Why does this code run infinitely?

From what I can see, this code should only run 20 time but it keeps generating rooms forever.

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

public class RoomGeneration : MonoBehaviour {

 public int roomsGenerated; // How many rooms are generated

 public GameObject[] rooms;
 public GameObject doorTop;
 public GameObject doorBottom;
 public GameObject doorLeft;
 public GameObject doorRight;
 public GameObject rayUp;
 public GameObject rayDown;
 public GameObject rayLeft;
 public GameObject rayRight;

 int genX; // Room X coord
 int genY; // Room Y coord

 // Use this for initialization
 void Start()
 {

     genX = 0;
     genY = 0;

     Instantiate(rooms[0], new Vector2(genX, genY), Quaternion.identity);
 }

 // Update is called once per frame
 void Update()
 {
    for(int i = 0; i <= 20; i++)
     {
         CheckAndBuild();
     }
 }

 void CheckAndBuild()
 {
     int randomRoomDir = Random.Range(0, 4);
     int randomRoom = Random.Range(1, 6);

     RaycastHit2D hitUp = Physics2D.Raycast(rayUp.transform.position, transform.up, 3f);
     RaycastHit2D hitDown = Physics2D.Raycast(rayDown.transform.position, transform.up * -1, 3f);
     RaycastHit2D hitLeft = Physics2D.Raycast(rayLeft.transform.position, transform.right * -1, 5f);
     RaycastHit2D hitRight = Physics2D.Raycast(rayRight.transform.position, transform.right, 5f);
     Debug.DrawRay(rayUp.transform.position, transform.up * 3f, Color.red, 1000f);
     Debug.DrawRay(rayDown.transform.position, transform.up * -1 * 3f, Color.red, 1000f);
     Debug.DrawRay(rayLeft.transform.position, transform.right * -1 * 5f, Color.red, 1000f);
     Debug.DrawRay(rayRight.transform.position, transform.right * 5f, Color.red, 1000f);


     if (randomRoomDir == 0 && hitUp.collider == null) // Generate room above
     {
         Instantiate(doorTop, new Vector2(genX, genY + 3.55f), Quaternion.identity);
         genY += 10;
         Vector2 roomGenPos = new Vector2(genX, genY);
         transform.position = roomGenPos;
         Instantiate(rooms[randomRoom], roomGenPos, Quaternion.identity);
         Instantiate(doorBottom, new Vector2(genX, genY - 3.55f), Quaternion.identity);

         print(hitUp.collider);
     }

     if (randomRoomDir == 1 && hitDown.collider == null) // Generate room below
     {
         Instantiate(doorBottom, new Vector2(genX, genY - 3.55f), Quaternion.identity);
         genY -= 10;
         Vector2 roomGenPos = new Vector2(genX, genY);
         transform.position = roomGenPos;
         Instantiate(rooms[randomRoom], roomGenPos, Quaternion.identity);
         Instantiate(doorTop, new Vector2(genX, genY + 3.55f), Quaternion.identity);
         print(hitUp.collider);
     }

     if (randomRoomDir == 2 && hitLeft.collider == null) // Generate room to the left
     {
         Instantiate(doorLeft, new Vector2(genX - 7.4f, genY), Quaternion.identity);
         genX -= 20;
         Vector2 roomGenPos = new Vector2(genX, genY);
         transform.position = roomGenPos;
         Instantiate(rooms[randomRoom], roomGenPos, Quaternion.identity);
         Instantiate(doorRight, new Vector2(genX + 7.4f, genY), Quaternion.identity);
         print(hitUp.collider);
     }

     if (randomRoomDir == 3 && hitRight.collider == null) // Generate room to the right
     {
         Instantiate(doorRight, new Vector2(genX + 7.4f, genY), Quaternion.identity);
         genX += 20;
         Instantiate(rooms[randomRoom], new Vector2(genX, genY), Quaternion.identity);
         Instantiate(doorLeft, new Vector2(genX - 7.4f, genY), Quaternion.identity);
         print(hitUp.collider);
     }
 }

}