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, so a cell is on the board while both its x and its z are between -7 and 7. 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. Then a snake that hits a wall stays on the last cell of the board instead of ending up inside the border. A rook on a chess board numbered 0 to 7 is checked the same way:

Vector3 target = transform.position + move;
if (target.x < 0 || target.x > 7 || target.z < 0 || target.z > 7)
{
    return;   // off the board: the rook stays
}
transform.position = target;

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

Challenge

Easy

Add public int edge = 7; (the board's cells run from -edge to edge) and public bool alive = true;.
In Step(), after setting direction, work out next, the cell the head is about to enter. If it is off the board, call Die() and return without moving; otherwise move the head to next.
Write void Die(), which sets alive to false.
Start Update() with a return when the snake is not alive, so a dead snake neither turns nor steps.

Press Play, steer into a wall, then press 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;
    }
}