function write_file(filename)
%
% Create a file on disk, and then ask the user for data.
%   Copy everything the user types into the file.
%   Quit when the user types the Return key (an empty line),
%   signalling no more data.
%   
% Arguments:
%      
%         filename          (input)  name of disk file to create
%                   

  % first, we try to open the file for writing (which will
  %   create it if it doesn't exist yet)
  fid = fopen(filename, 'w');
  if (fid == -1) 
    error('write_file: cannot open file for writing');
  end

  done_yet = 0;
  while (done_yet == 0)

    % we use the optional second argument 's' to the INPUT function,
    %   which means that everything the user types is kept together
    %   as a string, instead of being broken up into an array of values.
    user_string = input('gimme stuff for the file ','s');

    % if the size of the user's string is zero, 
    %    then the user didn't type anything, and we should quit
    input_size = size(user_string);
    if (input_size(1) == 0) 
      done_yet = 1;
    else
      fprintf(fid, '%s\n', user_string);
    end
    
  end

  % remember to close the file before we return;
  %   otherwise, some of the material might not be written onto disk
  fclose(fid);