What needs changing to make a Fortran 90 Subroutine “PURE”?

Question

I've defined a code module (a Fortran F90 file) and supplied it with these two subroutines:

pure Subroutine XRotation (x, y, z, Xrot, xp, yp, zp)
! Rotate about X-axis through the angle Xrot

real*4, INTENT(IN) :: x, y, z, xrot
real*4, INTENT(OUT) :: xp, yp, zp
real*4 xrad

Xrad = Xrot * 3.141592627 / 180 ! Convert to radians
yp = y * COS(Xrad) + z * SIN(Xrad)
xp = x
zp = z * COS(Xrad) - y * SIN(Xrad)


END


Pure Subroutine DummyDummy()

Call XRotation(1,2,3,4,5,6,7)

End Subroutine

The compiler (Intel Fortran 12, targeting 32-bit Windows, for whatever that's worth) returns a single error message:

error #7137: Any procedure referenced in a PURE procedure, including one referenced via a defined operation or assignment, must be explicitly declared PURE. [XROTATION]

I'm a little bit stumped. How can I change this code so that the PURE Subroutine "DummyDummy" compiles?

Answer 1

i don't know much about fortran 90 (back in my day etc etc) but if you place that in a module it works. i get the impression that modules are needed for various "modern" fortran features.

anyway, someone smarter (or younger?) than me can explain, but that should help you continue.

module foo
  implicit none
contains

  pure Subroutine XRotation (x, y, z, Xrot, xp, yp, zp)
  ....

end module foo

ah, here's an explanation (see the answers) - Writing and calling pure subroutines in Fortran 90 using gfortran

ps i also get a type warning as you're passing in integers to float arguments.