Unity in Practice 0007 – Very First Unity C# Code to Move and Jump a 2D Ball

This tutorial walks you through creating a simple Unity project where a 2D ball can move left/right and jump using keyboard input. It’s perfect for beginners who want to learn how to work with physics, input detection, and scripting in Unity.

🔄 What You’ll Build

A 2D ball that moves left/right using the A/D or arrow keys.
Pressing Space makes the ball jump upward.
The movement and jump behavior are handled in a custom C# script.

🎨 Step 1 – Create Your Scene

Create a 2D Sprite for the Ball:
- In the Unity Hierarchy, right-click → 2D Object → Sprite → Circle, then name it Ball.
- Add a Rigidbody2D and a CircleCollider2D component to the Ball.
Create 2 Ground:
- Right-click → 2D Object → Sprite → Square, then name it VGround and HGround.
- Stretch it wide at the bottom of the scene.
- Add a BoxCollider2D to make it solid.

refer previous article for detail: Unity in Practice 0004 – Unity Rigidbody – Wonderful Code See

💻 Step 2 – Write the Movement and Jump Script

Create a new C# script: BallScript.cs
Attach it to the Ball GameObject.
Code:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class BallScript : MonoBehaviour
{

    public Rigidbody2D rb;

    public float moveSpeed;
    public float jumpForce;

    private float xInput;

    // Start is called before the first frame update
    void Start()
    {
    }

    // Update is called once per frame
    void Update()
    {
        // Get horizontal input
        xInput = Input.GetAxis("Horizontal");

        // Apply horizontal movement
        rb.velocity = new Vector2(xInput * moveSpeed, rb.velocity.y);


        // Jump on Space key press
        if (Input.GetKeyDown(KeyCode.Space))
        {
            rb.velocity = new Vector2(rb.velocity.x, jumpForce);
        }
    }
}

🛠 Step 3 – Set Up References in the Inspector

Drag the Ball GameObject into the rb field.

▶️ Step 4 – Run the Scene

Press the Play button.
Use the keyboard:
- Press A/D or ←/→ to move the ball left and right.
- Press Space to jump when the ball is on the ground.

✅ Summary

You now have a basic but functional 2D movement and jumping system. This is a great starting point for platformers, side-scrolling games, or physics experiments.

🧠 Advance：Line-by-Line Detail Explanation:

xInput = Input.GetAxis("Horizontal");

Purpose: Reads the player’s horizontal movement input.
How it works:
- Unity’s built-in "Horizontal" axis maps:
  - A or ← : returns -1
  - D or → : returns +1
  - No key pressed : returns 0
- It also applies smoothing for gradual acceleration/deceleration.
Result: xInput becomes a float value between -1 and 1, indicating the direction and intensity of horizontal input.

rb.velocity = new Vector2(xInput * moveSpeed, rb.velocity.y);

Purpose: Moves the ball horizontally.
Explanation:
- rb refers to the Rigidbody2D component attached to the GameObject.
- rb.velocity sets the object’s current velocity.
- new Vector2(...) creates a 2D velocity vector.
Details:
- xInput * moveSpeed determines the speed along the X-axis.
  - For example, if xInput = 1 and moveSpeed = 5, the velocity becomes 5 to the right.
- rb.velocity.y preserves the current Y-axis velocity (such as jumping or gravity).
Result: The ball moves left/right based on input, without affecting vertical movement.

if (Input.GetKeyDown(KeyCode.Space))

Purpose: Detects if the Space key was just pressed.
Difference:
- GetKeyDown: Returns true only on the frame the key is initially pressed.
- GetKey: Returns true every frame the key is held down.
- GetKeyUp: Returns true when the key is released.
Use Case: Ideal for triggering a jump only once per press.

rb.velocity = new Vector2(rb.velocity.x, jumpForce);

Purpose: Makes the ball jump by applying upward velocity.
Explanation:
- rb.velocity.x keeps the existing horizontal movement.
- jumpForce (usually a positive number) sets the jump height.
- This line overrides the Y velocity, making the object jump instantly.
- Note: This code does not check if the object is on the ground, so it allows infinite jumping in mid-air. To fix this, ground-check logic is needed.