title: accessing specific intervals in list
body: <p>I have a list containing the system time from a machine. Since the list contains only the milliseconds part, the values don´t go beyond 1000.
  For better viewing I want to add i*1000 to certain intervals in this list for each time the list skips 1000. For better understanding i will give my input list and what my output list should look like:</p>
  
  <pre><code>inputlist = [300,600,900,200,500,800,100,400]
  </code></pre>
  
  <p>etc, the output list should look like this:</p>
  
  <pre><code>outputlist = [300,600,900,1200,1500,1800,2100,2400] 
  </code></pre>
  
  <p>since I want the list to start with zero i subtracted the first element of the list from each element giving me a new inputlist:</p>
  
  <pre><code>inputlist_new = [0,300,600,-100,200,500,-200,100]
  </code></pre>
  
  <p>which should give me a new outputlist like:</p>
  
  <pre><code>outputlist_new = [0,300,600,900,1200,1500,1800,2100]
  </code></pre>
  
  <p>I tried creating a list containing the indices of each element &lt; 0 to cut the list into intervals, to multiply the thousands on each interval but I am not able to do so. My code for this index list is this:</p>
  
  <pre><code>list_index = [i for i, j in enumerate(inputlist_new) if j &lt; 0]
  </code></pre>