How to instantiate an array of prefabs into an array of empty gameobject positions

Firstly I'm a complete Noob. I'm trying to instantiate an array of prefabs into a predefined array of positions randomly. The element I'm trying to get with the PositionOFPlaceholders is a value for "pos" so that when I Instantiate the GameObjects, it will bring in my 34 prefabs into the 34 placeholders randomly. I'm getting the "Array out of Range" exception at this line;

 placeholders[rndpos].transform.position = placeholderPositions[i];

Here is my code so far;

 public class TileScript : MonoBehaviour {

 public GameObject[] movableTiles;
 private GameObject tempGO;
 public Vector3[] placeholderPositions;
 public GameObject[] placeholders;
 private Vector3 pos;
 

 void Start()
 {
     Shuffle();
     PositionOfPlaceholder();          
     Instantiate(tempGO, pos, Quaternion.Euler(0, Random.Range(0, 4) * 90, 0));
 }
 public void Shuffle()
 {
     for (int i = 0; i < movableTiles.Length; i++)
     {
         int rnd = Random.Range(0, movableTiles.Length);
         tempGO = movableTiles[rnd];
         movableTiles[rnd] = movableTiles[i];
         movableTiles[i] = tempGO;
     }
 }
 public void PositionOfPlaceholder()
 {
     placeholders = GameObject.FindGameObjectsWithTag("Placeholder");

     for (int i = 0; i < placeholders.Length; i++)
     {             
         int rndpos = Random.Range(0, placeholders.Length);
         pos = placeholders[rndpos].transform.position;
         placeholders[rndpos].transform.position = placeholderPositions[i];
         placeholderPositions[i] = pos;                      
     }
 }       

}

Any ideads?