Fortran: is_power2(a) returns .true. if a is power of 2, i.e. a==2**n.

is_power2.f

! is_power2.f -*-f90-*-
! is_power2(a) returns .true. if a is power of 2, i.e. a==2**n.
! Author: Takeshi Nishimatsu
!!
logical function is_power2(a)
  implicit none
  integer, intent(in) :: a
  integer             :: x
  x = a
  if (x<=0) then
     is_power2 = .false.
  else
     do while (mod(x,2).eq.0)
        x = x/2
     end do
     if (x.eq.1) then
        is_power2 = .true.
     else
        is_power2 = .false.
     end if
  end if
end function is_power2

is_power2_check.f

! is_power2_check.f -*-f90-*-
! check is_power2.f
! Author: Takeshi Nishimatsu
!!
program is_power2_check
  implicit none
  logical is_power2
  external is_power2
  if (is_power2(-2)) stop 1
  if (is_power2(-1)) stop 2
  if (is_power2( 0)) stop 3
  if (is_power2( 6)) stop 4
  if (is_power2(48)) stop 5

  if (.not.is_power2( 1)) stop 6
  if (.not.is_power2( 2)) stop 7
  if (.not.is_power2( 4)) stop 8
  if (.not.is_power2( 8)) stop 9
end program