libnpy

libnpy is a simple C++ library for reading and writing of numpy's .npy files.

Refer to format.py for a detailed description of the .npy format.

This libraries primary purpose is writing numerical data easily and efficiently into the .npy format. It also allows reading .npy files, although only a very limited subset of data types are supported.

Features

• Writing C++ vectors (std::vector) to .npy files
• Reading (some) simple .npy files into C++ vectors

Supported data types and mapping

Only scalar numeric data types are supported. There is no natural way to represent more complex objects and parsing the header becomes tremendously more complex. Supported types:

• unsigned integer
• signed integer
• floating point
• complex floating point (std::complex, ...)

Usage

libnpy is a header only library. You only need to download npy.hpp into your include path. Neither special compiler flags nor a specific build system are required.

Optional: If you use meson, you can use the provided meson.build file to declare the dependency on libnpy.

The API has changed in the last release. The old C-style API is still available, but might get removed in the future.

Reading data:

#include "npy.hpp"
#include <vector>
#include <string>

int main() {
  
  const std::string path {"data.npy"};
  npy::npy_data d = npy::read_npy<double>(path);

  std::vector<double> data = d.data;
  std::vector<unsigned long> shape = d.shape;
  bool fortran_order = d.fortran_order;
}

Writing data:

#include "npy.hpp"
#include <vector>
#include <string>

int main() {
  const std::vector<double> data{1, 2, 3, 4, 5, 6};

  npy::npy_data d;
  d.data = data;
  d.shape = {2, 3};
  d.fortran_order = false; // optional

  const std::string path{"out.npy"};
  npy::write_npy(path, d);
}

This will involve an additional copy of the data, which might be undesireable for larger data. The copy can be avoided by using npy::npy_data_ptr as follows.

#include "npy.hpp"
#include <vector>
#include <string>

int main() {
  const std::vector<double> data{1, 2, 3, 4, 5, 6};

  npy::npy_data_ptr<double> d;
  d.data_ptr = data.data();
  d.shape = {2, 3};
  d.fortran_order = false; // optional

  const std::string path{"out.npy"};
  npy::write_npy(path, d);
}

See test/ for further examples. C++14 is required.

Tests

The tests can be build with meson>=0.55 and depend on catch2.

cd tests
meson setup builddir
meson test -Cbuilddir

Known limitations

1. Only a few data types are supported.

2. The numpy header is a literal Python dictionary and the Python syntax is very permissive. libnpy's parser was only tested with numpy's implemenation of the .npy format.

Contributing

Feel free to send me a pull request, open an issue, or contact me directly.

The code is formatted with clang-format. Please test your changes by running the tests and static analysis. Meson automatically builds a target for clang-tidy:

cd tests
meson setup builddir
ninja -C builddir clang-tidy

License

The project is licensed under the MIT license