How to generate a different number every time?

I am making a connect 4 game. I made an array that stores all the x-positions for the disks to fall in. I want the computer to generate a random x-position for the disk to fall in every time. Here is my code.

#pragma strict

// Disks that we're playing with public var RedDisk : GameObject;

// Converts array of numbers into a variable private var choices : float[] = [-26.92, -17.8, -9.2, 0, 9.2, 17.8, 26.92]; private var howManyThings : float = choices.length; private var myRandomIndex : float; myRandomIndex = Random.Range (0, howManyThings); private var result : float;

private var PlayerTurn : boolean = false;

// Happens every frame function Update () {

// If it is the enemy's turn, if (PlayerTurn == false) {

   result = choices[myRandomIndex];

   // Create a red disk in a random spot
   Instantiate (RedDisk, new Vector3 (result, 10, 0), Quaternion.identity);

  // User can now put his disk
 PlayerTurn = true;

}

}