Reference to derived type element by index?

I have a derived type, e.g.

type circle_datatype
real :: radius
real :: x
real :: y
real :: z
character(len=*) :: circle_label
endtype circle_datatype
type(circle_datatype) :: circle


and want to refer to the individual elements via some indexing scheme, say in this example I want to change the value of circle%radius I would use index #1, or the z-coordinate I would use #4. Is there some method by which this is possible?

I think IDL has a system for this, such that circle.radius can be referenced by circle.(0), but I really need Fortran's horsepower for this code!
.