?

How many lives are left?

Has the player found the key?

How many shots are left?

Tom's shield smashes every rock for the rest of the game after one star. What is missing?An if in on startA block that sets shield back to falseAnother set shield to trueA second star

ModifyChange the code on purpose and check the result.

Modify

Make the shield break after it smashes one rock.

Need a hint?

The shield should break when it smashes a rock: inside if shield, after the points.

Another hint

set shield to false, and set the background back to 15 (black).

If the shield is already on, a star is worth 3 points.

Need a hint?

In the star overlap, before shield is set to true: if shield, change score by 3.

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The shield breaks after one smash.

This ticks by itself when your blocks do what is asked.

A star caught while the shield is already on is worth 3 points.

This ticks by itself when your blocks do what is asked.

InvestigateTrace how each line of code connects to what you see.

Mix it up

colours is 9, 4, 2, 7 and level is 2. What number does colours[level] give?

MakeBuild something new using what you have learned. Expect a few tries; that is normal.

Make

A ghost chases you. A power pellet appears every 6 seconds. Make it a power-up:

a variable hunting is false at the start,
catching a pellet sets it to true (show it with a background colour),
touching the ghost while hunting scores 5 and sets hunting back to false; otherwise it costs a life.

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It takes a few seconds to load. Your work in this step is kept.

Need a hint?

A new variable, hunting, set to false in on start.

Another hint

The pellet overlap: destroy the pellet, set hunting to true, and change the background.

Another hint

The ghost overlap: if hunting, score 5, set hunting to false and put the background back; else lose a life. Either way, send the ghost to the corner.

A power pellet: catch it and the next ghost you touch is worth 5 points, not a life.

This ticks by itself when your blocks do what is asked.

ModifyChange the code on purpose and check the result.

Change a key and a door

Find the key, then reach the door. Right now the door opens with no key. Use hasKey so the door only opens if you have the key, and if you don't, splash "Locked!" and move the explorer back a step (or the splash comes back every frame).

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For teachers

Students use a true or false variable to remember whether a shield is on, see that it stays true until a block sets it back, and build a power pellet. About 45 minutes.

Open the teacher guide: answers, solutions, slides and a worksheet.

Students will be able to

I can use a true or false variable to remember whether a power-up is on.
I can set it back to false at the right moment.
I can build a power-up from a variable, an event and a rule.

Before class

A variable can hold true or false (Logic). A power-up is three parts: a variable that remembers it (shield), an event that changes it (catching a star sets it to true) and a rule that uses it (if shield in the rock overlap).

The misconception: a true or false variable stays as it is until a block changes it, so a shield set once lasts for ever. Modify fixes it the simplest way (break after one smash); next lesson makes it run out with a countdown.

Running the lesson

Predict and Run: the shield never ends. Ask: is this fair? What would make it fair?
Investigate: the match separates yes-or-no from how many.
Modify: one smash needs set shield to false inside the if.
Mix it up: lists from last lesson.
Make: ask students to name the variable, the event and the rule before they start.

The hard part (on purpose)

Setting it back. When a power-up never ends, ask which block sets it to false.

When students get stuck

The shield never switches on. Ask: Where is shield set to true, and does that event ever run? Fix: Inside the Player–Food overlap; check the star is made as Food.
The shield is on from the start. Ask: What is shield set to in on start? Fix: false.
Hunting lasts for ever. Ask: Which block sets hunting back to false? Fix: Add set hunting to false inside if hunting, in the ghost overlap.

Stretch and support

Stretch: Two pellets in a row: count the ghosts eaten while hunting, and give 5, then 10.

Support: Give students the hunting variable and the pellet overlap, so they write only the if in the ghost overlap.

Where it leads

Next: run out, a number that counts down so a power-up lasts a set time. Skillmap: Save the Galaxy (arcade.makecode.com/skillmaps) adds power-ups to a shooter.

What each step is for

Investigate: A true or false variable remembers whether something is on; an event sets it, an if asks it, and it stays as it is until a block sets it back.
Modify: Setting the variable back to false ends the power-up; any rule can ask the same variable.
Mix it up: A list holds what changes per level, looked up with the level as the place, counted from 0.

Now you can

Use a true or false variable to remember whether a power-up is on.
Set it back to false at the right moment.
Build a power-up from a variable, an event and a rule.

Next lesson9. Run outA number that counts down limits a power-up, and the event checks it first, so it never goes below zero.

Source: content/code/arcade-advanced/arcade-flag/lesson.md