% function to read in a file and apply Caesar cipher
function   caesar(filename)
   fileID=fopen(filename);
   input_text=fread(fileID)';
   fclose(fileID);
   plain_text=input_text(0<= input_text <= 127);
   new_text = plain_text;
   find(new_text==13);
   char(plain_text)
    %  find(plain_text==13)
   for k=1:length(new_text)
       letter=new_text(k);
       if   (65 <= letter)&&(letter <= 90)
           new_text(k) = 65+mod(3+letter-65,26);
       elseif (97 <= letter)&&(letter <=122)
           new_text(k) = 97+mod(3+letter-97,26);
       end
   end % for k
   char(new_text)
   find(new_text==13)
return
