Array length 0 only when I try to do something with it

I'm building some ui that spawns buildings and shows a 3d preview of the selected building. The problem is when try to switch to cycle to the left, my array of preview objects is length 0. Every other frame on update says that it's length 3, which is what it should be.

I'm calling left() through a ui button.

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class buildingSpawner : MonoBehaviour
6. {
7.     public StatueScript ss;
8.     public GameObject[] buildings;
9.     private int size;
10.     public GameObject[] buildPreview;
11.     public GameObject thePreview;
12.     public string[] names;
13.     public int[] unlockPrices;
14.     public int selectIndex;
15.     // Start is called before the first frame update
16.     void Start()
17.     {
18.         selectIndex = 0;
19.         //size = buildPreview.Length;
20.         //Debug.Log("Size: " + size);
21.     }
22.  
23.     public void openBuildMenu()
24.     {
25.         Cursor.visible = true;
26.         gameObject.SetActive(true);
27.         thePreview = Instantiate(buildPreview[selectIndex],transform);
28.         Debug.Log("size on openBuildMenu: " + buildPreview.Length);
29.     }
30.  
31.     public void left()
32.     {
33.         Debug.Log("going left selectIndex = " + selectIndex);
34.         Debug.Log("size when going left: " + buildPreview.Length);
35.         if (selectIndex > 0)
36.         {
37.             selectIndex -= 1;
38.  
39.         }
40.         else
41.         {
42.             Debug.Log("selectIndex = 0, setting to size-1: " +buildPreview.Length +" -1");
43.             selectIndex = buildPreview.Length-1;
44.         }
45.         Debug.Log("Selecting: " + selectIndex);
46.  
47.         //Destroy(thePreview);
48.         thePreview = Instantiate(buildPreview[selectIndex],transform);
49.  
50.     }
51.  
52.     public void right()
53.     {
54.         if (selectIndex < buildPreview.Length-1)
55.         {
56.             selectIndex += 1;
57.  
58.         }
59.         else
60.         {
61.             selectIndex = 0;
62.         }
63.         //Destroy(thePreview);
64.         Debug.Log("Selecting: "+selectIndex);
65.         Debug.Log("bp Length= " + buildPreview.Length);
66.         thePreview = Instantiate(buildPreview[selectIndex],transform);
67.  
68.     }
69.  
70.     public void spawnBuilding()
71.     {
72.         Instantiate(buildings[selectIndex]);
73.         PauseMenu.isPaused = false;
74.         Cursor.visible = false;
75.         Time.timeScale = 1f;
76.         gameObject.SetActive(false);
77.  
78.     }
79.  
80.     public void closeBuildMenu()
81.     {
82.         Cursor.visible = false;
83.         PauseMenu.isPaused = false;
84.         ss.choosing = false;
85.         //Destroy(thePreview);
86.         Time.timeScale = 1f;
87.         gameObject.SetActive(false);
88.     }
89.  
90.     // Update is called once per frame
91.     void Update()
92.     {
93.         Debug.Log("Size on update = "+buildPreview.Length);
94.     }
95. }
96.  

I've thought that maybe I'm changing something in another script, but I've checked the usual suspects and I can't find anything. If anyone has any ideas I'd be grateful. Attached is a png of the debug console.

 

Well right before the error it shows your "selectIndex" is -1

So you're trying to access buildPreview array's index of "-1" which doesn't exist (cause arrays go from 0 up).

 

Yes but I don't know what's causing the array.length, which is what determines the array index to go to 0, for instance if i manually select 2 as my index, I'll get array index out of bounds because there's nothing in the array for that frame

 

What is that script attached to? From your console log, it appears that buildPreview goes from length 3 to length 0, then back to length 3. One explanation might be that you are not looking at the same instance of your buildingSpawner class from inside left as you are from Update. That is, whatever buildingSpawner is attached to might be instanced a second time at run time. If that second instance runs left, it would probably have a zero-length buildingSpawner at that time.

Try adding

Debug.Log(gameObject.name)

to Update and also to left, and let's see if they are being called for the same instance.

 

Even better would be to use a second parameter: Debug.Log(gameObject.name, gameObject).

That way when you click on the log statement in your console, it will select the corresponding game object in your object hierarchy. This makes it easier to find the object in question, and is much more helpful if for any reason you have multiple objects with the same name.

 

Seems to be the same object. I've set selectIndex to always be 0 but that doesn't work.

 

Alright so I tried this, and what happens is when I click on the update debug log it highlights my buying panel object in yellow in the object hierarchy.

When I click on the debug log called on left, nothing is highlighted.

Searching for "buying panel" in the hierarchy only brings up the one.

 

Antistone's suggestion is a good one. As Denisowator mentioned, no index is going to avoid the exception when your array is of length zero. We need to figure out if it really is changed and, if so, why.

Let's try something to confirm you are where you think you are: have your left method pause the editor by adding

Debug.Break()

as its first line. When your code hits that line, you can use the Inspector to verify if your array has actually changed length. I'm sure it has (or you wouldn't be seeing what you're seeing), but let's try that and see if it shows us anything else, like some other objects in the Project tree, etc.

 

That still has me thinking you've got a fleeting instance of some object coming and going. Try my break trick and let us know what you get.

 

Ok fixed it,

turns out I was referencing the prefab buying panel instead of the buying panel in the scene, so you were right about there being two different versions.

The fix was when selecting the on click() object for my button, to select scene and the buying panel object in that one.

 

Glad you solved it! Now, there's an important lesson here for all of us: the bug wasn't in the code you posted, it was in the caller to that code. It's a hard thing to decide just how much code to post when you have a lot of it. In this case, you posted a small amount, but the problem was elsewhere. If you hadn't found the problem yet, my next suggestion would have been to post the calling code, as that's where my suspicions were. But, you got there on your own, so well done!

 

I'm a bit surprised that Unity even allows click events in a scene to point to methods on a prefab...

 

Well, we never saw his calling code, so we can't be sure exactly what was happening. But there may be some reason to operate directly on a prefab, maybe in particular with the new prefab workflow. Something to keep in mind, though.