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pythonarraysctypes
Eric
Eric

Reputation: 199

Python ctypes combine two c_double arrays

I have a Python program that needs to pass an array to a .dll that is expecting an array of c doubles. This is currently done by the following code, which is the fastest method of conversion I could find:

from array import array
from ctypes import *
import numpy as np

python_array = np.array(some_python_array)
temp = array('d', python_array.astype('float'))
c_double_array = (c_double * len(temp)).from_buffer(temp)

...where 'np.array' is just there to show that in my case the python_array is a numpy array. Let's say I now have two c_double arrays: c_double_array_a and c_double_array_b, the issue I'm having is I would like to append c_double_array_b to c_double_array_a without reconverting to/from whatever python typically uses for arrays. Is there a way to do this with the ctypes library?

I've been reading through the docs here but nothing seems to detail combining two c_type arrays after creation. It is very important in my program that they can be combined after creation, of course it would be trivial to just append python_array_b to python_array_a and then convert but that won't work in my case.

Thanks!

P.S. if anyone knows a way to speed up the conversion code that would also be greatly appreciated, it takes on the order of 150ms / million elements in the array and my program typically handles 1-10 million elements at a time.

Upvotes: 3

Views: 1118

Answers (1)

Davis Herring
Davis Herring

Reputation: 39758

Leaving aside the construction of the ctypes arrays (for which Mark's comment is surely relevant), the issue is that C arrays are not resizable: you can't append or extend them. (There do exist wrappers that provide these features, which may be useful references in constructing this.) What you can do is make a new array of the size of the two existing arrays together and then ctypes.memmove them into it. It might be possible to improve the performance by using realloc, but you'd have to go even lower than normal ctypes memory management to use it.

Upvotes: 2

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