[SciPy-user] Newbie broadcasting question

[Robin]

On Tue, Mar 4, 2008 at 6:00 PM, John Reid <j.reid at mail.cryst.bbk.ac.uk> wrote:
> This works fine:
>
>  In [49]: numpy.zeros((3,2))+numpy.array([1,2])
>  Out[49]:
>  array([[ 1.,  2.],
>         [ 1.,  2.],
>         [ 1.,  2.]])
>
>
>  but this doesn't:
>  In [50]: numpy.zeros((3,2))+numpy.array([1,2,3])
>  ---------------------------------------------------------------------------
>  ValueError                                Traceback (most recent call last)
>
>  C:\Dev\MyProjects\Bio\Python\site_dpm\<ipython console> in <module>()
>
>  ValueError: shape mismatch: objects cannot be broadcast to a single shape
>   > <ipython console>(1)<module>()
>
>
>
>  what is the simplest way to get this addition to work? I would like the
>  following
>
>  array([[ 0.,  0.],
>         [ 0.,  0.],
>         [ 0.,  0.]])
>
>
>  +
>
>  array([1,2,3])
>
>  =
>
>  array([[ 1.,  1.],
>         [ 2.,  2.],
>         [ 3.,  3.]])
>
>  In general my arrays are held in variables and are not constructed on
>  the fly so while this works
>
>  numpy.zeros((3,2)).T+numpy.array([1,2,3])
>
>  which works fine with the '+' operator but not with '+='. This doesn't work:
>  In [54]: a=zeros((3,2))
>
>  In [55]: a.T += numpy.array([1,2,3])
>  ---------------------------------------------------------------------------
>  AttributeError                            Traceback (most recent call last)
>
>  C:\Dev\MyProjects\Bio\Python\site_dpm\<ipython console> in <module>()
>
>  AttributeError: attribute 'T' of 'numpy.ndarray' objects is not writable
>   > <ipython console>(1)<module>()
>
>
>  My guess is that I should do
>  a = (a.T + numpy.array([1,2,3])).T
>
>  but I wonder if there is a more efficient way. Am I missing something?
>
>  Thanks,
>  John.
>
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>

You could try:
aT = a.T
aT += numpy.array([1,2,3])
Since aT is a view the data is the same as a, so a itself will be updated.