Activity 1.3

Variables and Types

Key Concepts

A variable gives a value a useful name. Its type tells C# what kind of value the variable can store.

Where We Left Off

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Raylib.DrawCircle(400, 190, 35, Color.Red);
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Raylib.DrawCircle(400, 300, 35, Color.Yellow);
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Raylib.DrawCircle(400, 410, 35, Color.Green);

The Problem With Hardcoded Numbers

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Raylib.DrawCircle(400, 190, 35, Color.Red);
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Raylib.DrawCircle(400, 300, 35, Color.Yellow);
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Raylib.DrawCircle(400, 410, 35, Color.Green);

To move the signal, you must find and change the same idea in several places.

Give the Value a Name

int lightX = 400;

Read the declaration in three parts:

1. int means this variable stores a whole number.
2. lightX is the name.
3. = 400 assigns its starting value.

The Type Stays Fixed

A variable's value may change, but its type does not. An int variable can store a different whole number, but it cannot store text.

int score = 0;
score = "ten";

error CS0029: Cannot implicitly convert type 'string' to 'int'

Use the Name Wherever the Idea Appears

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int lightX = 400;
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int lightY = 190;
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int radius = 35;
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int gap = 110;
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Raylib.DrawCircle(lightX, lightY, radius, Color.Red);
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Raylib.DrawCircle(lightX, lightY + gap, radius, Color.Yellow);
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Raylib.DrawCircle(lightX, lightY + gap * 2, radius, Color.Green);

Names Can Be Used in Arithmetic

Raylib.DrawCircle(lightX, lightY + gap * 2, radius, Color.Green);

C# calculates each argument before Raylib draws the circle.

Assignment Has a Direction

int score = 40;

score = score + 10;

Assignment Has a Direction

int score = 40;

score = score + 10;

If score holds 40, the right side becomes 50. Then 50 goes back into score.

One Edit Can Move a Whole Scene

Good Names Are Part of the Program

lightX tells the reader what 400 means. x1 only saves typing.

Months from now, you will explain your own program on the AP exam. Name values for the idea they represent.

Today's Objectives

• Create variables with clear names.
• Identify int as the whole-number type.
• Use variables in drawing calls and arithmetic.
• Translate assignment into exam notation.

Key Terms

Variable

A named storage location whose value may change.

Assignment

The instruction that puts a value into a variable.

Type

A category that determines what kind of value a variable stores.

Integer

A whole number. C# uses the type name int.

'F' → Fullscreen

Build

You will make one small program, then refactor yesterday's traffic signal without changing its appearance.

Task 1: One Circle, Three Names

1. Copy template-game.
2. Rename the copy 1-03-circle.
3. Add circleX, circleY, and radius above the Raylib setup.
4. Use the three names in the circle drawing call.
5. Run the program.
6. Change circleX to 150 and run again.
7. Change radius and describe what stays fixed.

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using Raylib_cs;
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int screenWidth = 800;
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int screenHeight = 600;
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int circleX = 400;
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int circleY = 300;
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int radius = 40;
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Raylib.InitWindow(screenWidth, screenHeight, "One Circle");
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Raylib.SetTargetFPS(60);
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while (!Raylib.WindowShouldClose())
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{
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    Raylib.BeginDrawing();
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    Raylib.ClearBackground(Color.Black);
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    Raylib.DrawCircle(circleX, circleY, radius, Color.SkyBlue);
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    Raylib.EndDrawing();
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}
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Raylib.CloseWindow();

Task 2: Name the Signal's Numbers

1. Open yesterday's 1-02-signal folder.
2. Copy it and rename the copy 1-03-signal.
3. Add lightX, lightY, radius, and gap above the setup.
4. Replace one hardcoded number at a time. Run after every replacement.
5. Stop when the picture looks exactly as it did before. An unchanged picture is the success condition.

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using Raylib_cs;
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int screenWidth = 800;
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int screenHeight = 600;
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int lightX = 400;
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int lightY = 190;
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int radius = 35;
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int gap = 110;
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Raylib.InitWindow(screenWidth, screenHeight, "Traffic Signal");
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Raylib.SetTargetFPS(60);
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while (!Raylib.WindowShouldClose())
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{
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    Raylib.BeginDrawing();
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    Raylib.ClearBackground(Color.Black);
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    Raylib.DrawRectangle(lightX - 50, lightY - 60, 100, 340, Color.Gray);
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    Raylib.DrawCircle(lightX, lightY, radius, Color.Red);
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    Raylib.DrawCircle(lightX, lightY + gap, radius, Color.Yellow);
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    Raylib.DrawCircle(lightX, lightY + gap * 2, radius, Color.Green);
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    Raylib.EndDrawing();
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}
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Raylib.CloseWindow();

The rectangle uses lightX - 50 because rectangles are placed by a corner while circles are placed by their centers.

Task 3: Move the Whole Signal

1. Change only lightX and run.
2. Count how many shapes moved.
3. Change only lightY and run.
4. Try the four values below. Explain why the rectangle leaves extra room at the bottom.

int lightX = 650;
int lightY = 125;
int radius = 30;
int gap = 95;

Challenge (Optional): Break the Type

1. Add the two lines below anywhere above the Raylib setup.
2. Run and record the error code.
3. Explain how the message shows that an int variable can only store whole numbers.
4. Delete both lines.

int score = 0;
score = "ten";

error CS0029: Cannot implicitly convert type 'string' to 'int'

Challenge (Optional): Break the Name

1. Change one use of lightX to lowercase lightx.
2. Run and compare the error with the type error.
3. Repair the capital letter.

Raylib.DrawCircle(lightx, lightY, radius, Color.Red);

error CS0103: The name 'lightx' does not exist in the current context

Same Idea, Exam Notation

The AP exam uses an arrow for assignment. It does not include C#'s type declaration because the exam notation does not declare types.

int lives = 3;
lives = lives - 1;
Console.WriteLine(lives);

lives ← 3
lives ← lives - 1
DISPLAY(lives)

On the exam, = means “is equal to.” In C#, = is assignment. The comparison operator arrives later.

Checkpoint

Test your final 1-03-signal program:

• Changing lightX alone moves all four shapes left or right.
• Changing lightY alone moves all four shapes up or down.
• Changing radius changes all three lights.
• Changing gap changes the spacing between the lights.
• The program compiles without an error.

Reflection

Answer the following questions before submitting your work.

1. The refactored signal initially looked identical to yesterday's version. What did the variables improve if the picture did not change?
2. If score holds 40, explain how C# evaluates score = score + 10; and state the final value.
3. Why can a variable declared with int score = 0; not later store the text "ten"?

Submit

Submit your completed activity sheet. Keep 1-03-circle and 1-03-signal in your workspace.

Activity Complete