title: What does .shape[] do in "for i in range(Y.shape[0])"?
body: <p>I'm trying to break down a program line by line. <code>Y</code> is a matrix of data but I can't find any concrete data on what <code>.shape[0]</code> does exactly.</p>
  
  <pre><code>for i in range(Y.shape[0]):
      if Y[i] == -1:
  </code></pre>
  
  <p>This program uses numpy, scipy, matplotlib.pyplot, and cvxopt.</p>