[Numpy-discussion] problem with view() and strided arrays?

[Zachary Pincus]

Hi all,

> Actually -- it seems like view() doesn't work with strided arrays at
> all. (?)
>
> In : a = numpy.ones((4,32), dtype=numpy.uint8)
>
> In : a.view(numpy.uint16).shape
> Out: (4, 16)
>
> In : a[:,:16].view(numpy.uint16)
> ValueError: new type not compatible with array.
>
> I think this might be a recent regression, because before I updated my
> numpy installation to the latest SVN version (to check if the bug was
> fixed!), I'm pretty sure this kind of operation worked.

The problem starts out in arrayobject.c:6392, in array_descr_set(),  
where the following test is performed:
     if ((newtype->elsize != self->descr->elsize) &&         \
         (self->nd == 0 || !PyArray_ISONESEGMENT(self) || \
          newtype->subarray)) goto fail;

I *think* I could fix it by relaxing the restrictions to only require  
that the array have at least one dimension where self->strides[dim] ==  
self->descr->elsize, and then adjust the size of that dimension.

Here's my perhaps-fix. The old code is:
     if ((newtype->elsize != self->descr->elsize) &&         \
         (self->nd == 0 || !PyArray_ISONESEGMENT(self) || \
          newtype->subarray)) goto fail;

     if (PyArray_ISCONTIGUOUS(self)) index = self->nd - 1;
     else index = 0;


My suggested fix is:
     if ((newtype->elsize != self->descr->elsize) &&         \
         (self->nd == 0 || newtype->subarray)) goto fail;

     if (PyArray_ISCONTIGUOUS(self)) index = self->nd - 1;
     else if (PyArray_ISFORTRAN(self)) index = 0;
     else {
       int index_found = FALSE;
       for (index = 0; index < self->nd; index++) {
         if (self->strides[index] == self->descr->elsize) {
           index_found = TRUE;
           break;
         }
       }
       if (!index_found) goto fail;
     }

Could someone look this over? If it looks basically right, I'll make  
this a proper patch and post it to trac.

Thanks,
Zach