On Wed, Jul 14, 2010 at 9:16 PM, Fernando Perez <fperez....@gmail.com> wrote:
> On Wed, Jul 14, 2010 at 7:09 PM, Robert Bradshaw
> <rober...@math.washington.edu> wrote:
>> Still
>> waiting for someone to put up a wiki page on what they like from weave
>> and what should go into Cython.
>
> Ahem :)
>
> A simple example of the blitz expressions which are nice:
>
> In [2]: import numpy as np
>
> In [3]: from scipy import weave
>
> In [4]:
>
> In [5]: a = np.arange(10)
>
> In [6]: x = np.empty_like(a)
>
> In [7]: weave.blitz('x=a+2*a*a')
> creating
> /tmp/fperez/python26_intermediate/compiler_8dc7dab30c62f7787b818e8ffdc19f8c
>
> In [8]: print x
> [ 0 3 10 21 36 55 78 105 136 171]
>
> In [9]: print x-(a+2*a*a)
> [0 0 0 0 0 0 0 0 0 0]
>
>
> Interactively, weave gives you a feel of 'interactive C++' thanks to
> its tying of the module name to the code hash:
>
> In [17]: src = 'std::cout << "a is " << a << std::endl;'
>
> In [18]: a = 10
>
> In [19]: inline(src, ['a']) # compilation happens here
> a is 10
>
> In [20]: a = 4.99
>
> In [21]: inline(src, ['a']) #compiling again, for a float
> a is 4.99
>
> In [22]: a = 12
>
> In [23]: inline(src, ['a']) # no compilation, code for a int is reused
> a is 12
>
>
> Some examples that use the blitz indexing support:
>
> # Returning a scalar quantity computed from an array.
> def trace(mat):
> """Return the trace of a matrix.
> """
> nrow,ncol = mat.shape
> code = \
> """
> double tr=0.0;
>
> for(int i=0;i<nrow;++i)
> tr += mat(i,i);
> return_val = tr;
> """
> return inline(code,['mat','nrow','ncol'],
> type_converters = converters.blitz)
>
> # In-place operations on arrays in general work without any problems
> def in_place_mult(num,mat):
> """In-place multiplication of a matrix by a scalar.
> """
> nrow,ncol = mat.shape
> code = \
> """
> for(int i=0;i<nrow;++i)
> for(int j=0;j<ncol;++j)
> mat(i,j) *= num;
>
> """
> inline(code,['num','mat','nrow','ncol'],
> type_converters = converters.blitz)
>
>
> But at least I don't care if this *specific syntax* survives, I only
> care about the ability to produce fast code to manipulate numpy arrays
> with clean syntax. Weave with inline/blitz helps really well in
> practice on this front, and if we can get these abilities with Cython,
> that would be great.
I think we can.
> One more use case I have had in the past: I've auto-generated complex
> C++ codes with dimension-specific numbers of loops and other
> parameters of a problem, to produce objects which, once instantiated,
> had a few key methods run very fast. This proved quite easy to do
> with weave, because I could compute the code as a string with nice
> python code at object initialization time (when the
> dimensions/parameters became known), and then during the object's
> life it would always use the compiled version. So while 'coding in
> strings' is in most situations awful, in this case it actually was
> very handy to have the flexibility to use string-generated code and
> weave handle all the gory details for me.
:) I know all about auto-generated code.
> I hope this is useful, sorry for not having provided these details before.
Yes very. I got a bit of this in our conversation a couple of weeks
back, but thanks for putting this down in a concrete form. (Also,
there's nothing like actually seeing code.)
- Robert
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