title: AttributeError: 'numpy.ndarray' object has no attribute 'append'
body: <p>I am trying to run the code presented on the second page:</p>
  
  <p><a href="http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-00-introduction-to-computer-science-and-programming-fall-2008/video-lectures/lecture-20/lec20.pdf" rel="noreferrer">http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-00-introduction-to-computer-science-and-programming-fall-2008/video-lectures/lecture-20/lec20.pdf</a></p>
  
  <p>At the bottom of the code you have to add these lines:</p>
  
  <pre><code>simFlips(100,100)
  
  show()
  </code></pre>
  
  <p>Here is the error that I get when I run it on ubuntu:</p>
  
  <pre><code>Traceback (most recent call last):
    File "coin.py", line 36, in &lt;module&gt;
      simFlips(100,100)
    File "coin.py", line 16, in simFlips
      diffs.append(abs(heads - tails))
  AttributeError: 'numpy.ndarray' object has no attribute 'append'
  </code></pre>
  
  <p>Please tell me what I'm doing wrong that gives me the last error.  Thanks in advance!</p>