Problems accessing shader parameter from within another script

Hello, I have a little problem here: each time I try to setFloat to my shader parameter, it gives me this error
"UnassignedReferenceException: The variable rn of Glowinteract has not been assigned.
You probably need to assign the rn variable of the Glowinteract script in the inspector.
Glowinteract.Update () (at Assets/Scripts/Glowinteract.cs:26)". But the strangest thing is that getFloat method works. So I am wondering what is wrong?
Here's my script that sets the value:

Code (CSharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class Glowinteract : MonoBehaviour {
5.  
6.     private float power;
7.     private Renderer rn;
8.     private bool forward;
9.     void Start () {
10.         Renderer rn = gameObject.GetComponent<Renderer> ();
11.         power = rn.material.GetFloat("_RimPower");
12.         print(rn.material.GetFloat("_RimPower"));
13.     }
14.  
15.     void Update () {
16.         if (power >= 6.0f)
17.                 forward = false;
18.         if (power >= 1.0f && power < 6.0f)
19.                 forward = true;
20.         if (forward)
21.             power = power +Time.deltaTime;
22.         else
23.             power = power -Time.deltaTime;
24.       rn.material.SetFloat("_RimPower", power);
25.     }
26. }
27.  

 

I realised that if I put SetFloat() inside Start(), it works, but I want it to be more dynamic, no just one defined value. So why would the update function no longer recognize my renderer?

 

OKay, solved the issue.. No more programming at 2am. It was gameOject.getComponent<Renederer>().material.SetFloat() that I had to use.. Sorry for trouble.