Numeric, how to do in place full-array modification?

[Janko Hauser]

"Ionel Simionescu" <ionel at psy.uva.nl> writes:

> Hi,
> 
> I want to do the following:
> 
> for t in range( T ):
>    net = dot( C, x )
>    x   = A*net*(1-net)
> 
> Because these arrays might turn to be large, I want to pre-alloate <net> and
> write both <net> and <x> in place rather than creating new arrays.
> 
> 
> Intuitively, I would write it like:
> 
> net = zeros(shape, typecode)
> for t in range( T ):
>     net[:] = dot( C, x)
>     x[:]   = A*net*(1-net)
> 
This would not buy you much. Because you only substitute inplace, the
dot function is always generating a new array, which gets copied into
net in your case, which would be more expensive than only to reference 
the result with

net = dot( C, x) 

But dot is mainly implemented in C, so this should be no memory problem.

The second one can be changed to inplace modification.

x=multiply(multiply(A,net,x),subtract(1,net,x),x)

That looks ugly, right? :-)

HTH

__Janko

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