MATLAB: Creating a memory mapped file

MATLABmemmapfile

I am trying to create a new memory-mapped 'big data' file by using sequential writes of structured binary data, where I write a vector of values for each variable separately. First, can I start with an empty file that I have opened for write? I cannot seem to write an array into mapped location unless data already exists at that location. See below
%%define the  memory map for the output binary file
  OutFile = 'testOutFile';
  fid = fopen('test.lls','w');
  mmLLS = memmapfile('test.lls',...
    'Format',{          ...
    'single' 1 'lat';   ...
    'single' 1 'lon'},...
    'Repeat',1,'Writable',true);
Lat = single(32.3);
Lon = single(-110.2);
mmLLS.Data.lat = Lat;
Error using memmapfile/subsasgn (line 732)
A subscripting operation on the Data field attempted to create a
comma-separated list. The memmapfile class does not support the use
of comma-separated lists when subscripting.

Best Answer

http://www.mathworks.com/help/matlab/ref/memmapfile.html
"memmapfile does not expand or append to a mapped file. Use instead standard file I/O functions like fopen and fwrite."
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