Embedding Python into C++ calling two functions and passing array around

Question

I'm trying to embedding python functions into my c++ code for doing some symbolic and algebraic operations.

My idea: I want to create a matrix with symbolic variables depending on some input variables. This matrix I want to use afterwards in another function. This function is generating some coordinates out of the symbolic matrix. And i want then to pass this results to my c++ code back.

My problem: I have a problem to get this matrix out of python to my c++ code and back to the other function as input. Because i want to pass the symbolic matrix via c++ to the other python function which should generate out of this matrix the results. Just to try my python code and the integration into c++ i wrote the code below.

Edit: To make my question more specific I wrote the whole work I want to do with this functions in Python. To clarify I need one function which is generating the symbolic matrix and the other has to calculate the values. And for the calculation the values are coming from the inside of my C++ code.

Here the example written in Python:

from sympy import *
import numpy as np


def main():
    start_pos = [1,2,3]
    end_pos = [4,5,6]

    generic_function = generate_symbolic_transformation(start_pos,end_pos)
    calculate_symbolic_transformation(generic_function,90,0.5);

# Calculation of the symbolic transformation depending on the input
def calculate_symbolic_transformation(generic_function,thetay_value,k_value):
    thetay = symbols('thetay')
    k = symbols('k')
    transf_matrix = MatrixSymbol('tm',4,4)
    transf_matrix = Matrix(transf_matrix)
    transf_matrix = sympify(generic_function)
    transf_matrix = transf_matrix.subs([(thetay,thetay_value),(k,k_value)])
    print transf_matrix
    return 1

# Generation of the symbolic transformation depending on the input
def generate_symbolic_transformation(Start_pos_coords,End_pos_coords):

    # Symbolic startposition
    Start_pos = MatrixSymbol('S',3,1)
    Start_pos = Matrix(Start_pos)
    Start_pos[0] = Start_pos_coords[0]
    Start_pos[1] = Start_pos_coords[1]
    Start_pos[2] = Start_pos_coords[2]

    print Start_pos

    # Symbolic endposition
    End_pos = MatrixSymbol('E',3,1)
    End_pos = Matrix(End_pos)
    End_pos[0] = End_pos_coords[0]
    End_pos[1] = End_pos_coords[1]
    End_pos[2] = End_pos_coords[2]

    print End_pos

    # Symbolic rotation matric
    R = MatrixSymbol('R',3,3)

    # Symbolic transformation matric
    T = MatrixSymbol('T',4,4)

    # Necessary symbolic variabls
    k = symbols('k')
    thetax = symbols('thetax')
    thetay = symbols('thetay')
    thetaz = symbols('thetaz')

    # For rotation of EulerAngles RzRyRx:
    Rx = MatrixSymbol('Rx',3,3)
    Ry = MatrixSymbol('Ry',3,3)
    Rz = MatrixSymbol('Rz',3,3)

    # Filling Rx rotation matric
    #   |   1           0                   0         |
    #   |   0       -cos(thetax)      sin(thetax)     |
    #   |   0        sin(thetax)      cos(thetax)     |

    Rx = Matrix(Rx)
    Rx[0,0] = 1
    Rx[0,1] = 0
    Rx[0,2] = 0
    Rx[1,0] = 0
    Rx[1,1] = cos(thetax)
    Rx[1,2] = -sin(thetax)
    Rx[2,0] = 0
    Rx[2,1] = sin(thetax)
    Rx[2,2] = cos(thetax)

    # Filling Ry rotation matric
    #   |    cos(thetay)        0      sin(thetay)     |
    #   |          0            1           0          |
    #   |   -sin(thetay)        0      cos(thetay)     |

    Ry = Matrix(Ry)
    Ry[0,0] = cos(thetay)
    Ry[0,1] = 0
    Ry[0,2] = sin(thetay)
    Ry[1,0] = 0
    Ry[1,1] = 1
    Ry[1,2] = 0
    Ry[2,0] = -sin(thetay)
    Ry[2,1] = 0
    Ry[2,2] = cos(thetay)

    # Filling Rz rotation matric
    #   |    cos(thetaz)   -sin(thetaz)      0     |
    #   |    sin(thetaz)    cos(thetaz)      0     |
    #   |          0            0            1     |

    Rz = Matrix(Rz)
    Rz[0,0] = cos(thetaz)
    Rz[0,1] = -sin(thetaz)
    Rz[0,2] = 0
    Rz[1,0] = sin(thetaz)
    Rz[1,1] = cos(thetaz)
    Rz[1,2] = 0
    Rz[2,0] = 0
    Rz[2,1] = 0
    Rz[2,2] = 1

    # Generating the rotation matric
    R = Rz*Ry*Rx

    # Generating the linear translation
    # Symbolic 3D line function
    Translation = MatrixSymbol('Tl',3,1)
    Translation = Start_pos + k * (End_pos-Start_pos)

    # Integrate it into the transformation matric
    #   |    R      T    |
    #   |   000     1    |
    T = Matrix(T)

    i=0
    for r in range(4):
        for c in range(4):
            if (c < 3 and r < 3):
                T[r,c] = R[r,c]
            elif (c == 3 and r < 3):
                T[r,c] = Translation[i]
                ++i
            elif (c < 3 and r == 3):
                T[r,c] = 0
            else:
                T[r,c] = 1

    ## Save the created matrics with symbolic variables into global object
    T = T.subs([(thetax,0),(thetaz,0)])
    return T

if __name__ == "__main__":
    main()

You'll have to extract a minimal example. Please also fix the spelling, it's C++ instead of c++, Python instead of python (unless referring to the animal) and I instead i (when referring to yourself). Oh, and please search the web for the meaning of that error you receive, your question is not new at all! — Ulrich Eckhardt
– Ulrich Eckhardt, Commented Jul 27, 2015 at 7:56
Sorry for my bad spelling. If my question is not new, then i'm too dumb to find the right answer. I only know that there is some problem with the variable tmatrix I want to put back into my Python function. And if I comment out the "print tmatrix" everything is working fine! I'm trying to get this running since one week if searched the web and didn't get any answer! — Noxious
– Noxious, Commented Jul 27, 2015 at 8:21
Can you please tell me what my problem is?I'm searching the whole web since more than a week and i don't get any progress. It's not helping to say my question is not new! — Noxious
– Noxious, Commented Jul 29, 2015 at 9:38
You need to reduce the amount of code that causes problems. It doesn't matter if the code doesn't do what it's actually supposed to do, as long as it demonstrates the issue. Post that minimal example code here, so that people can reproduce it. Chances are that you find your own mistake on the way. Further, make sure you know what "Access Violation" means. Lastly, it doesn't help that you switched the content of your question around. If one problem is solved and others come up, as a new question. — Ulrich Eckhardt
– Ulrich Eckhardt, Commented Jul 29, 2015 at 20:59

sterin · Accepted Answer · 2015-07-27 09:48:02Z

1

There are several errors here:

The Python function generate_symbolic_transformation throws an exception, which makes resultObj be NULL. This propagates further and leads to the crash.
Even if resultObj was not NULL, it would not have been returned properly to its caller, as the last two lines of CallPlugIn_generate_symbolic_transformation make sure that a value is only returned it it is NULL.

But these are specific problems. I would also make a few general suggestion that might help you catch problems early, saving time and effort:

Handle Python errors. At the bare minimum, if a Python C/API function returns NULL, print the error. For example:

if (!resultObj) { PyErr_Print(); }

This would have resulted in an error message similar to:

Traceback (most recent call last): File "/home/sterin/ClionProjects/numpy1/numpy_test.py", line 15, in generate_symbolic_transformation T = T.subs([(thetax,0),(thetaz,0)]) NameError: global name 'thetax' is not defined

Helping you to catch the error much earlier.

Make sure no NULL values are provided as PyObject* arguments to Python C/API calls (unless specifically allowed). This would have caught the place where the NULL result have been used.

answered Jul 27, 2015 at 9:48

sterin

1,96816 silver badges13 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Noxious Over a year ago

Sry I switched my code just to simplify it. But I changed back to the first one, there i don't have the problem that there is nothing not defined. Maybe you could check it again. But I really don't know where I'm doing something wrong maybe I have to create the right PyObject to send it back to the other Python function?

Collectives™ on Stack Overflow

Embedding Python into C++ calling two functions and passing array around

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related