Start and Update methods in Unity - Step-by-Step Execution

Concept Flow - Start and Update methods

Game Object Created

↓

Start Method Called Once

↓

Game Loop Begins

↓

Update Method Called Every Frame

↓

Repeat Update Each Frame Until Game Ends

When a game object appears, Start runs once to set up. Then Update runs every frame to keep things moving.

Execution Sample

Unity

void Start() {
    Debug.Log("Game Started");
}

void Update() {
    Debug.Log("Frame Updated");
}

This code prints a message once at start, then prints a message every frame.

Execution Table

Step	Method Called	Action	Output
1	Start	Runs once when object is created	"Game Started" printed
2	Update	Runs first frame update	"Frame Updated" printed
3	Update	Runs second frame update	"Frame Updated" printed
4	Update	Runs third frame update	"Frame Updated" printed
...	Update	Runs every frame after	"Frame Updated" printed each time
Exit	Game Ends	Update stops running	No more output

💡 Game ends or object destroyed, so Update no longer runs

Variable Tracker

Variable	Start	After Frame 1	After Frame 2	After Frame 3	...final
Game State	Initialized	Running	Running	Running	Running until game ends

Key Moments - 2 Insights

Why does Start run only once but Update runs many times?

Start runs once when the object is created to set things up (see execution_table step 1). Update runs every frame to keep the game active (steps 2 and after).

What happens if you put code in Start that you want to run every frame?

It will only run once at the beginning, so the game won't update continuously. You need to put repeating code in Update (see execution_table steps 2+).

Visual Quiz - 3 Questions

Look at the execution table, what is printed at step 1?

A "Frame Updated"

B "Game Started"

C Nothing

D "Game Ended"

💡 Hint

Check the output column for step 1 in the execution_table.

At which step does Update first run?

A Step 3

B Step 1

C Step 2

D Exit

💡 Hint

Look at the Method Called column in the execution_table.

If the game ends after frame 3, what happens to Update calls?

A Update stops running

B Start runs again

C Update runs again at step 4

D Update runs twice at step 3

💡 Hint

See the exit_note and last row in execution_table.

Concept Snapshot

Start() runs once when the object is created to initialize.
Update() runs every frame to update game logic.
Start is for setup; Update is for ongoing actions.
Update keeps running until the game or object ends.
Put repeating code in Update, not Start.

Full Transcript

In Unity, the Start method runs once when a game object is created. It is used to set up initial states or variables. After that, the Update method runs every frame, which means it repeats many times per second to keep the game active and responsive. The Start method is like turning on a machine once, while Update is like the machine running continuously. If you put code that should repeat inside Start, it will only run once, so the game won't update properly. The Update method keeps running until the game ends or the object is destroyed.

Practice

1. What is the main purpose of the Start() method in a Unity script?

easy

A. It runs every frame to update game logic.

B. It runs once at the beginning when the script is enabled.

C. It handles user input events.

D. It is used to draw graphics on the screen.

Solution

1. Step 1: Understand the role of Start()

   The Start() method is called once when the script first runs, usually at the beginning of the game or when the object is enabled.

2. Step 2: Compare with Update()

   Unlike Update(), which runs every frame, Start() only runs once to initialize things.

3. Final Answer:

   It runs once at the beginning when the script is enabled. -> Option B

4. Quick Check:

   Start() runs once = A [OK]

Hint: Start() runs once at start, Update() runs every frame [OK]

Common Mistakes:

• Confusing Start() with Update()
• Thinking Start() runs multiple times
• Assuming Start() handles input every frame

2. Which of the following is the correct syntax for the Update() method in a Unity C# script?

easy

A. void Update() { }

B. public void update() { }

C. private int Update() { return 0; }

D. void update() { }

Solution

1. Step 1: Recall Unity method syntax

   The Update() method must be void and have no parameters. It is usually declared as void Update().

2. Step 2: Check method name casing and return type

   Unity requires exact casing: Update() with capital U. It should not return a value.

3. Final Answer:

   void Update() { } -> Option A

4. Quick Check:

   Correct syntax is void Update() [OK]

Hint: Update() must be void and capital U [OK]

Common Mistakes:

• Using lowercase 'update()'
• Adding return type other than void
• Making Update() public or private unnecessarily

3. Consider this Unity C# script attached to a GameObject:

int counter = 0;

void Start() {
    counter = 5;
}

void Update() {
    counter++;
    Debug.Log(counter);
}

What will be printed to the console during the first two frames after the game starts?

medium

A. 5 then 6

B. 0 then 1

C. 6 then 7

D. 1 then 2

Solution

1. Step 1: Analyze Start() initialization

   At the start, counter is set to 5 once by Start().

2. Step 2: Analyze Update() increments and logs

   On the first frame, Update() runs, increments counter to 6, then logs 6. On the second frame, it increments to 7 and logs 7.

3. Final Answer:

   6 then 7 -> Option C

4. Quick Check:

   Start sets 5, Update adds 1 before log [OK]

Hint: Start sets initial value, Update increments before logging [OK]

Common Mistakes:

• Assuming counter logs before increment
• Thinking Start() runs every frame
• Confusing initial counter value

4. This script is attached to a GameObject:

void Start() {
    Debug.Log("Start called");
}

void Update() {
    Debug.Log("Update called");
}

Why might "Start called" never appear in the console?

medium

A. The script is disabled or the GameObject is inactive.

B. Update() is called before Start(), so Start() is skipped.

C. Debug.Log only works inside Update().

D. Start() must return a value to run.

Solution

1. Step 1: Understand when Start() runs

   Start() runs once when the script is enabled and the GameObject is active.

2. Step 2: Identify why Start() might not run

   If the script component is disabled or the GameObject is inactive, Start() will not be called, so no log appears.

3. Final Answer:

   The script is disabled or the GameObject is inactive. -> Option A

4. Quick Check:

   Start() runs only if enabled and active [OK]

Hint: Start() runs only if script and object are active [OK]

Common Mistakes:

• Thinking Update() runs before Start()
• Believing Debug.Log only works in Update()
• Expecting Start() to return a value

5. You want to create a Unity script that moves a GameObject up by 1 unit every second, but only after an initial 3-second delay. Which approach correctly uses Start() and Update() to achieve this?

hard

A. Use Update() to set the timer and move the object immediately.

B. Move the object up by 1 unit inside Start() and do nothing in Update().

C. Move the object up by 1 unit every frame inside Start().

D. Use Start() to set a timer start time, then in Update() move the object if 3 seconds passed.

Solution

1. Step 1: Use Start() to record start time

   In Start(), save the current time as a reference point for the delay.

2. Step 2: Use Update() to check elapsed time and move

   In Update(), check if 3 seconds have passed since start time; if yes, move the object up by 1 unit per second.

3. Final Answer:

   Use Start() to set a timer start time, then in Update() move the object if 3 seconds passed. -> Option D

4. Quick Check:

   Start() sets timer, Update() moves after delay [OK]

Hint: Start() sets delay start, Update() moves after delay [OK]

Common Mistakes:

• Moving object only once in Start()
• Moving object every frame without delay
• Trying to set timer inside Update() only