Cannot convert Cython memoryviewslice to ndarray - multidimensional-array

Question

I am trying to write an explicit Successive Overrelaxation Function over a 2D matrix. In this case for an electrostatic potential.
When trying to optimize this in Cython I seem to get an error that I am not quite sure I understand.
%%cython
cimport cython
import numpy as np
cimport numpy as np
from libc.math cimport pi
#SOR function
#cython.boundscheck(False)
#cython.wraparound(False)
#cython.initializedcheck(False)
#cython.nonecheck(False)
def SOR_potential(np.float64_t[:, :] potential, mask, int max_iter, float error_threshold, float alpha):
#the ints
cdef int height = potential.shape[0]
cdef int width = potential.shape[1] #more general non quadratic
cdef int it = 0
#the floats
cdef float error = 0.0
cdef float sor_adjustment
#the copy array we will iterate over and return
cdef np.ndarray[np.float64_t, ndim=2] input_matrix = potential.copy()
#set the ideal alpha if user input is 0.0
if alpha == 0.0:
alpha = 2/(1+(pi/((height+width)*0.5)))
#start the SOR loop. The for loops omit the 0 and -1 index\
#because they are *shadow points* used for neuman boundary conditions\
cdef int row, col
#iteration loop
while True:
#2-stencil loop
for row in range(1, height-1):
for col in range(1, width-1):
if not(mask[row][col]):
potential[row][col] = 0.25*(input_matrix[row-1][col] + \
input_matrix[row+1][col] + \
input_matrix[row][col-1] + \
input_matrix[row][col+1])
sor_adjustment = alpha * (potential[row][col] - input_matrix[row][col])
input_matrix[row][col] = sor_adjustment + input_matrix[row][col]
error += np.abs(input_matrix[row][col] - potential[row][col])
#by the end of this loop input_matrix and potential have diff values
if error<error_threshold:
break
elif it>max_iter:
break
else:
error = 0
it = it + 1
return input_matrix, error, it
and I used a very simple example for an array to see if it would give an error output.
test = [[True, False], [True, False]]
pot = np.array([[1.0, 2.0], [3.0, 4.0]], dtype=np.float64)
SOR_potential(pot, test, 50, 0.1, 0.0)
Gives out this error:
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
Cell In [30], line 1
----> 1 SOR_potential(pot, test, 50, 0.1, 0.0)
File _cython_magic_6c09a5060df996862b8e35adacc0e25c.pyx:21, in _cython_magic_6c09a5060df996862b8e35adacc0e25c.SOR_potential()
TypeError: Cannot convert _cython_magic_6c09a5060df996862b8e35adacc0e25c._memoryviewslice to numpy.ndarray
But when I delete the np.float64_t[:, :] part from
def SOR_potential(np.float64_t[:, :] potential,...)
the code works. Of course, the simple 2x2 matrix will not converge but it gives no errors. Where is the mistake here?
I also tried importing the modules differently as suggested here
Cython: how to resolve TypeError: Cannot convert memoryviewslice to numpy.ndarray?
but I got 2 errors instead of 1 where there were type mismatches.
Note: I would also like to ask, how would I define a numpy array of booleans to put in front of the "mask" input in the function?

Answer 1

A minimal reproducible example of your error message would look like this:
def foo(np.float64_t[:, :] A):
cdef np.ndarray[np.float64_t, ndim=2] B = A.copy()
# ... do something with B ...
return B
The problem is, that A is a memoryview while B is a np.ndarray. If both A and B are memoryviews, i.e.
def foo(np.float64_t[:, :] A):
cdef np.float64_t[:, :] B = A.copy()
# ... do something with B ...
return np.asarray(B)
your example will compile without errors. Note that you then need to call np.asarray if you want to return a np.ndarray.
Regarding your second question: You could use a memoryview with dtype np.uint8_t
def foo(np.float64_t[:, :] A, np.uint8_t[:, :] mask):
cdef np.float64_t[:, :] B = A.copy()
# ... do something with B and mask ...
return np.asarray(B)
and call it like this from Python:
mask = np.array([[True, True], [False, False]], dtype=bool)
A = np.ones((2,2), dtype=np.float64)
foo(A, mask)
PS: If your array's buffers are guaranteed to be C-Contiguous, you can use contiguous memoryviews for better performance:
def foo(np.float64_t[:, ::1] A, np.uint8_t[:, ::1] mask):
cdef np.float64_t[:, ::1] B = A.copy()
# ... do something with B and mask ...
return np.asarray(B)