[GitHub] aidan-plenert-macdonald commented on issue #9380: nd.stack causes abort for differently sized arrays

[GitBox <gi...@apache.org>]

aidan-plenert-macdonald commented on issue #9380: nd.stack causes abort for differently sized arrays
URL: https://github.com/apache/incubator-mxnet/issues/9380#issuecomment-358758416
 
 
   @eric-haibin-lin So this wasn't so much about it doing the right thing as simply not killing Python. This error shouldn't be fatal. Not Numpy's behavior,
   
   ```
   $ python
   Python 3.6.3 (default, Oct  4 2017, 06:09:15) 
   [GCC 4.2.1 Compatible Apple LLVM 9.0.0 (clang-900.0.37)] on darwin
   Type "help", "copyright", "credits" or "license" for more information.
   >>> import numpy as np
   >>> a = np.arange(20).reshape((20, 2))
   >>> a = np.arange(20).reshape((10, 2))
   >>> b = np.arange(10).reshape((10, 1))
   >>> np.stack([a, b])
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "/usr/local/lib/python3.6/site-packages/numpy/core/shape_base.py", line 354, in stack
       raise ValueError('all input arrays must have the same shape')
   ValueError: all input arrays must have the same shape
   >>> np.hstack([a, b])
   array([[ 0,  1,  0],
          [ 2,  3,  1],
          [ 4,  5,  2],
          [ 6,  7,  3],
          [ 8,  9,  4],
          [10, 11,  5],
          [12, 13,  6],
          [14, 15,  7],
          [16, 17,  8],
          [18, 19,  9]])
   ```
   
   Numpy doesn't kill the session when it throws the error. But beyond that, MXNet lacks an `nd.hstack` function, so it would be nice if `nd.stack` could handle stacking of different shapes.
   
   For example, pseudocode outlining the shape assertions.
   ```python
   def stack(A, B, axis=0):
       for i, (s0, s1) in enumerate(zip(A.shape, B.shape)):
           if axis != i:
               assert A.shape[i] == B.shape[i]
       # Actually do the stacking
       C = actual_stacking(A, B, axis=axis)
       for i, (s0, s1) in enumerate(zip(A.shape, B.shape, C.shape)):
           if axis != i:
               assert A.shape[i] == B.shape[i]
               assert A.shape[i] == C.shape[i]
          else:
               assert C.shape[i] == A.shape[i] + B.shape[i]
       return C
   ```

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