Converting python arguments into fortran arrays F2PY3

Question

I have finally managed to wrap some Fortran into Python using numpy's f2py. I am going to need to write a small library of subroutines that I can then use for a larger project, but I am already having trouble with the example subroutine I just compiled.

This is a simple algorithm to solve linear systems of equations in Fortran 90:

SUBROUTINE solve_lin_sys(A, c, x, n)
! =====================================================
! Uses gauss elimination and backwards substitution
! to reduce a linear system and solve it.
! Problem (0-div) can arise if there are 0s on main diag.
! =====================================================
  IMPLICIT NONE
  INTEGER:: i, j, k
  REAL*8::fakt, summ
  INTEGER, INTENT(in):: n
  REAL*8, INTENT(inout):: A(n,n)
  REAL*8, INTENT(inout):: c(n)
  REAL*8, INTENT(out):: x(n)

  DO i = 1, n-1                   ! pick the var to eliminate
    DO j = i+1, n                 ! pick the row where to eliminate
      fakt = A(j,i) / A(i,i)      ! elimination factor
      DO k = 1, n                 ! eliminate
        A(j,k) = A(j,k) - A(i,k)*fakt
      END DO
      c(j)=c(j)-c(i)*fakt         ! iterate on known terms
    END DO
  END DO

   ! Actual solving:
    x(n) = c(n) / A(n,n)          ! last variable being solved
    DO i = n-1, 1, -1
      summ = 0.d0
      DO j = i+1, n
        summ = summ + A(i,j)*x(j)
      END DO
      x(i) = (c(i) - summ) / A(i,i)
    END DO

END SUBROUTINE solve_lin_sys

I successfully made it into a Python module and I can import it, but I can't figure out what kind of arguments I should pass it into python.

This is what I've tried:

import numpy as np
import fortran_operations as ops

sys = [[3, -0.1, -0.2],
       [0.1, 7, -0.3],
       [0.3, -0.2, 10]]

known = [7.85, -19.3, 71.4]

A = np.array(sys)
c = np.array(known)

x = ops.solve_lin_sys(A, c, 3)

I tried using both the normal list and the numpy array as an argument for the Fortran function, but in all cases I get the error:

x = ops.solve_lin_sys(A, c, 3) ------ ValueError: failed in converting 1st argument `a' of fortran_operations.solve_lin_sys to C/Fortran array

I thought you were supposed to simply use numpy arrays. Printing solve_lin_sys. doc also states that I should use rank 2 arrays for A and rank 1 for C. Isn't that already the shape of the arrays when I defined them like that?

Sorry if this has a stupid solution, I'm a beginner with all of this.

Thank you in advance

Answer 1

Arrays of dimension 2 and higher must be continuous with 'Fortran' order of elements.

import ops

sys = [[3, -0.1, -0.2],
       [0.1, 7, -0.3],
       [0.3, -0.2, 10]]

known = [7.85, -19.3, 71.4]

A = np.array(sys, dtype='d', order='F')
c = np.array(known, dtype='d')

x = ops.solve_lin_sys(A, c, 3)