How to find position in an array

I have an array:

   r1.rp_parts:
        [<Rp1....>][<Rp1....>][<Rp1....>][<Rp4....>]

If I do:
    r1.rp_parts[2]
I obtain
    Rp1...

I would like to obtain de position of r1.rp_parts[2] in the r1.rp_parts
array. I know that it's position is 2, but I wonder if I can obtain the
position from r1.rp_parts[2].

John Smith wrote:

I have an array:

   r1.rp_parts:
        [<Rp1....>][<Rp1....>][<Rp1....>][<Rp4....>]

If I do:
    r1.rp_parts[2]
I obtain
    Rp1...

I would like to obtain de position of r1.rp_parts[2] in the r1.rp_parts
array. I know that it's position is 2, but I wonder if I can obtain the
position from r1.rp_parts[2].

Use the index method of Array:

some_part = r1.rp_parts[2]
r1.rp_parts.index(some_part) #=> 2