title: Add digit of number
body: <p>I have to build a method to add every digit of a string given in parameter and so till there are only 1 digit left, e.g. <code>1234 = (1+2+3+4) = 10 = (1+0) = 1</code>.</p>
  
  <p>At first, I thought a recursive call or a while loop should be fine. But Is there a smarter way? Using the modulo perhaps?</p>
  
  <p><code>1234 % 9 = 1</code> That's seems to work... But no: <code>9%9</code> is not equal to <code>9</code> but it is to <code>0</code>.</p>
  
  <p>Is there a way to build this function without recursive/for/ while?</p>