title: Invert negative values in a list
body: <p>I am trying to convert a list that contains negative values, to a list of non-negative values; inverting the negative ones. I have tried <code>abs</code> but it didn't work.</p>
  
  <p>My input is</p>
  
  <pre><code>x = [10,9,8,7,6,5,4,3,2,1,0,-1,-2,-3,-4,-5,-6,-7,-8,-9,-10]
  </code></pre>
  
  <p>How can I make it into this format as I am trying calculate the area</p>
  
  <pre><code>x = [10,9,8,7,6,5,4,3,2,1,0,1,2,3,4,5,6,7,8,9,10]
  </code></pre>