title: Replace every nth letter in a string
body: <p>I'm writing a function to replace every n-th letter from a string </p>
  
  <pre><code>def replaceN(str, n):
     for i in range(len(str)):
       n=str[i]
       newStr=str.replace(n, "*")
       return newStr
  </code></pre>
  
  <p>but when i execute it, only the first letter is replaced with a *. I'm sure there is a mistake, but i can't see it. </p>