[Numpy-discussion] Array views

Pauli Virtanen pav at iki.fi
Sat Mar 26 15:13:43 EDT 2011

On Sat, 26 Mar 2011 12:32:24 -0500, srean wrote:
> Is there anyway apart from using ufuncs that I can make updater() write
> the result directly in b and not create a new temporary column that is
> then copied into b?  Say for the matrix vector multiply example. I can
> write the matrix vector product in terms of ufuncs but will lose out in
> terms of speed.

Well, you can e.g. write

def updater(b, col_idx):
   b[:,col_idx] *= 3 # <- modifies b[:,col_idx] in place

And ditto for sparse matrices --- but maybe this is not what you asked.

If you want to have control over temporaries, you can make use of the 
out= argument of ufuncs (`numpy.dot` will gain it in 1.6.1 --- you can 
call LAPACK routines from scipy.lib in the meantime, if your data is in 
Fortran order).

Also numexpr is probably able to write the output directly to a given 
array --- using it is an alternative way to avoid temporaries, and 
probably easier to write than doing things via the out= arguments.

For sparse matrices, things then depend on how they are laid out in 
memory. You can probably alter the `.data` attribute of the arrays 
directly, if you know how the underlying representation works.

Pauli Virtanen

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