function result = convertMorseAthruF(inputString)
%convertMorseAthruF Convert letters A through F to Morse Code
%
% consumes: inputString, a string of characters (only spaces and A-F)
% produces: result, a string in morse code
%
% notes: 
%   The output string will have . for dits,
%   --- for a dash (with a duration of three dits)
%   and spaces for a silent period equal to one dit.
%
% Examples:
%      >> convertMorseAthruF('error');
%      Error: all chars in input string must be A-F
%      >> convertMorseAthruF('FEED');
%      ans =
%            '. . --- .   .   .   --- . .'
%      >> convertMorseAthruF('FEED ABBA BEEF');
%      ans =  
%           ['. . --- .   .   .   --- . .       ' ...
%            '. ---   --- . . .   --- . . .   . ---       ' ...
%            '--- . . .   .   .   . . --- .']
%      >> 
%

  % the vector "letters" is a vector of the legal characters.
  % the space character is treated as the inter-word-space.
  
    letters = 'ABCDEF ';
  
  % set up table of letters A through F, and inter-word-space
    
  morse{1} = '. ---';       % A
  morse{2} = '--- . . .';   % B
  morse{3} = '--- . --- .'; % C
  morse{4} = '--- . .';     % D
  morse{5} = '.';           % E
  morse{6} = '. . --- .';   % F
  morse{7} = '       ';     % inter-word-space
  
  % loop through inputString.
  % for each character, find its position in the letters string.
  % add the corresponding element of the cell array morse to the
  % result.
  % if the character is NOT in the letters string, throw an error
  
  result = '';
  
  for k=1:length(inputString)
    position = strfind(letters,inputString(k));
    if (length(position) == 0)
      error(['Character "' inputString(k) '" in position ' num2str(k) ...
	     ' is not permitted']);
    else
      
      % if this isn't the first character, and the previous 
      % character wasn't a space, and this character isn't a space
      % then add the space-between-chars
      
      if (k>1 && inputString(k-1)~=' ' && inputString(k)~=' ')
	result = [ result '   ' ];
      end % if 
      
      % add this character into the string
      result = [ result morse{position} ];
    end % if
  end % for

  return;
end