Game Builder: Change the Rules with Variables

Before you start: try it once yourself

Break and fix Meadow Run yourself once before you start, about ten minutes. The arrow keys run, Space jumps, R starts the level over and Esc goes back to the editor.

1. Open the lesson. Go to mytekdev.com/try/gamemaker?lesson=change-the-rules-with-variables&ref=fam-gamebuilder-1. Meadow Run opens in the editor with a short guide along the bottom; press Play, run to the star, then press Esc.
2. Read the variables. Click Player on the right. The Movement tab shows Gravity 0.5, Jump Power 10 and Move Speed 4.
3. Break it. Set Jump Power to 8, press Done, then Play, and run right. The player drops into the low valley with the mushroom and cannot jump back out of it.
4. See it in the code. Set Jump Power back to 10 and press Done. Click the Code Preview bar under the level, then click Game Settings on the right: the code shows player.velocityY = -10 next to jump power and += 0.5 next to gravity; press Done.
5. Gravity. Set Gravity to 0.8 and play: the player is trapped in the valley again. Then try 0.3, where the player floats and jumps much higher, and set it back to 0.5.

Jump Power 8: the same level, and the player cannot get out of the valley.

The ideas, in plain words

A variable is a name a program uses for a value it keeps, so the program can read the value wherever it needs it. In Game Builder, Gravity, Jump Power and Move Speed are variables. Each rule of the game is written once and reads those names, so changing one number changes that rule everywhere in the game, on every level, at once. The level itself, the tiles and the objects, is separate: it does not change when a number does.

The rules run 60 times a second, once a frame. Every frame, gravity is added to the player's falling speed. When Space is pressed and the player is on the ground, its upward speed is set to the Jump Power, and while an arrow key is held, its sideways speed is set to the Move Speed. The rules and the level have to fit each other: the valley in Meadow Run has walls 2 tiles tall, a Jump Power of 10 climbs them, and 8 climbs less than 2 tiles. A bigger gravity does the same thing from the other side, by taking the speed away sooner.

Do it together

1. Open mytekdev.com/try/gamemaker?lesson=change-the-rules-with-variables&ref=fam-gamebuilder-1 together. Meadow Run opens in the editor with a short guide along the bottom; follow it together. The arrow keys run, Space jumps, R starts the level over and Esc goes back to the editor. The guide has your child play to the star, open Player, set Jump Power to 8 and play again.

   If you get stuck: Esc switches between the editor and the game, so a second press starts the game again.

   You'll know it worked when: Running right, the player drops into the low valley with the mushroom and cannot jump back out.

2. Talk about what happened: the level did not change, only one number did. Gravity, Jump Power and Move Speed are variables, named numbers the game reads every time the player moves. The valley walls are 2 tiles tall; a Jump Power of 10 climbs them, and 8 climbs less than 2. Press Esc, click Player, set Jump Power back to 10 and press Done.

   You'll know it worked when: The line under Player on the right shows Jump 10 again, and your child can say why 8 got the player stuck.

3. Find the numbers in the code. Click the Code Preview bar under the level, then click Game Settings on the right. Press Done when you have looked.

   If you get stuck: Code Preview shows the gravity and jump code only after Game Settings is clicked, not Player.

   You'll know it worked when: The code shows player.velocityY = -10 next to jump power and += 0.5 next to gravity: the same numbers as in the Player box.

4. Predict, then test. Before touching the app, your child says or writes what Gravity 0.8 will do. Then click Player, set Gravity to 0.8, press Done, press Play and run to the valley. Do the same with 0.3, then set Gravity back to 0.5.

   If you get stuck: Check the line under Player after every test, because a number that was not set back spoils the next test.

   You'll know it worked when: Gravity 0.8 traps the player too, because it pulls harder and the jump is lower. At 0.3 the player floats and jumps much higher.

5. Now your child makes the game their own, with three rules: change at least two numbers, one at a time; press Play after every change; and a player must still be able to reach the star.

   You'll know it worked when: The game feels different, and your child can still run it from the start to the star.

6. The real question: when a change makes some part impossible to get past, ask your child to name two different ways to fix it. One is to change the number back a little. The other is to change the level: pick a tile under Tiles on the left and click on the level to draw it, for example a step in the valley wall. Right-click erases.

   You'll know it worked when: Your child names both, a number fix and a level fix. That means they see that the rules and the level work together, and that a variable is one place to change a rule for the whole game.

7. Trade seats. Your child presses Play first, because the Player box shows the numbers. You play their game for about 3 minutes without pressing Esc and guess which numbers they changed, and whether each went up or down. Then they tell you what really changed. Finish with the big question: how can changing one number change a whole game?

   You'll know it worked when: Your child explains each change they made and what it did to the way the game feels.

Talk about it

Ask these while you work together. A child who has understood the lesson can answer them without help:

• Which number did you change, and what did you predict?
• Why does Jump Power 8 trap the player in the valley?
• Gravity 0.8 traps the player too. Why does a bigger gravity make the jump lower?
• Show me the jump power in Code Preview (click Game Settings, not Player).
• Your player cannot get past this part. Name two different ways to fix it.

Bring it into other subjects

• Science: gravity on the Moon is about one sixth of gravity on Earth. Set Gravity to 0.1, the smallest it goes, and play. Is a Moon level easier or harder? (Answers vary: the player jumps very high and floats, so landing on a small platform takes care.)
• Math: measure how high each Jump Power reaches, in tiles. Does doubling the jump double the height?
• English: write the rules of your game for a player who has never played, in three sentences or fewer, including how high and how fast the player moves.
• Computer science: open Code Preview and click Game Settings. Find the line for move speed and say what happens to it when the left arrow is pressed. (The code sets the speed to minus the Move Speed, so the player moves left.)

Quick check

Name: ______________________________

1. With Jump Power 8 the player is stuck in the valley. What changed? (A) the level, (B) the value of a variable the game reads, (C) the player's picture.

2. Gravity goes from 0.5 to 0.8. The jump gets: (A) higher, (B) lower, (C) the same.

3. In someone's game, the player cannot climb a wall. Name two different ways to fix it.

Answers