(Javascript) Object reference not set to an instance of an object

Nightkilla · October 20, 2013, 8:26pm

I am making a simple car game. I have trouble making the AI though. Here is my script:

var FrontLeftWheel : WheelCollider;
var FrontRightWheel : WheelCollider;
var BackLeftWheel : WheelCollider;
var BackRightWheel : WheelCollider;

// These variables are for the gears, the array is the list of ratios. The script
// uses the defined gear ratios to determine how much torque to apply to the wheels.
private var GearRatio : float[];
var DifferentialRatio : float = 3.1;
var CurrentGear : int = 0;

// These variables are just for applying torque to the wheels and shifting gears.
// using the defined Max and Min Engine RPM, the script can determine what gear the
// car needs to be in.
var EngineTorque : float = 600.0;
var MaxEngineRPM : float = 3000.0;
var MinEngineRPM : float = 1000.0;
private var EngineRPM : float = 0.0;

// Here's all the variables for the AI, the waypoints are determined in the "GetWaypoints" function.
// the waypoint container is used to search for all the waypoints in the scene, and the current
// waypoint is used to determine which waypoint in the array the car is aiming for.
var waypointContainer : GameObject;
private var waypoints : Array;
private var currentWaypoint : int = 0;

// input steer and input torque are the values substituted out for the player input. The 
// "NavigateTowardsWaypoint" function determines values to use for these variables to move the car
// in the desired direction.
private var inputSteer : float = 0.0;
private var inputTorque : float = 0.0;

function Start () {
	// I usually alter the center of mass to make the car more stable. I'ts less likely to flip this way.
	rigidbody.centerOfMass.y = -.45;
	rigidbody.centerOfMass.z = -0.15;
	
	// Call the function to determine the array of waypoints. This sets up the array of points by finding
	// transform components inside of a source container.
	GetWaypoints();
}

function Update () {
	
	// This is to limith the maximum speed of the car, adjusting the drag probably isn't the best way of doing it,
	// but it's easy, and it doesn't interfere with the physics processing.
	rigidbody.drag = rigidbody.velocity.magnitude / 250;
	
	// Call the funtion to determine the desired input values for the car. This essentially steers and
	// applies gas to the engine.
	NavigateTowardsWaypoint();
	
	// Compute the engine RPM based on the average RPM of the two wheels, then call the shift gear function
	EngineRPM = Mathf.Abs(BackLeftWheel.rpm + BackRightWheel.rpm)/2 * GearRatio[CurrentGear] * DifferentialRatio;
	if ( EngineRPM>10000) {EngineRPM =10000;}
	if ( EngineRPM<0) {EngineRPM =0;}
	ShiftGears();

	// set the audio pitch to the percentage of RPM to the maximum RPM plus one, this makes the sound play
	// up to twice it's pitch, where it will suddenly drop when it switches gears.
	audio.pitch = Mathf.Abs(EngineRPM / MaxEngineRPM) + 0.5 ;
	// this line is just to ensure that the pitch does not reach a value higher than is desired.
	if ( audio.pitch > 1.5 ) {
		audio.pitch = 1.5;
	}
	
	// finally, apply the values to the wheels.	The torque applied is divided by the current gear, and
	// multiplied by the calculated AI input variable.
	//FrontLeftWheel.motorTorque = EngineTorque / GearRatio[CurrentGear]*DifferentialRatio * inputTorque;
	//FrontRightWheel.motorTorque = EngineTorque / GearRatio[CurrentGear]*DifferentialRatio * inputTorque;
	BackLeftWheel.motorTorque = -EngineTorque / GearRatio[CurrentGear]*DifferentialRatio * inputTorque;
	BackRightWheel.motorTorque = -EngineTorque / GearRatio[CurrentGear]*DifferentialRatio * inputTorque;
	
	// the steer angle is an arbitrary value multiplied by the calculated AI input.
	FrontLeftWheel.steerAngle = 35 * inputSteer;
	FrontRightWheel.steerAngle = 35 * inputSteer;
}

function ShiftGears() {
	// this funciton shifts the gears of the vehcile, it loops through all the gears, checking which will make
	// the engine RPM fall within the desired range. The gear is then set to this "appropriate" value.
	if ( EngineRPM >= MaxEngineRPM ) {
		var AppropriateGear : int = CurrentGear;
		
		for ( var i = 0; i < GearRatio.length; i ++ ) {
			if ( BackLeftWheel.rpm * GearRatio_*DifferentialRatio < MaxEngineRPM ) {_

```
 		AppropriateGear = i;*
```
```
 		break;*
```
```
 	}*
```
```
 }*
```
```
 CurrentGear = AppropriateGear;*
```
}*
if ( EngineRPM <= MinEngineRPM ) {*
```
 AppropriateGear = CurrentGear;*
```

 for ( var j = GearRatio.length-1; j >= 0; j -- ) {*

_ if ( BackLeftWheel.rpm * GearRatio[j]*DifferentialRatio > MinEngineRPM ) {_

```
 		AppropriateGear = j;*
```
```
 		break;*
```
```
 	}*
```
```
 }*
```
```
 CurrentGear = AppropriateGear;*
```
}*
}

function GetWaypoints () {

// Now, this function basically takes the container object for the waypoints, then finds all of the transforms in it,*
// once it has the transforms, it checks to make sure it’s not the container, and adds them to the array of waypoints.*
var potentialWaypoints : Array = waypointContainer.GetComponentsInChildren( Transform );*
waypoints = new Array();*
for ( var potentialWaypoint : Transform in potentialWaypoints ) {*

 if ( potentialWaypoint != waypointContainer.transform ) {*

 	waypoints[ waypoints.length ] = potentialWaypoint;*

```
 }*
```
}*
}

function NavigateTowardsWaypoint () {

// now we just find the relative position of the waypoint from the car transform,*
// that way we can determine how far to the left and right the waypoint is.*
var RelativeWaypointPosition : Vector3 = transform.InverseTransformPoint( Vector3(*

 										waypoints[currentWaypoint].position.x,*

```
 										transform.position.y,* 
```

 										waypoints[currentWaypoint].position.z ) );*

// by dividing the horizontal position by the magnitude, we get a decimal percentage of the turn angle that we can use to drive the wheels*
inputSteer = RelativeWaypointPosition.x / RelativeWaypointPosition.magnitude;*
// now we do the same for torque, but make sure that it doesn’t apply any engine torque when going around a sharp turn…*
if ( Mathf.Abs( inputSteer ) < 1.0 ) {*

 inputTorque = RelativeWaypointPosition.z / RelativeWaypointPosition.magnitude - Mathf.Abs( inputSteer );*

}else{*
```
 inputTorque = 0.0;*
```
}*
// this just checks if the car’s position is near enough to a waypoint to count as passing it, if it is, then change the target waypoint to the*
// next in the list.*
if ( RelativeWaypointPosition.magnitude < 30 ) {*
```
 currentWaypoint ++;*
```

 if ( currentWaypoint >= waypoints.length ) {*

```
 	currentWaypoint = 0;*
```
```
 }*
```
}*

}
On line 55 it gives me the error: Object reference not set to an instance of an object.
How to I fix this?

robertbu · October 20, 2013, 8:37pm

This is essentially the same question and the same bug as this question:

http://answers.unity3d.com/questions/559117/array-index-out-of-range-javascript.html

You’ve made ‘GearRatio’ private in this version of the script, but you still don’t initialize it anywhere. That is when you have a deceleration like:

private var GearRatio : float[];

…no space is created for the array. So when you try and access the array it is null and generates a null reference exception. You need to do something to both create space and to initialize the array.

But I don’t think going private is the way to solve your problem. Since I’ve not done the tutorial, I’m just guessing but I think you want to do the following:

Make GearRatio public again.
Select the game object with this script in the Inspector
Click on the triangle to the left of GearArray to open it up
Set it size to the number of gears this vehicle has.
For each entry in the array, enter a value for the gear ratio

I have no idea what values should be entered. The tutorial should indicate what values.