How an I find grid outer square positions by formula by script?

Question

I mean automatic to find the outer grid square :

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

public class GenerateGrid : MonoBehaviour
{
    public GameObject gridPrefab;

    public int gridWidth = 10;
    public int gridHeight = 10;
    public int gap;
    public float spawnSpeed = 0;
    public bool gridGenerated = false;

    void Start()
    {
        StartCoroutine(CreateGrid());
    }

    IEnumerator CreateGrid()
    {
        for (int x = 0; x < gridWidth; x++)
        {
            yield return new WaitForSeconds(spawnSpeed);

            for (int z = 0; z < gridHeight; z++)
            {
                yield return new WaitForSeconds(spawnSpeed);

                GameObject block = Instantiate(gridPrefab, Vector3.zero, gridPrefab.transform.rotation) as GameObject;
                block.transform.parent = transform;
                block.transform.localPosition = new Vector3(x + x * gap, 0, z + z * gap);
                block.tag = "Grid Block";
            }
        }

        gridGenerated = true;
    }
}

I marked them selected them in the screenshot to show what I mean by the outer square. I want to find automatic by some code logic only the outer square of the grid no matter what is the grid size if it's 10 on 10 or 3 on 7

This is working but with two problems that prevent from it to be automatic :

In another script I'm getting all the grid blocks positions :

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

public class GetGridPos : MonoBehaviour
{
    public GenerateGrid generateGrid;

    private List<Vector3> gridBlocksPositions = new List<Vector3>();

    private List<Vector3> gridOuterBlocks = new List<Vector3>();

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

    }

    // Update is called once per frame
    void Update()
    {
        if (generateGrid.gridGenerated)
        {
            var blocks = GameObject.FindGameObjectsWithTag("Grid Block");

            foreach (GameObject pos in blocks)
            {
                gridBlocksPositions.Add(pos.transform.position);

                if (pos.transform.position.x == 0 && pos.transform.position.y == 0
                    || pos.transform.position.z == 18 || pos.transform.position.x == 18
                    || pos.transform.position.y == 0 && pos.transform.position.z == 0)
                {
                    gridOuterBlocks.Add(pos.transform.position);
                }
            }

            generateGrid.gridGenerated = false;
        }
    }
}

The problems :

1. In this case I know the grid is 10 x 10 so there are 100 blocks and the gap between each blocks is 1 so the 4 sides length of the outer grid square will be 18 in this case but I want to find it automatic not to type the length on my own.

2. I need to add to the calculation also the gap factor.

So I'm still missing the right formula to get the grid outer blocks positions.

Answer 1

I would recommend you to use the same class for generation and for usage of the Grid, because, firstly, it is the same objects you are working with, secondly, because you can avoid usage of Update() for now:

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

public class Grid : MonoBehaviour
{
    public GameObject gridPrefab;

    private Transform[,] gridBlocks;

    private List<Vector3> gridBlocksPositions = new List<Vector3>();
    private List<Vector3> gridOuterBlocks = new List<Vector3>();

    public int gridWidth = 10;
    public int gridHeight = 10;
    public int gap;
    public float spawnSpeed = 0;
    public bool gridGenerated = false;

    void Start()
    {
        StartCoroutine(CreateGrid());
    }

    IEnumerator CreateGrid()
    {
        gridBlock=new Transform[gridWidth,gridHeight]();
        for (int x = 0; x < gridWidth; x++)
        {
            yield return new WaitForSeconds(spawnSpeed);

            for (int z = 0; z < gridHeight; z++)
            {
                yield return new WaitForSeconds(spawnSpeed);

                GameObject block = Instantiate(gridPrefab, Vector3.zero, gridPrefab.transform.rotation) as GameObject;
                block.transform.parent = transform;
                block.transform.localPosition = new Vector3(x + x * gap, 0, z + z * gap);
                block.tag = "Grid Block";
                gridBlocks[x,z] = block.transform;
                gridBlocksPositions.Add(block.transform.position);
                if((x==0)||(x==gridWidth-1)||(z==0)||(z==gridHeight-1){
                    gridOuterBlocks.Add(block.transform.position);
                }
            }
        }

        gridGenerated = true;
    }
}

And even if you are going to use different classes, it would be useful to create public Transform[,] gridBlocks inside class GenerateGrid and add all blocks there, in order to avoid usage of the FindGameObjectsWithTag() because it is very resource-consuming, and finding elements at the corners would be a lot easier.