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# Run out
Arcade advanced skills · Power-ups
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## Today
**We are learning to** a number that counts down limits a power-up, and the event checks it first, so it never goes below zero.
- 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.
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## Why it matters
> Ammo, fuel, a shield that flickers out: the best power-ups run out. Behind each one is a number counting down.
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💬 Discussion
## Predict
**ammo starts at 3. You press A five times. What happens?**
- 5 shots fly, and the ship says -2
- 5 shots fly, and the ship says 3
- 3 shots fly, then the game ends
- 3 shots fly, then A does nothing
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✅ Check
## What happened?
* 5 shots fly and ammo goes 2, 1, 0, -1, -2. Nothing checks there is ammo left, so the count goes below zero.
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✍️ Activity
## Run it
Open the lesson at this step and try it together.
[studio.learningwaypoints.com/tutorials/code/arcade-advanced/arcade-run-out/?full=step-2#step-2](https://studio.learningwaypoints.com/tutorials/code/arcade-advanced/arcade-run-out/?full=step-2#step-2)
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💬 Discussion
## Investigate
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✅ Check
## Shots left
**This version fires only if ammo > 0. Fill in each press of A.**
| ammo before | a shot? | ammo after |
|---|---|---|
| 3 | yes | 2 |
| 1 | ? | ? |
| 0 | ? | ? |
* Answer: yes, 0, no, 0
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✅ Check
## Why > 0?
**Why ammo > 0, and not ammo ≥ 0?**
- There is no difference
- ≥ is not allowed in Arcade
- > 0 makes it fire faster
- With ≥ 0, a shot fires at 0 and ammo goes to -1: one shot too many
* Answer: With ≥ 0, a shot fires at 0 and ammo goes to -1: one shot too many
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✅ Check
## Lily's blaster
**Lily put the if around change ammo by -1 only, not around the shot. What happens after 3 shots?**
- A does nothing
- Shots keep firing, though ammo stays at 0
- The game ends
- Ammo goes below zero
* Answer: Shots keep firing, though ammo stays at 0
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✍️ Activity
## Three sweets (3 min)
A student has three counters. Each "fire" hands one over, but only if they still have one.
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✍️ Activity
## Modify
- **Refill:** A star **refills** the ammo to 5.
- **Top up:** Now a star **adds 3** to what you have. Which is better for a player with 4 shots left?
[Open this step](https://studio.learningwaypoints.com/tutorials/code/arcade-advanced/arcade-run-out/?full=step-4#step-4)
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✅ Check
## 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 again
- It scores 5 too
- Nothing happens
- It costs a life: hunting is now false
* Answer: It costs a life: hunting is now false
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✍️ Activity
## Make
The shield from last lesson. A new variable, `shieldTime`, is set to **5** when you catch a star.
1. Every second while the shield is on, take 1 off `shieldTime`.
2. When `shieldTime` reaches **0**, set `shield` to **false** and put the black background back.
[Open this step](https://studio.learningwaypoints.com/tutorials/code/arcade-advanced/arcade-run-out/?full=step-6#step-6)
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✍️ Activity
## 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.
[Open this step](https://studio.learningwaypoints.com/tutorials/code/arcade-advanced/arcade-run-out/?full=step-7#step-7)
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✍️ Activity
## Fair or not? (4 min)
Play the next pair's sprint boost. Is three boosts fair?
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✅ Check
## Exit ticket
**Why does the A event ask ammo > 0 before it fires?**
* So it only fires while there are shots left. Without the check, ammo goes below zero and the shots never run out.