How to define a n variable function in fortran using array

Question

I actually want to solve an n variable hamilton's sets of equations. In fortran,to define a function, we, generally do the following.

function H(x,p) result(s)
real::x,p,s
s=x**2+p**2
end function H

Now, if I wish to solve an n variable hamilton's equation, I need to define an n variable H(x(i),p(i)) where i runs from 1 to n. Suppose p(i) are the variables and H is p(i)^2, summed over i from 1 to n.

What are the possible ways of defining a function with an array as input? It is not possible to write H(x1,x2....x100...) manuaaly each time.

In fortran,to define a function, we, generally do the following. .... s=x^2+p^2 not with those funny little hat symbols we don't. — High Performance Mark
– High Performance Mark, Commented Jan 19, 2020 at 10:45
@kvantour perhaps elemental would be a better option ? But it's difficult to tell. — High Performance Mark
– High Performance Mark, Commented Jan 19, 2020 at 10:46
@HighPerformanceMark If H(x,p) really is that simple, elemental was also my immediate thought. It takes a while to wrap your head around array functions, side effects, etc. — arclight
– arclight, Commented Jan 23, 2020 at 3:59

rvbarreto · Accepted Answer · 2020-01-19 14:13:51Z

2

module aa
  implicit none

  public :: H

contains
  function H(x,p) result(s)
    real, dimension(:), intent(in) :: x,p
    real, dimension(:), allocatable :: s
    integer :: i, n

    n = size(x, 1)
    allocate(s(n))

    do i=1, n
       s(i) = x(i)**2 + p(i)**2
    enddo
  end function H
end module aa

program test
  use aa

  real, dimension(10) :: x, p
  real, dimension(:), allocatable :: s
  integer :: n

  x(:) = 1.
  p(:) = 1.

  n = size(x, 1)
  allocate(s(n))
  s(:) = 0.

  s = H(x,p)

  print*, s
end program test

Compiled and tested with GNU Fortran (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0

This is not really a real case program example because if you know the dimensions of x and p, you also know the dimensions of s, so you could have just defined it instead of allocating. But it can be used to generalize modules once there is no reference to any dimension in the module.

For this to work, you will notice that s must be allocatable and must have been allocated before calling the function.

edited Jan 19, 2020 at 14:13

answered Jan 19, 2020 at 12:50

rvbarreto

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3 Comments

albert Over a year ago

Did you test this? See comment from High Performance Mark with the question.

rvbarreto Over a year ago

Fixed. I haven't compiled, was writing by heart because I've done this specific kind of function a lot, but I mixed the math part with other languages. Thanks!

rvbarreto Over a year ago

Changed for a version which can be compiled out of the box.

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