parsing arguments from python to c++

Question

I wanted to know how could I change the values of arguments in a C++/C program from python. In the program below I use the Newton-Raphson method to compute the roots of a function. What I want is to change the value of the variable xn (for the initial guess) to a certain value given in a python interface. This code is very simple but I am trying a bigger code where I need to change the values of some variables and I dont want to compile the code each time I change the values of these variables.

I can write an input file in order to avoid compiling the code again but what I want is
controlling the code even more, for instance, stop the program at a certain time step and change the values of the variables and the continue the execution. Could you tell me if there is a straightforward way to do that?

Regards.

#include <iostream>                                                                                                               
using namespace std;                                                                                                                

double first_derivative(double x)
{
    double dummy;
// Parabolic Function
    dummy = 2.0 * x + 7.0;
    return (dummy);
}

double second_derivative(double x)
{
    double dummy;
//Parabolic Function
    dummy = 2.0 ;
    return(dummy);
}


int main ()                                                                                                                         
{                                                                                                                                   
    double xn,fd,sd;
    int i,n;                                                                                                                        
        i=0;    
        xn=4.0 ; 

    cout << i << "    " << xn << "\n" ;

    for(i=0; i<10 ; i++) {
        fd =  first_derivative(xn);
        sd =  second_derivative(xn);
        xn = xn - fd/sd ;
        cout << i << "    " << fd << "    " << sd << "     " << xn << "\n" ;
    }
    return 0;
}

It's unclear what you're asking. Do you want to simply execute the program from the command line via Python? If so, google "argv" and "python os.system" — Chriszuma
– Chriszuma, Commented Oct 6, 2011 at 20:53
I want to get the values for the variables from a python interface and then run the program written in C/C++. The problem of using just C/C++ is that I must compile each time variables are changed. — armando
– armando, Commented Oct 6, 2011 at 20:59

Adam · Accepted Answer · 2011-10-06 20:52:44Z

1

If you want it to act like Python then it's best to just write it in Python.

You cannot just "change the numbers" of a compiled program. What you CAN do, however, is make the program accept parameters that you then change at runtime. You can do this in C++ with command-line parameters, configuration files, or asking for input. All are easier than involving Python.

If for some reason parts of this code have to be in C++ and you really want Python, then you can expose the C++ routines as a library to Python. One popular approach to do this is SWIG. What you could then do is to make functions that you'd call from Python which would accept your changeable parameters. Your Python script would simply call them.

answered Oct 6, 2011 at 20:52

Adam

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4 Comments

armando Over a year ago

I have seen a code from the internet where the guys parse the values of some variables from a Tcl interface to a C code. They wrote a script with the values of the variables and the functions used in the simulations. The problem with Tcl is that it is very slow. I want to perform a molecular dynamics simulation, set up the values for number of particles, temperature, external field, mass, etc. And then change some of these values at some point and continue with the simulation.

Chriszuma Over a year ago

This is what I was trying to say. You can easily set up your C++ program to accept command-line inputs. Just google for C++ command line inputs

Adam Over a year ago

@armando, You can do that with Python (and I'm currently involved in a project that does something similar), but interfacing these two languages is not something that you can do in 5 minutes. The easiest solution would be to have those parameters in a config file which the sim checks once in a while. Or you can write it with Python (and numpy/scipy). If speed is an issue try pypy or Cython.

Adam Over a year ago

@armando . But if you want something like what those guys did with TCL, then set up a C++ library that does the sim and accepts arguments then wrap it with SWIG or Boost.Python and call it from Python.

Dave Hillier · Accepted Answer · 2011-10-06 21:18:21Z

1

One approach is to use Boost Python:

Boost.Python, a C++ library which enables seamless interoperability between C++ and the Python programming language.

Sounds like it fits you requirements.

Alternatively, as your C++ program does not accept any arguments. You could change the your main function to accept arguments.

int main(int argc, char *argv[])

argc contains the number of arguments and argv is pointer to the argument strings. You can parse it yourself or use something like Boost.program_options.

This program can then be invoked from python using Python subprocess.

answered Oct 6, 2011 at 21:18

Dave Hillier

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Comments

Brian McFarland · Accepted Answer · 2011-10-06 20:53:06Z

0

I would assume this is for testing? Since that's the only sane reason to "stop the program at a certain time step and change the values of the variables and the continue the execution".

If so, this could be close to what you're looking for: http://sourceware.org/gdb/wiki/PythonGdb. Of course that only works if you're using gdb.

answered Oct 6, 2011 at 20:53

Brian McFarland

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1 Comment

armando Over a year ago

well I want to do a molecular dynamics simulation, one starts with some configuration for the system and let the simulation run for a certain time. Then, I would like to stop the simulation and change some parameters like temperature or external field and let the simulation continue. I can do that in other ways but I just wanted to know if it is possible from a python interface.

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