Traffic light script errors

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1

Hi developers, I want to make a traffic light using 3 point lights that turn on and off. This is my script :

public class TrafficLight : MonoBehaviour {
	public GameObject Red;
	public GameObject Green;
	public GameObject Yellow;

	// Use this for initialization
	void Start () {
	 
		//GameObject.Yellow.SetActive(false);                               
		GameObject.FindGameObjectWithTag("_YellowLight").SetActive(false);
	}
	
	IEnumerator Example() {
		GameObject.FindGameObjectWithTag("_GreenLight").SetActive(true);
		GameObject.FindGameObjectWithTag("_RedLight").SetActive(false);
		//GameObject.Green.SetActive(true);  
		//GameObject.Red.SetActive(false);  
		yield return new WaitForSeconds(30);
		GameObject.FindGameObjectWithTag("_YellowLight").SetActive(true);
		GameObject.FindGameObjectWithTag("_GreenLight").SetActive(false);
		//GameObject.Yellow.SetActive(true);  
		//GameObject.Green.SetActive(false);  
		yield return new WaitForSeconds(4);
		GameObject.FindGameObjectWithTag("_RedLight").SetActive(true);
		GameObject.FindGameObjectWithTag("_YellowLight").SetActive(false);
		//GameObject.Red.SetActive(true);
		//GameObject.Yellow.SetActive(false);  
		yield return new WaitForSeconds (30);
	}
	
	
	// Update is called once per frame
	void Update () {
			StartCoroutine (Example());

	}
}

And this is the error that I get :

NullReferenceException: Object reference not set to an instance of an object
TrafficLight+c__Iterator3.MoveNext () (at Assets/Scripts/TrafficLight.cs:20)
UnityEngine.MonoBehaviour:StartCoroutine(IEnumerator)
TrafficLight:Update() (at Assets/Scripts/TrafficLight.cs:45)

If you notice I have tried another solution which is commented out but again I get another error :

Yellow, Green, Red are not defined.

Any help?
Thanks in advance for your time

2 Answers

2
2

You can use the commented approach, but you need to correct the syntax. Instead of doing:

GameObject.Yellow.SetActive(false);

You need to change it to:

Yellow.SetActive(false);

Also, It would be better if you checked whether the object exists or not before calling SetActive on it:

if(Yellow != null) {
   Yellow.SetActive(false);
}

Which is exactly the mistake you are making in the non-commented approach. You are trying to call SetActive on whatever GameObject.FindGameObjectWithTag returns, which may be null (therefore causing the error). To avoid it, first get the return value into a GameObject, then check if that GameObject is null and then call SetActive (same way as in the other approach).

Also, make sure you link your objects to the public variables in the script (the Yellow, Red and Green).

By the way, besides the logic of the lights (I haven't checked), don't start the coroutine inside Update function (as it will happen once every frame), just start it iside your Start function.

 –

Thank you for your time. when i use Yellow.SetActive(false); i get an error Yellow is not defined which is defined as gameobject. I don't understand when an object is null. To me if the object is on the scene then it is not null ? I changed the coroutine and put it in as you suggested I was suspecting it was better put in the start()

 –

For the object not to be null, it has to be linked to the script (by dragging it in the editor, or by using the Find function). Make sure you link the objects, and post the full error, so we can figure out how to solve it.

 –
3

It can’t work , when i apply this script to traffic signals it totally disappear the whole signals. please help me to solve this problem.
i’m using unity3d , 2017
and for Coding = Mono develop,