Matlab(tm) 7.3 file format is actually hdf5 and can be read from other languages like python

Posted by Soeren Sonnenburg on November 19, 2009

It looks like that matlab version 7.3 and later are capable of writing out objects in the so called matlab 7.3 file format. While at first glance it looks like another proprietary format - it seems to be in fact the Hierarchical Data Format version 5 or in short hdf5.

So you can do all sorts of neat things:

1. Lets create some matrix in matlab first and save it:

   >> x=[[1,2,3];[4,5,6];[7,8,9]]
   
   x =
   
    1     2     3
    4     5     6
    7     8     9
   
   >> save -v7.3 x.mat x
   

2. Lets investigate that file from the shell:

   $ h5ls x.mat 
   x                        Dataset {3, 3}
   
   $ h5dump x.mat 
   HDF5 "x.mat" {
    GROUP "/" {
     DATASET "x" {
       DATATYPE  H5T_IEEE_F64LE
       DATASPACE  SIMPLE { ( 3, 3 ) / ( 3, 3 ) }
       DATA {
       (0,0): 1, 4, 7,
       (1,0): 2, 5, 8,
       (2,0): 3, 6, 9
       }
       ATTRIBUTE "MATLAB_class" {
          DATATYPE  H5T_STRING {
                STRSIZE 6;
                STRPAD H5T_STR_NULLTERM;
                CSET H5T_CSET_ASCII;
                CTYPE H5T_C_S1;
             }
          DATASPACE  SCALAR
          DATA {
          (0): "double"
          }
        }
      }
     }
   }
   

3. And load it from python:

   >>> import h5py
   >>> import numpy
   
   >>> f = h5py.File('x.mat')
   >>> x=f["x"]
   >>> x
   <HDF5 dataset "x": shape (3, 3), type "<f8">
   
   >>> numpy.array(x)
   array([[ 1.,  4.,  7.],
      [ 2.,  5.,  8.],
      [ 3.,  6.,  9.]])
   

So it seems actually to be a good idea to use matlab's 7.3 format for interoperability.