[Numpy-discussion] Fortran ordering

[Dag Sverre Seljebotn]

Paul Northug wrote:
> I'd like to use numpy fortran ordering in order to use some external
> libraries more easily, but it looks like I don't understand how it
> works and it is causing transposes that are confusing me.
> 
> When I define an array as:
> 
> a = np.array([[1.,2.],[3.,4.]], order='F', type=np.float32)
> 
> how is it stored in memory, as [1, 2, 3, 4] or [1, 3, 2, 4]?
> 
> a.strides == (4, 8), suggesting [1, 3, 2, 4]. I can also check by
> stepping through:
> 
> ap = a.ctypes.data_as(POINTER(c_float))
> ap[:4]  == [1., 3., 2., 4.]
> 
> But when I call external libraries through ctypes with a.ctypes.data
> as argument, the external function gets [1, 2, 3, 4] (I think).
> Eventually, I plan to use cython's buffer interface and a.data. Will
> it be [1, 2, 3, 4] there as well? Or is my external code broken.

Using Cython's a.data, it will definitely be [1, 3, 2, 4].

Declare your array in Cython as np.ndarray[float, mode='fortran'], that 
way you'll get a ValueError unless the ordering is correct.

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Dag Sverre