title: Filling a 2D matrix in numpy using a for loop
body: <p>I'm a Matlab user trying to switch to Python.</p>
  
  <p>Using Numpy, how do I fill in a matrix inside a <code>for</code> loop?</p>
  
  <p>For example, the matrix has 2 columns, and each iteration of the <code>for</code> loop adds a new row of data.</p>
  
  <p>In Matlab, this would be:</p>
  
  <pre><code>n = 100;
  matrix = nan(n,2); % Pre-allocate matrix
  for i = 1:n
      matrix(i,:) = [3*i, i^2];
  end
  </code></pre>