title: Why do I get "ufunc 'multiply' did not contain a loop with signature matching types dtype('S32') dtype('S32') dtype('S32')" with values from raw_input
body: <p>I am trying to create a really simple program that will plot a plot a parabola where <code>v</code> is velocity, <code>a</code> is acceleration and <code>x</code> is time. The user will input values for <code>v</code> and <code>a</code>, then <code>v</code> and <code>a</code> and <code>x</code> will determine <code>y</code>.</p>
  <p>I attempted to do this with this:</p>
  <pre><code>x = np.linspace(0., 9., 10)
  a = raw_input('Acceleration =')
  v = raw_input('Velocity = ')
  y = v * x - 0.5 * a * x**2.
  </code></pre>
  <p>But, I keep getting this error:</p>
  <blockquote>
  <p>TypeError: ufunc 'multiply' did not contain a loop with signature matching types dtype('S32') dtype('S32') dtype('S32')</p>
  </blockquote>
  <p>What does this mean?</p>