title: Python two arrays, get all points within radius
body: <p>I have two arrays, lets say x and y that contain a few thousand datapoints. 
  Plotting a scatterplot gives a beautiful representation of them. Now I'd like to select all points within a certain radius. For example r=10</p>
  
  <p>I tried this, but it does not work, as it's not a grid.</p>
  
  <pre><code>x = [1,2,4,5,7,8,....]
  y = [-1,4,8,-1,11,17,....]
  RAdeccircle = x**2+y**2
  r = 10
  
  regstars = np.where(RAdeccircle &lt; r**2)
  </code></pre>
  
  <p>This is not the same as an nxn array, and <code>RAdeccircle = x**2+y**2</code> does not seem to work as it does not try all permutations. </p>