how to place a block on the terrain where the mouse is?

[ApplezGaming]

I am trying to make a game where you have a flat world and you use a,w,s,d and up and down arrow to rotate the camera round and zoom in and out and i have all that done and what i am trying to do is get it so the person playing the game can place and destroy blocks on the terrai nwith in the grid. but all i can get it to do is place them at the centre of the screen could some one please give me some help? as i am not very good at coding.

[Seth-Bergman]

like this:

var prefab : Transform;

function Update(){
    if(Input.GetMouseButtonUp(0)){
        var ray : Ray = camera.ScreenPointToRay (Input.mousePosition);
        var hit : RaycastHit;

        if (Physics.Raycast(ray, hit,Mathf.Infinity)){

            if(hit.collider.tag == "Terrain"){
                 var placePos : Vector3 = hit.point;
                 placePos.y += .5;
                 placePos.x = Mathf.Round(placePos.x);
                 placePos.z = Mathf.Round(placePos.z);
                   Instantiate (prefab, placePos, Quaternion.identity);
            }
        }
    }
}

you will need to set up the tag on your terrain for this to work, or get rid of that line…

EDIT: updated the code to attempt to achieve the desired grid behavior… I think this should pretty much work…