Writing binary in c++ and read in python

Question

I would like to store a series of numbers to a binary file using c++, to open later using python. Currently I have the following script in c++ named *writer.cpp:

#include <fstream>

int main()
{
  std::ofstream outFile;
  outFile.open("binaryFile.bin", std::ios::binary);
  int number = 0;
  for (int i=0; i<10; i++){
    number += i;
    outFile.write(reinterpret_cast<const char*>(&number), sizeof(int));
  }
  outFile.close();

  return 0;
}

Which when compiling as

g++ -o writerTest.x writer.cpp

and run as

./writerTest.x

produce a binary file named "binaryFile.bin".

I then try to read this using python with the following script named reader.py:

import numpy as np
from sys import argv

path = str(argv[1])
x = np.fromfile(path)
print x, type(x)

Running this as python reader.py binaryFile.bin produce the following result

[  2.12199579e-314   1.27319747e-313   3.18299369e-313   5.94158822e-313
9.54898106e-313] <type 'numpy.ndarray'>

Which obviously is not what I was hoping for. What am I doing wrong, and how should this be done properly?

Answer 1

You have to specify the type of the values you're going to be reading, numpy has no way to guess that as there is no metadata stored in the file itself. So in your case you have to do something like:

x = np.fromfile(path, dtype=int)

If you're doing things like this, it's highly recommended to use fixed-size integers instead of just int, e.g. in C++ you can use int32_t from <cstdint> and in Python you can specify int32 as dtype.

Answer 2

fromfile assumes floating point numbers by default. If you want to change this behaviour, you need to pass in the type in the dtype named parameter.

As you're writing ints, this should work:

x = np.fromfile(path, dtype=int)