[Numpy-discussion] Re: [SciPy-user] doctest fortran I/O synchronization

[Pearu Peterson]

Hi Todd,

On 10 Oct 2003, Todd Miller wrote:

> I'm trying to make a doctest to verify that the different flow patterns
> of f2py interfaces work with different varieties of numarrays (normal,
> byte-swapped, misaligned, dis-contiguous, type-converted).  I'm trying
> to test this out under Linux with g77, and (it seems like) I'm having
> trouble synchronizing the Fortran I/O with Python's C I/O.
> 
> Given foo.f:
> 
>       subroutine in_c(a,m,n)
>       real*8 a(n,m)
> Cf2py intent(in,c) a
> Cf2py depend(a) ::  n=shape(a,0),  m=shape(a,1)
>       do j=1,m
>          do i=1,n
>             write (6,1) a(i,j)
>  1          format( $, 1F3.0, ', ')
>          enddo
>          print *,''
>       enddo
>       end
> 
> And given f2py_tests.py:

> """
> >>> foo.in_f(a)
<snip>

I could not run given tests as they were not complete and had typos.
For instance, foo.f defines in_c but in test string you are using in_f.
Also AFAIK, Python I/O will not catch I/O from Fortran.

Any way, when I modify in_c to in_f then the following code

  a = numarray.arange(15., shape=(3,5))
  print a
  foo.in_f(a)

outputs:

[[  0.   1.   2.   3.   4.]
 [  5.   6.   7.   8.   9.]
 [ 10.  11.  12.  13.  14.]]
 0.,  1.,  2.,  
 3.,  4.,  5.,  
 6.,  7.,  8.,  
 9., 10., 11.,  
12., 13., 14.,  

You probaly would not expect this. This is related to different storage 
order in C and Fortran, of cource, and you have disabled f2py ability to 
take into account this by using intent(c).

So, when you would not use intent(c), that is, in foo.f is a line

  Cf2py intent(in) a

then the following output occurs:

[[  0.   1.   2.   3.   4.]
 [  5.   6.   7.   8.   9.]
 [ 10.  11.  12.  13.  14.]]
 0.,  5., 10.,  
 1.,  6., 11.,  
 2.,  7., 12.,  
 3.,  8., 13.,  
 4.,  9., 14.,  

The arrays look transposed because in Fortran your row index varies 
faster, that is, a transposed array is displayed.

To sum up, don't use intent(c) and change the order of loops in in_f 
function to get matching results.

HTH,
  Pearu