The Walls

Lesson 4 of 14 in Coddy's Unity Game: Snake 3D course.

The board's cells run from -7 to 7 on x and on z. With the board centred on 0, one test covers both sides: Mathf.Abs(x) > 7 is true past either edge.

Test the cell the head is about to enter, before it moves there, so a snake that hits a wall stays on the last cell instead of ending inside the border. On a smaller board with edge 4:

Vector3 next = transform.position + direction;
if (Mathf.Abs(next.x) > 4) { Die(); return; }
transform.position = next;

Hitting a wall stops the snake for good. A bool that Update() tests first does that: a return at the top skips the keys and the timer alike.

Challenge

Easy

Hitting a wall ends the game.

Add public int edge = 7; and public bool alive = true;
In Step(), after setting direction, Vector3 next = transform.position + direction;. Off the board (Mathf.Abs(next.x) > edge, or z): Die() and return; else move to next.
void Die() { alive = false; }; Update() starts with if (!alive) return;

Press Play and steer into a wall: the snake stops on the last cell. Then Check.

Try it yourself

using UnityEngine;
using UnityEngine.InputSystem;

public class Snake : MonoBehaviour
{
    public float tick = 0.15f;   // seconds between two steps
    private float timer;
    private Vector3 direction = Vector3.right;       // the way the head moved on its last step
    private Vector3 nextDirection = Vector3.right;   // the turn waiting for the next step

    void Update()
    {
        Keyboard keyboard = Keyboard.current;
        if (keyboard != null)
        {
            if (keyboard.upArrowKey.wasPressedThisFrame || keyboard.wKey.wasPressedThisFrame) Turn(Vector3.forward);
            if (keyboard.downArrowKey.wasPressedThisFrame || keyboard.sKey.wasPressedThisFrame) Turn(Vector3.back);
            if (keyboard.leftArrowKey.wasPressedThisFrame || keyboard.aKey.wasPressedThisFrame) Turn(Vector3.left);
            if (keyboard.rightArrowKey.wasPressedThisFrame || keyboard.dKey.wasPressedThisFrame) Turn(Vector3.right);
        }

        timer += Time.deltaTime;
        if (timer >= tick)
        {
            timer -= tick;
            Step();
        }
    }

    void Turn(Vector3 newDirection)
    {
        if (newDirection != -direction)
        {
            nextDirection = newDirection;
        }
    }

    void Step()
    {
        direction = nextDirection;
        transform.position += direction;
    }
}