[VRChat] The Four Parts of a Program: Variables, if, for, and Functions

You've made an Udon script and got a Cube moving. But you were copying sample code, and what it says is still unclear.

There isn't that much to learn. Most world gimmicks are made from just four parts.

This article uses those four in order, inside a single example. We'll build a button that changes three lights' color on each press and returns to the start after three.

What You'll Learn

• Variables: boxes that hold values
• if: a fork in the road, yes or no
• for: repeating the same thing
• Functions: giving a name to a piece of logic

Start from being able to create scripts, from getting started with UdonSharp.

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It's made from four parts

Think of programs as incantations and the amount to learn looks infinite. In practice there are only four kinds of thing going on.

Part	What it does	In one word
Variable	Stores a value	A box
if	Forks by condition	A fork in the road
for	Repeats the same thing	Repetition
Function	Names a piece of logic	A named procedure

Combine these four and you can write doors, lighting, and counters.

New gimmicks can be thought through within these four too. What do I store, where does it fork, what do I repeat, where do I split it. Answer those four and the code takes shape naturally.

Looking at them one at a time

Variables: boxes that hold values

A variable is a box. You put a value in and later take it out or overwrite it.

int pressCount = 0;   // Made a "box for whole numbers" and put 0 in it

int is "the kind of box," pressCount is "the box's name," and 0 is "the value it starts with."

Boxes have kinds. Text doesn't go in a whole-number box.

Written as	What it holds	Examples
int	Whole numbers	0 3 -5
float	Decimals	0.5f 3.14f
bool	Yes / no	true false
string	Text	"Opened"

The f on float values is a C# rule. Forgetting it gives an error, but the error tells you, so don't worry about it.

A box's contents can be changed as often as you like.

pressCount = pressCount + 1;   // Add 1 to what's in there and put it back

= doesn't mean "equals" — it means "put the thing on the right into the box on the left." That's the first hurdle, so keep it in mind.

if: a fork in the road, yes or no

An if is a fork. If the condition is yes, the inside runs; if no, it's skipped.

if (pressCount >= 3)
{
    pressCount = 0;   // Three or more, so back to 0
}

What goes in the ( ) is a question whose answer is yes or no.

Written as	Meaning
a == b	Are a and b the same
a != b	Are a and b different
a > b / a < b	Is a greater than b, less than b
a >= b / a <= b	At least, at most

Two == is for comparing; one = is for putting in. Everyone gets this wrong once too.

To do something on "no" as well, add an else.

if (isOpen)
{
    // What happens when open
}
else
{
    // What happens when closed
}

for: repeating the same thing

A for is repetition. It runs the same logic a set number of times.

The first time you need a for in world building is when there are several of the same thing. Three lights, five chairs, four signs. You could write each one out, but adding more means rewriting.

for (int i = 0; i < 3; i++)
{
    // This runs 3 times. i goes 0, 1, 2
}

The ( ) splits into three parts.

Part	Meaning
int i = 0	Make a counting box, starting at 0
i < 3	Repeat while this condition holds
i++	Add 1 after each pass

i++ is shorthand for i = i + 1.

Combined with arrays, it shows its worth.

for (int i = 0; i < lights.Length; i++)
{
    lights[i].enabled = true;   // Turn every light on
}

lights.Length is "how many are in the array." Since you don't write the count as a number, adding lights doesn't require editing the code.

Functions: giving something a name

A function is a piece of logic with a name. It groups a procedure you use repeatedly so you can call it by name.

private void ApplyColor(Color color)
{
    // Logic that applies the color
}

Reading it out:

Part	Meaning
private	Used only inside this script
void	Returns no value when called
ApplyColor	The name
(Color color)	What you pass when calling it

Calling it means just writing the name.

ApplyColor(Color.red);

There are two reasons to split things out. You avoid writing the same logic repeatedly. And the name becomes the explanation.

Seeing ApplyColor tells you "ah, it applies a color" without reading the body. Code is something you read back six months later. Naming things helps that version of you.

Hands-On: A button that changes color by press count

Build a button that changes three lights' color on each press and returns to the start after three. All four parts appear.

1. Place the lights and the button

In a scene with a floor, place the following.

Name	How to make it, and settings
Light1 Light2 Light3	Point Light. (-2, 2.5, 3) (0, 2.5, 3) (2, 2.5, 3). Range 6, Mode "Realtime"
ColorButton	Cube. (0, 1, 1), Scale (0.4, 0.4, 0.4)

2. Write the code

In Assets/Scripts, choose "Create → U# Script" and make ColorCycleButton.

using UdonSharp;
using UnityEngine;

[UdonBehaviourSyncMode(BehaviourSyncMode.None)]
public class ColorCycleButton : UdonSharpBehaviour
{
    [SerializeField] private Light[] targetLights;   // The three lights
    [SerializeField] private Color[] colors;         // Three colors

    private int pressCount = 0;                      // (1) Variable

    public override void Interact()
    {
        pressCount = pressCount + 1;

        // (2) if: once it hits 3 or more, go back to the start
        if (pressCount >= colors.Length)
        {
            pressCount = 0;
        }

        // (4) Call the function
        ApplyColor(colors[pressCount]);

        Debug.Log("[ColorCycle] color index " + pressCount);
    }

    // (4) Function: apply the color to every light
    private void ApplyColor(Color color)
    {
        // (3) for: repeat for as many lights as there are
        for (int i = 0; i < targetLights.Length; i++)
        {
            if (targetLights[i] == null) continue;   // (2) if: skip empty slots
            targetLights[i].color = color;
        }
    }
}

All four parts are gathered in one place.

Number	Part	Where and what it does
(1)	Variable	Stores the press count in pressCount
(2)	if	Returns to 0 at 3 or more. Skips empty light slots
(3)	for	Repeats for as many lights as there are, applying the color to all
(4)	Function	Names the color-applying procedure ApplyColor

continue means "skip this pass and go to the next." It keeps an empty light slot from erroring.

Because it uses colors.Length, growing to four colors requires no code change. Writing >= 3 would have. That's why you use Length.

3. Assign and confirm

Add a Udon Behaviour to ColorButton and set ColorCycleButton.

Field	What to set
Target Lights	Set Size to 3 and drag in Light1 through Light3
Colors	Set Size to 3 and set white, blue, and orange
Interaction Text	Change color

Press Play and try it.

Press	Light color	Console output
First	Blue (index 1)	color index 1
Second	Orange (index 2)	color index 2
Third	Back to white (index 0)	color index 0
Fourth	Blue	color index 1

It returns to the start after three. That's proof the if is working.

Deliberately clear the Light3 assignment and press. No error, and only the remaining two change color. That's the continue if doing its job.

Common mistakes

Here are the first ones beginners hit. Everyone goes through all of them.

• Mixing up = and == → One to put in, two to compare
• Forgetting f on a float → Write 0.5f, not 0.5
• Forgetting ; → Lines end with ;. The error's line number sometimes points one line down
• Mismatched { } → Every open needs a close. The editor auto-indents, so suspect it when the indentation looks off
• Going past the end of an array → An array of 3 has no [3]. It runs 0 to 2
• Misspelling a variable name → Case matters. pressCount and presscount are different things

Errors are a good thing. Failing silently is far more troublesome. Read the line number in the error message and suspect the area around it.

Bonus: Good to Know Up Front

• You don't have to memorize: Forget the syntax and you can look back at this article or code you wrote before. What to remember is just the framework: there are four parts
• Long names are fine: pressCount reads better later than pc. Cut the effort of reading, not the effort of typing
• Write comments: Anything after // doesn't execute. Writing down what something does helps you six months from now
• Try small: Rather than writing it all at once, start by writing one Debug.Log and running it. Growing something that works is faster
• Arrays are next: We used them here, and variables and data types covers them in detail

Summary

Programs are made from four parts.

• Variables: boxes that hold values, and they have kinds
• if: forks by condition. Use == to compare
• for: repeats the same thing. Strong when paired with arrays
• Functions: name a piece of logic. The name becomes the explanation

The question to ask when building something new is: "What do I store, where does it fork, and what do I repeat?" Answer that and the shape of the code is already visible.

To learn value types in detail, go to variables and data types. To call logic selectively, go to methods and custom events.