Numpy: Retriving an array from an array of pointers

Question

I am writing a code, where I create an numpy array of pointers. They point to other arrays.

I can succesfully (no exception is generated) store a pointer inside one element of the array-of-pointers. But I cannot re-convert this pointer to a numpy array.

The problem specifically appears when pointers are stored in an numpy array of pointers. I can succesfully store and retrive an array when I store the pointer in a normal pyhon variable.

Note that I cannot just create a python list of pointers because of performances reasons.

this code works:

import numpy, ctypes
ic = numpy.array([[1,2],[3,4]],dtype=numpy.int32)
pointer = ic.__array_interface__['data'][0]
v = numpy.ctypeslib.as_array(ctypes.cast(pointer,ctypes.POINTER(ctypes.c_int)),shape=(2,2))
print(v)

and v will return the initial array set in ic.

this code does not work:

import numpy, ctypes
ic = numpy.array([[1,2],[3,4]],dtype=numpy.int32)
pointers = numpy.zeros(5, dtype=ctypes.POINTER(ctypes.c_int))
pointers[0] = ic.__array_interface__['data'][0]
numpy.ctypeslib.as_array(ctypes.cast(pointers[0],ctypes.POINTER(ctypes.c_int)),shape=(2,2))

The last line will give the following exception:

File "/opt/intel/intelpython3/lib/python3.5/ctypes/__init__.py", line 484, in cast
return _cast(obj, obj, typ)
ctypes.ArgumentError: argument 1: <class 'TypeError'>: wrong type

Question: how do I store and retrive a numpy-array from/to a numpy-array of pointers?

hpaulj · Accepted Answer · 2017-09-04 20:52:22Z

2

Indexing the array returns a np.int32 object, not a native Python int:

In [118]: type(pointer)
Out[118]: int
In [119]: type(pointers[0])
Out[119]: numpy.int32

Use item to extract the int:

In [120]: type(pointers[0].item())
Out[120]: int

You could also first convert the array to list

In [121]: type(pointers.tolist()[0])
Out[121]: int

pointers as you construct it is a np.int32 dtype

In [123]: pointers = numpy.zeros(5, dtype=ctypes.POINTER(ctypes.c_int))
In [124]: pointers.dtype
Out[124]: dtype('int32')

Alternatively make a object dtype array

In [125]: pointers = numpy.zeros(5, dtype=object)
In [126]: pointers[0] = pointer
In [127]: pointers
Out[127]: array([157379928, 0, 0, 0, 0], dtype=object)
In [128]: type(pointers[0])
Out[128]: int

answered Sep 4, 2017 at 20:52

hpaulj

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