練習課題：アリーナが形になる

using System.Collections.Generic;
using UnityEngine;

public class ArenaBuilder : MonoBehaviour
{
    [Header("Grid")]
    [SerializeField] private GameObject blockPrefab;
    [SerializeField] private int columns = 4;
    [SerializeField] private int rows = 4;
    [SerializeField] private float spacing = 1.6f;
    [SerializeField] private Transform blockParent;

    // every block we made, so we can count and clear them
    private readonly List<GameObject> blocks = new List<GameObject>();

    public int BlockCount => blocks.Count;

    void Start()
    {
        Build();
    }

    void Update()
    {
        // press R to rebuild with fresh random colours
        if (Input.GetKeyDown(KeyCode.R))
            Build();
    }

    public void Build()
    {
        Clear();

        float offsetX = (columns - 1) * spacing * 0.5f;
        float offsetZ = (rows - 1) * spacing * 0.5f;

        for (int x = 0; x < columns; x++)
        {
            for (int z = 0; z < rows; z++)
            {
                Vector3 pos = new Vector3(
                    x * spacing - offsetX,
                    0.5f,
                    z * spacing - offsetZ);

                GameObject b = Instantiate(blockPrefab, pos, Quaternion.identity, blockParent);
                b.name = $"Block_{x}_{z}";

                // TODO 1: give it a random colour
                // TODO 2: give it a random height between 0.5 and 2.5

                blocks.Add(b);
            }
        }
    }

    public void Clear()
    {
        foreach (GameObject b in blocks)
            if (b != null) Destroy(b);

        blocks.Clear();
    }
}

using UnityEngine;

public class ArenaHud : MonoBehaviour
{
    [SerializeField] private ArenaBuilder builder;
    [SerializeField] private CharacterSwitcher switcher;

    // OnGUI is the quick-and-dirty way to draw text. Fine for an exercise,
    // never ship it — real UI lives on a Canvas, like the HudPanel from the last lesson.
    void OnGUI()
    {
        GUI.skin.label.fontSize = 20;
        GUI.Label(new Rect(20, 20, 400, 30), "Blocks: " + builder.BlockCount);
        GUI.Label(new Rect(20, 50, 400, 30), "Active: " + switcher.ActiveCharacter.name);
        GUI.Label(new Rect(20, 80, 400, 30), "R = rebuild");
    }
}