Modify contents of HDF5 file but keep it readable by external programs

A FORTRAN/C++ program uses an input file with extension *.h5. I want to change the contents of one array in this h5 file to single precision and re-run the FORTRAN/C++ program. I've taken the following steps in MATLAB/Octave:

p=load('data.h5');
p.second.values=cast(p.second.values,'single');
save('data.h5','p');

The FORTRAN/C++ program crashes when looking for the data in the data.h5 produced by MATLAB in this way. I came to find that p=load('data.h5') does not store the data in p.second.values, but in p.p.second.values.

I just want the h5 file read by the FORTRAN/C++ program to be the exact same as before, except with second.values to be an array of single-precision numbers rather than double. Is there an easy way?

[Nike Dattani]

I never thought I'd say it was easier to obtain a python solution, but here it is:

    import h5py
    import numpy as np
    FILENAME = 'data.h5'
    f = h5py.File(FILENAME, 'a')
    f['/second/values'][:] = f['/second/values'].value.astype(np.float32)

The key is the letter 'a' on the 4th line, which means "append" the file, don't just read it. How shall I do it in MATLAB or OCTAVE?

[per isakson]

"an input file with extension *.h5" I assume that's a HDF5 file.

In Matlab, use h5read and h5write. See High-Level Functions, Easily view, read, and write HDF5 files

Or

save( 'data.h5', 'p', '-v7.3' );

but that creates a HDF5-file with a very special format, which would require a FORTRAN-program that recognizes this format.