title: How to randomly change n characters in a string
body: <p>I am wondering how to randomly change <code>n</code> characters in a string, e.g.</p>
  
  <pre><code>orig = 'hello'
  
  mod = 'halle'
  </code></pre>
  
  <p>that I want to randomly select two positions (<code>orig[1]</code> and <code>orig[4]</code>) in the string, and replace the chars in the positions of the original string (<code>hello</code>) with randomly selected chars (<code>a</code> and <code>e</code> here), results in a new string <code>halle</code>.</p>