Edit every Nth item in a list
<p>I want to perform an arithmetic operation (e.g. doubling the value) on a list of integers, every n places.</p>

<p>For example, given the list <code>[1,2,3,4,5,6,7]</code>, I want to double values every three places. In that case, we would have <code>[1,2,6,4,5,12,7]</code>.</p>

<p>How can I do it? </p>