title: How to access the elements of a 2D array?
body: <p>I would like to understand how one goes about manipulating the elements of a 2D array.</p>
  <p>If I have for example:</p>
  <pre><code>a= ( a11 a12 a13 )  and b = (b11 b12 b13) 
       a21 a22 a23             b21 b22 b23
  </code></pre>
  <p>I have defined them in python as for example:</p>
  <pre><code>a=[[1,1],[2,1],[3,1]]
  b=[[1,2],[2,2],[3,2]]
  </code></pre>
  <p>I saw that I cannot refer to <code>a[1][1]</code> but to <code>a[1]</code> which gives me a result of <code>[2,1]</code>.
  So, I don't understand how do I access the second row of these arrays? That would be <code>a21, a22, a23, b21, b22, b23</code>?
  And how would I do in order to multiply them as <code>c1 = a21*b21, c2 = a22*b22</code>, etc ?</p>