Variables and Interactive Stories

30 min

Let's begin

Niyonzima wants to make a Scratch story called Keza and the Lost Goat. He wants the reader to click on things and choose where Keza searches, and he wants the story to count the clues Keza finds. To count, he needs a variable. To let the reader take part, he needs events.

What is a variable?

A variable is a named box that stores a value, and the value can change while the program runs: a score, the number of lives, the time left, or a player's name.

To make one: click Variables, then Make a Variable, type a name such as score, choose For all sprites or For this sprite only, and press OK. The variable appears on the stage.

Block	What it does	Example
set [score] to (0)	Puts an exact value in the box, replacing what was there.	Start every game at 0.
change [score] by (1)	Adds to the value already in the box.	Add 1 point each time you catch something. Use −1 to count down.
show variable / hide variable	Shows or hides the box on the stage.	Hide the score on the title screen.

Keeping score

In the Raspberry Pi project Ghostbusters, a ghost appears in random places and the player clicks it:

when green flag clicked
set [score] to (0)

when this sprite clicked
hide
change [score] by (1)

A countdown timer

when green flag clicked
set [time] to (30)
repeat until <(time) = (0)>
wait (1) seconds
change [time] by (-1)
broadcast [end]

Making a story interactive

A good story has a beginning, a middle and an end. Events blocks let the reader decide when things happen:

when this sprite clicked: runs when the reader clicks a sprite (see the picture below: each click turns the sprite 15 degrees).
when [space] key pressed: runs when a key is pressed, such as the left or right arrow.
when backdrop switches to [ ]: runs when the scene changes.
broadcast [message] and when I receive [message]: a broadcast is like an announcement over a loudspeaker. Every sprite hears it, and sprites with the matching when I receive block react.

Niyonzima's story plan

Part	What happens	Blocks used
Beginning	Village backdrop. Keza says "My goat is lost! Click me to start the search."	when green flag clicked, switch backdrop, say, set [clues] to (0)
Middle	The reader presses the left arrow for the forest or the right arrow for the river. Clicking hidden objects (a hoofprint, a rope, some chewed leaves) adds a clue.	when key pressed, switch backdrop, when this sprite clicked, change [clues] by (1)
End	When 3 clues are found, the goat appears and Keza says "Thank you for helping me!"	if (clues) = (3) then broadcast [found]; when I receive [found], show

Tip

SET once at the start, CHANGE every time something happens. If you forget to set the score to 0, a new game starts with the old game's points.

✦

Try it yourself

The score starts at 0, and change [score] by (1) runs 5 times. What is the score?
Why do we set the score to 0 when the green flag is clicked?
The time is set to 30 and changes by −1 every second. What is it after 10 seconds?
Which Events block lets the story move on when the reader clicks the character?
What does a broadcast block do?

Answers

5.
So that every new game starts from zero instead of carrying on from the last game's score.
30 − 10 = 20.
when this sprite clicked.
It sends a message that every sprite can hear; sprites with a matching when I receive block then run their scripts.

Quick check

What you learned today

A variable is a named box whose value can change; set gives it an exact value, change adds to it.
Variables keep scores and timers.
Events (sprite clicked, key pressed, broadcasts) turn a story with a beginning, middle and end into an interactive one.

Sources and credits

Ghostbusters, by the Raspberry Pi Foundation, Raspberry Pi Projects, CC BY-SA 4.0, adapted by Ganzaa.
Brain game, by the Raspberry Pi Foundation, Raspberry Pi Projects, CC BY-SA 4.0, adapted by Ganzaa.
Scratch 3.0 Screenshot Sprite Clicked Example, by Blink456, Wikimedia Commons, CC BY-SA 4.0, used unchanged.

The exercises (Try it yourself and the practice questions) are written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1What is a variable?

2Keeping score

3A countdown timer

4Making a story interactive

5Niyonzima's story plan