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npy_matlab

Code to read and write NumPy's NPY format (.npy files) in MATLAB.

This is experimental code and still work in progress. For example, this code:

  • Only reads a subset of all possible NPY files, specifically N-D arrays of certain data types.
  • Only writes little endian, fortran (column-major) ordering
  • Only writes with NPY version number 1.0.
  • Always outputs a shape according to matlab's convention, e.g. (10, 1) rather than (10,).

Feel free to open an issue and/or send a pull request for improving the state of this project!

For the complete specification of the NPY format, see the NumPy documentation.

Installation

After downloading npy_matlab as a zip file or via git, just add the npy_matlab directory to your MATLAB path:

Usage example

>> import npy_matlab.*
>> a = rand(5,4,3);
>> writeNPY(a, 'a.npy');
>> b = readNPY('a.npy');
>> sum(a(:)==b(:))
ans =

    60

Tests

  • Run tests in MATLAB: see tests/test_readNPY.m
  • Run tests in Python: see tests/npy.ipynb & tests\test_npy_roundtrip.py

Memory mapping npy files

See examples/exampleMemmap.m for an example of how to memory map a .npy file in MATLAB, which is not trivial when the file uses C-ordering (i.e., row-major order) rather than Fortran-ordering (i.e., column-major ordering). MATLAB's memory mapping only supports Fortran-ordering, but Python's default is C-ordering so .npy files created with Python defaults are not straightforward to read in MATLAB.