Run out

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

Ammo, fuel, a shield that flickers out: the best power-ups run out. Behind each one is a number counting down.

A power-up that runs out needs a number: shots left, seconds left. Each use takes 1 away. The ship says how many shots are left after each one.

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

ammo counts the shots left · Sound on for the voiceover

A power-up that runs out is a number counting down.

The event asks first: if ammo > 0, fire, then change ammo by -1. So it stops at zero.

For time, a timer takes 1 away every second, and at 0 switches the power-up off.

Show as code

let star: Sprite = null
let shot: Sprite = null
let rock: Sprite = null
let ammo = 3
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)
controller.A.onEvent(ControllerButtonEvent.Pressed, function () {
    if (ammo > 0) {
        shot = sprites.createProjectileFromSprite(img`
            4 4 4
            `, ship, 100, 0)
        ammo += -1
    }
    ship.sayText(ammo)
})
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
})
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
})
sprites.onOverlap(SpriteKind.Projectile, SpriteKind.Enemy, function (sprite, otherSprite) {
    sprite.destroy()
    otherSprite.destroy()
    info.changeScoreBy(1)
})
sprites.onOverlap(SpriteKind.Player, SpriteKind.Food, function (sprite, otherSprite) {
    otherSprite.destroy()
})

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 shot: Sprite = null
let rock: Sprite = null
let ammo = 3
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)
controller.A.onEvent(ControllerButtonEvent.Pressed, function () {
    shot = sprites.createProjectileFromSprite(img`
        4 4 4
        `, ship, 100, 0)
    ammo += -1
    ship.sayText(ammo)
})
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
})
sprites.onOverlap(SpriteKind.Projectile, SpriteKind.Enemy, function (sprite, otherSprite) {
    sprite.destroy()
    otherSprite.destroy()
    info.changeScoreBy(1)
})

Look at the on A pressed block. Does anything ask whether there is ammo left?

ammo starts at 3. You press A five times. What happens?5 shots fly, and the ship says -25 shots fly, and the ship says 33 shots fly, then the game ends3 shots fly, then A does nothing

RunRun it and compare the result with your prediction.

Run

Press A (space) again and again. Watch what the ship says.

Show as code

lesson.ts

let shot: Sprite = null
let rock: Sprite = null
let ammo = 3
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)
controller.A.onEvent(ControllerButtonEvent.Pressed, function () {
    shot = sprites.createProjectileFromSprite(img`
        4 4 4
        `, ship, 100, 0)
    ammo += -1
    ship.sayText(ammo)
})
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
})
sprites.onOverlap(SpriteKind.Projectile, SpriteKind.Enemy, function (sprite, otherSprite) {
    sprite.destroy()
    otherSprite.destroy()
    info.changeScoreBy(1)
})

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

I pressed A more than three times and compared with my prediction.

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

Investigate

This version checks first. Try to fire a fourth shot.

Show as code

lesson.ts

let star: Sprite = null
let shot: Sprite = null
let rock: Sprite = null
let ammo = 3
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)
controller.A.onEvent(ControllerButtonEvent.Pressed, function () {
    if (ammo > 0) {
        shot = sprites.createProjectileFromSprite(img`
            4 4 4
            `, ship, 100, 0)
        ammo += -1
    }
    ship.sayText(ammo)
})
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
})
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
})
sprites.onOverlap(SpriteKind.Projectile, SpriteKind.Enemy, function (sprite, otherSprite) {
    sprite.destroy()
    otherSprite.destroy()
    info.changeScoreBy(1)
})
sprites.onOverlap(SpriteKind.Player, SpriteKind.Food, function (sprite, otherSprite) {
    otherSprite.destroy()
})

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This version fires only if ammo > 0. Fill in each press of A.

ammo before	a shot?	ammo after
3	yes	2
1
0

Why ammo > 0, and not ammo ≥ 0?There is no difference≥ is not allowed in Arcade> 0 makes it fire fasterWith ≥ 0, a shot fires at 0 and ammo goes to -1: one shot too many

Lily put the if around change ammo by -1 only, not around the shot. What happens after 3 shots?A does nothingShots keep firing, though ammo stays at 0The game endsAmmo goes below zero

ModifyChange the code on purpose and check the result.

Modify

A star refills the ammo to 5.

Need a hint?

A star floats past every 5 seconds. Its overlap only destroys it.

Another hint

Add set ammo to 5 in the Player–Food overlap.

Now a star adds 3 to what you have. Which is better for a player with 4 shots left?

Need a hint?

Use change ammo by 3 in place of set.

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A star refills the ammo to 5.

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

A star adds 3 shots to what you have left.

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

hunting is true. The player touches a ghost, and the overlap scores 5 and sets hunting to false. A second ghost touches a moment later. What happens?hunting turns true againIt scores 5 tooNothing happensIt costs a life: hunting is now false

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

Make

The shield from last lesson. A new variable, shieldTime, is set to 5 when you catch a star.

Every second while the shield is on, take 1 off shieldTime.
When shieldTime reaches 0, set shield to false and put the black background back.

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Need a hint?

Catching a star already sets shieldTime to 5. In the timer that runs every 1000 ms, add: if shield, change shieldTime by -1.

Another hint

Inside that if, add another: if shieldTime = 0, set shield to false and set the background back to 15 (black).

The shield switches off by itself after 5 seconds.

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

ModifyChange the code on purpose and check the result.

Change a sprint boost

A runner with a sprint boost on B (enter on the keyboard): 3 seconds of extra speed, then back to normal. Make it fair: only 3 boosts for the whole game, counted down.

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For teachers

Students limit a blaster with a number that counts down, check it first so it never goes below zero, refill it with a star, and make a shield run out after 5 seconds. About 45 minutes.

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

Students will be able to

I can limit an action with a number that counts down.
I can check the number first, so it never goes below zero.
I can make a power-up last a set time with a timer.

Before class

A power-up that runs out is a number counting down. For ammo, the A event asks if ammo > 0 first, then fires and changes ammo by -1. Without the check, ammo goes below zero and the shots never stop (the misconception). With ≥ 0 there is one shot too many.

For time, a timer every 1000 ms counts shieldTime down while the shield is on, and at 0 sets the shield back to false: an if inside an if. Set (refill to 5) and change (add 3) are different, which Modify brings out.

Running the lesson

Predict: let students find the -2 in the Run.
Investigate: the trace shows the if saying no at 0. Lily's blaster checks the wrong block.
Modify: ask which refill is kinder to a player with 4 shots.
Mix it up: true or false from last lesson.
Make: read it aloud: every second, if the shield is on, count down; if it reached zero, switch it off.

The hard part (on purpose)

Checking before using. When the count goes below zero, ask what the event asks first.

When students get stuck

Shots never run out. Ask: Is the shot inside the if? Fix: Put both the shot and change ammo by -1 inside if ammo > 0.
The shield switches off straight away, or never. Ask: Where is the countdown, and when does that block run? Fix: In the 1000 ms timer, inside if shield; set it to false only inside if shieldTime = 0.
The countdown goes 5, 3, 1, -1. Ask: How many timers count it down? Fix: One: remove the second change.

Stretch and support

Stretch: Show the shield time with the ship's say block, every second.

Support: Give students the if shield in the timer, so they add only the countdown and the switch-off.

Where it leads

Next: the Power-ups challenge, with less help. Skillmap: Save the Galaxy (arcade.makecode.com/skillmaps).

What each step is for

Investigate: A number that counts down limits an action: the event asks it first, and only then uses one up.
Modify: Set throws away what is left; change by adds to it.
Mix it up: A true or false variable stays as it is until a block sets it back.

Now you can

Limit an action with a number that counts down.
Check the number first, so it never goes below zero.
Make a power-up last a set time with a timer.

Next lesson10. Power-upsChallenge: grow the shield game into a prize box: a blaster with limited ammo, a smart bomb, then a power-up of your own.

Source: content/code/arcade-advanced/arcade-run-out/lesson.md