• Kirill Smelkov's avatar
    lib.xnumpy.structured: New utility to create structured view of an array · 32ca80e2
    Kirill Smelkov authored
    Structured creates view of the array interpreting its minor axis as fully covered by a dtype.
    
    It is similar to arr.view(dtype) + corresponding reshape, but does
    not have limitations of ndarray.view(). For example:
    
      In [1]: a = np.arange(3*3, dtype=np.int32).reshape((3,3))
    
      In [2]: a
      Out[2]:
      array([[0, 1, 2],
             [3, 4, 5],
             [6, 7, 8]], dtype=int32)
    
      In [3]: b = a[:2,:2]
    
      In [4]: b
      Out[4]:
      array([[0, 1],
             [3, 4]], dtype=int32)
    
      In [5]: dtxy = np.dtype([('x', np.int32), ('y', np.int32)])
    
      In [6]: dtxy
      Out[6]: dtype([('x', '<i4'), ('y', '<i4')])
    
      In [7]: b.view(dtxy)
      ---------------------------------------------------------------------------
      ValueError                                Traceback (most recent call last)
      <ipython-input-66-af98529aa150> in <module>()
      ----> 1 b.view(dtxy)
    
      ValueError: To change to a dtype of a different size, the array must be C-contiguous
    
      In [8]: structured(b, dtxy)
      Out[8]: array([(0, 1), (3, 4)], dtype=[('x', '<i4'), ('y', '<i4')])
    
    Structured always creates view and never copies data.
    
    Here is original context where separately playing with .shape and .dtype
    was not enough, since it was creating array copy and OOM'ing the machine:
    
    klaus/wendelin@cbe4938b
    32ca80e2
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