Encoding ceaser using matlab

function Encoding_ceaser()
    msg='Enter the message to encode it :';
    plainText=input( msg , 's');
         for i =1 : length(plainText)
             if ( plainText(i)>="a" && plainText(i)<="z")
                  continue ;
             elseif (plainText(i) ~=" " )
                  disp('can not be encode this message ')
                  return ;
             end
         end

    k = input('Enter the value of K :');
    k = mod(k , 26 );

        for i =1 : length(plainText)
            if (plainText(i) ~=" " )
              plainText(i)= mod(plainText(i)-'a' + k  , 26) +'A';
            end
        end

  disp('the encoded message : ');
  disp( plainText);

end