How do I convert a Python list of lists of lists into a C array by using ctypes?

Question

As seen here How do I convert a Python list into a C array by using ctypes? this code will take a python array and transform it to a C array.

import ctypes
arr = (ctypes.c_int * len(pyarr))(*pyarr)

Which would the way of doing the same with a list of lists or a lists of lists of lists?

For example, for the following variable

list3d = [[[40.0, 1.2, 6.0, 0.3], [50.0, 4.2, 0, 0]], [[40.0, 1.2, 6.0, 0.3], [50.0, 4.2, 0, 0]], [[40.0, 1.2, 6.0, 0.3], [50.0, 4.2, 0, 0]]]

I have tried the following with no luck:

([[ctypes.c_double * 4] *2]*3)(*list3d)
# *** TypeError: 'list' object is not callable

(ctypes.c_double * 4 *2 *3)(*list3d)
# *** TypeError: expected c_double_Array_4_Array_2 instance, got list

Thank you!

EDIT: Just to clarify, I am trying to get one object that contains the whole multidimensional array, not a list of objects. This object's reference will be an input to a C DLL that expects a 3D array.

this is not a good solution. for a list a=[[1,2,3,4],[2,3,5,6]] , you can do arr = [ (ctypes.c_int * len(key))(*key) for key in a ] — Thothadri Rajesh
– Thothadri Rajesh, Commented Oct 17, 2013 at 18:59

Eryk Sun · Accepted Answer · 2013-10-17 23:55:28Z

2

It works with tuples if you don't mind doing a bit of conversion first:

from ctypes import *

list3d = [
    [[0.0, 1.0, 2.0, 3.0], [4.0, 5.0, 6.0, 7.0]], 
    [[0.2, 1.2, 2.2, 3.2], [4.2, 5.2, 6.2, 7.2]],
    [[0.4, 1.4, 2.4, 3.4], [4.4, 5.4, 6.4, 7.4]],
]

arr = (c_double * 4 * 2 * 3)(*(tuple(tuple(j) for j in i) for i in list3d))

Check that it's initialized correctly in row-major order:

>>> (c_double * 24).from_buffer(arr)[:]
[0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 
 0.2, 1.2, 2.2, 3.2, 4.2, 5.2, 6.2, 7.2, 
 0.4, 1.4, 2.4, 3.4, 4.4, 5.4, 6.4, 7.4]

Or you can create an empty array and initialize it using a loop. enumerate over the rows and columns of the list and assign the data to a slice:

arr = (c_double * 4 * 2 * 3)()

for i, row in enumerate(list3d):
    for j, col in enumerate(row):
        arr[i][j][:] = col

edited Oct 17, 2013 at 23:55

answered Oct 17, 2013 at 21:58

Eryk Sun

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Comments

quimnuss · Accepted Answer · 2014-06-25 10:02:18Z

0

I made the change accordingly

a = [[[40.0, 1.2, 6.0, 0.3], [50.0, 4.2, 0, 0]], [[40.0, 1.2, 6.0, 0.3], [50.0, 4.2, 0, 0]], [[40.0, 1.2, 6.0, 0.3], [50.0, 4.2, 0, 0]]]
arr = (((ctypes.c_float * len(a[0][0])) * len(a[0])) * len(a))
arr_instance=arr()
for i in range(0,len(a)):
  for j in range(0,len(a[0])):
    for k in range(0,len(a[0][0])):
      arr_instance[i][j][k]=a[i][j][k]

The arr_instance is what you want.

edited Jun 25, 2014 at 10:02

quimnuss

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answered Oct 17, 2013 at 19:07

Thothadri Rajesh

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2 Comments

pc05 Over a year ago

Unfortunately I am looking for only one object. I have added extra information. Thanks anyway!

pc05 Over a year ago

That´s it! However eryksun's answer is more concise. Thx

Nicholas Masso · Accepted Answer · 2024-03-11 21:23:47Z

0

I wanted a solution to use for arbitrarily-dimensioned lists of lists. Here is a recursive method.

import ctypes

def list_into_ctype(l, ctype):
    currentlevel = []
    for i in l:
        if type(i) is list:
            currentlevel.append(list_into_ctype(i, ctype))
        else:
            currentlevel.append(ctype(i))
    
    result = (type(currentlevel[0]) * len(l))(*currentlevel)
    
    return(result)

running on the example:

list3d = [
    [[0.0, 1.0, 2.0, 3.0], [4.0, 5.0, 6.0, 7.0]], 
    [[0.2, 1.2, 2.2, 3.2], [4.2, 5.2, 6.2, 7.2]],
    [[0.4, 1.4, 2.4, 3.4], [4.4, 5.4, 6.4, 7.4]],
]

out = list_into_ctype(list3d, ctypes.c_double)

>>> out
<__main__.c_double_Array_4_Array_2_Array_3 object at 0x0000024C95AB0340>

edited Mar 11, 2024 at 21:23

answered Mar 11, 2024 at 19:08

Nicholas Masso

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