title: Modifying a list with list comprehension
body: <p>I want to modify a list using list comprehension. Something like this:</p>
  
  <pre><code>c = [10,20,30,40,50,60,70,80,90]
  a = [0,1,2]
  b = [3,4,5]
  
  c[a[jj]] = [c[a[jj]]+c[b[jj]] for jj in range(len(a))]
  </code></pre>
  
  <p>My expected output:</p>
  
  <pre><code>c = [50,70,90,40,50,60,70,80,90]
  </code></pre>
  
  <p>But every time the error appears:</p>
  
  <pre class="lang-none prettyprint-override"><code>    c[a[jj]] = [c[a[jj]]+c[b[jj]] for jj in range(len(a))]
  NameError: name 'jj' is not defined
  </code></pre>
  
  <p>What am I doing wrong here?</p>
  
  <p>Edit:
  I am trying to replicate the statement down below without using a for loop:</p>
  
  <pre><code>for jj in range(len(a)):
      c[a[jj]] = c[a[jj]] + c[b[jj]]
  </code></pre>