Subroutines and Functions

Code

! Function with intent
function square(x) result(y)
  real, intent(in) :: x
  real :: y
  y = x * x
end function

! Subroutine with intent(out) — modifies argument
subroutine swap(a, b)
  real, intent(inout) :: a, b
  real :: temp
  temp = a; a = b; b = temp
end subroutine

! Pure function (no side effects)
pure function norm(vec) result(n)
  real, intent(in) :: vec(:)
  real :: n
  n = sqrt(sum(vec**2))
end function

! Elemental — works on scalars and arrays
elemental function deg2rad(deg) result(rad)
  real, intent(in) :: deg
  real :: rad
  rad = deg * 3.14159265 / 180.0
end function

! Usage
print *, square(5.0)         ! 25.0
call swap(x, y)              ! modifies x and y
print *, norm([3.0, 4.0])    ! 5.0
print *, deg2rad([0.0, 90.0, 180.0])  ! array version (elemental)

Explanation

intent(in) is input (read-only), intent(out) is output (overwritten), intent(inout) is both. pure functions have no side effects — compiler can optimize. elemental functions work on scalars and arrays element-wise (like NumPy ufuncs). Subroutines are called with call; functions return values.

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