Re: [Numpy-discussion] Getting a numpy array from a ctype pointer

2008-09-05 Thread Paulo J. S. Silva 

 x = np.frombuffer(int_asbuffer(C.addressof(x.contents), n*8))

I'll go with your faster solution. Very good. Thank you very much.

Paulo

___
Numpy-discussion mailing list
Numpy-discussion@scipy.org
http://projects.scipy.org/mailman/listinfo/numpy-discussion

[Numpy-discussion] Getting a numpy array from a ctype pointer

2008-09-04 Thread Paulo J. S. Silva 
Hello,

I am writing some code interfacing C and Python using ctypes. In a
callback function (in Python) I get in a parameter x which is c_double
pointer and a parameter n which is c_int representing the length of the
array. 

How can I transform this information into a numpy array?

Paulo


___
Numpy-discussion mailing list
Numpy-discussion@scipy.org
http://projects.scipy.org/mailman/listinfo/numpy-discussion

Re: [Numpy-discussion] Getting a numpy array from a ctype pointer

2008-09-04 Thread Travis E. Oliphant 
Paulo J. S. Silva wrote:
 Hello,

 I am writing some code interfacing C and Python using ctypes. In a
 callback function (in Python) I get in a parameter x which is c_double
 pointer and a parameter n which is c_int representing the length of the
 array. 

 How can I transform this information into a numpy array?

   
Something like this may work:

from numpy import ctypeslib

r = ctypeslib.as_array(x._type_ * n)

If that doesn't work, then you can create an array from an arbitrary 
buffer or any object that exposes the __array_interface__ attribute.   
So, if you can get the address of c_double then you can use it as the 
data for the ndarray.

Ask if you need more help.

-Travis

___
Numpy-discussion mailing list
Numpy-discussion@scipy.org
http://projects.scipy.org/mailman/listinfo/numpy-discussion

Re: [Numpy-discussion] Getting a numpy array from a ctype pointer

2008-09-04 Thread Paulo J. S. Silva 
Em Qui, 2008-09-04 às 15:01 -0500, Travis E. Oliphant escreveu:
 Paulo J. S. Silva wrote:

 Something like this may work:
 
 from numpy import ctypeslib
 
 r = ctypeslib.as_array(x._type_ * n)
 

Unfortunately, it didn't work.

 If that doesn't work, then you can create an array from an arbitrary 
 buffer or any object that exposes the __array_interface__ attribute.   
 So, if you can get the address of c_double then you can use it as the 
 data for the ndarray.
 
 Ask if you need more help.

I think I need help. I discovered how to create a Python string from the
pointer using ctypes' string_at function:

r = C.string_at(x, n*C.sizeof(x._type_))

Now I can use numpy fromstring to get a array version of it. However
this is not enough for my application as fromstring actually copies the
data, hence change to the array are not reflected in the original array
that goes back to the Fortran code.

Any hints?

Paulo 

___
Numpy-discussion mailing list
Numpy-discussion@scipy.org
http://projects.scipy.org/mailman/listinfo/numpy-discussion

Re: [Numpy-discussion] Getting a numpy array from a ctype pointer

2008-09-04 Thread Paulo J. S. Silva 
After some trial and erros, I found a solution, described below. Is this
the best one? Looks a little convoluted to me (C represents ctypes
module and np numpy):

Array = n*C.c_double
x = Array.from_address(C.addressof(x.contents))
x = np.ctypeslib.as_array(x)

Thanks,

Paulo

Em Qui, 2008-09-04 às 17:31 -0400, Paulo J. S. Silva escreveu:
 Em Qui, 2008-09-04 às 15:01 -0500, Travis E. Oliphant escreveu:
  Paulo J. S. Silva wrote:
 
  Something like this may work:
  
  from numpy import ctypeslib
  
  r = ctypeslib.as_array(x._type_ * n)
  
 
 Unfortunately, it didn't work.
 
  If that doesn't work, then you can create an array from an arbitrary 
  buffer or any object that exposes the __array_interface__ attribute.   
  So, if you can get the address of c_double then you can use it as the 
  data for the ndarray.
  
  Ask if you need more help.
 
 I think I need help. I discovered how to create a Python string from the
 pointer using ctypes' string_at function:
 
 r = C.string_at(x, n*C.sizeof(x._type_))
 
 Now I can use numpy fromstring to get a array version of it. However
 this is not enough for my application as fromstring actually copies the
 data, hence change to the array are not reflected in the original array
 that goes back to the Fortran code.
 
 Any hints?
 
 Paulo 
 
 ___
 Numpy-discussion mailing list
 Numpy-discussion@scipy.org
 http://projects.scipy.org/mailman/listinfo/numpy-discussion

___
Numpy-discussion mailing list
Numpy-discussion@scipy.org
http://projects.scipy.org/mailman/listinfo/numpy-discussion

Re: [Numpy-discussion] Getting a numpy array from a ctype pointer

2008-09-04 Thread Travis E. Oliphant 
Paulo J. S. Silva wrote:
 After some trial and erros, I found a solution, described below. Is this
 the best one? Looks a little convoluted to me (C represents ctypes
 module and np numpy):

 Array = n*C.c_double
 x = Array.from_address(C.addressof(x.contents))
 x = np.ctypeslib.as_array(x)
   

That's a pretty simple approach.   There is a faster approach which uses 
the undocumented function int_asbuffer function from numpy.core.multiarray

(i.e. from numpy.core.multiarray import int_asbuffer)

Then:

x = np.frombuffer(int_asbuffer(C.addressof(x.contents), n*8))

If you don't like the hard-coded '8', then you can get that number from

np.dtype(float).itemsize

There may also be a way to get it from ctypes.


-Travis




___
Numpy-discussion mailing list
Numpy-discussion@scipy.org
http://projects.scipy.org/mailman/listinfo/numpy-discussion