title: Find the indices of elements greater than x
body: <p>Given the following vector, </p>
  
  <pre><code>a = [1, 2, 3, 4, 5, 6, 7, 8, 9]
  </code></pre>
  
  <p>I need to identify the indices of "a" whose elements are >= than 4, like this: </p>
  
  <pre><code>idx = [3, 4, 5, 6, 7, 8] 
  </code></pre>
  
  <p>The info in "idx" will be used to delete the elements from another list X (X has the same number of elements that "a"):</p>
  
  <pre><code>del X[idx] #idx is used to delete these elements in X. But so far isn't working.
  </code></pre>
  
  <p>I heard that numpy might help. Any ideas?
  Thanks!</p>