title: Why isn't arange defined?
body: <p>I have no programming experience and am learning Python for a class. I'm going through conditionals now and have a problem that I'm not understanding the cause of. </p>
  
  <p>I'm basically just trying to let someone plot x^2 within a desired interval of x with the restriction that x isn't negative. If the user inputs a negative input, they'll get a message saying it's not allowed. Otherwise, x vs. x^2 should be plotted.</p>
  
  <pre><code>N = input('Enter upper limit of x:');
  if N &lt; 0:
      print "Negative input isn't allowed."
  else:
      x = arange(N)
      y = x*x
      plot(x,y) 
  </code></pre>
  
  <p>The above gives me the error:</p>
  
  <pre><code>NameError: name 'arange' is not defined 
  </code></pre>
  
  <p>Which I don't understand because <code>arange()</code> is normally defined from my (very little) understanding. </p>