title: Append numbers to vectors with NumPy
body: <p>I want to store multiple forces at certain positions with NumPy.</p>
  
  <p>I need a vector with the positions <code>positions</code> and a vector with the forces <code>forces</code> at the certain positions. I am prompting the user to input these positions and forces with <code>input("Please input position: ")</code> and <code>input("Please input force: ")</code>.</p>
  
  <p>How can I append these inputted values to two separate vectors with NumPy? I've tried</p>
  
  <pre><code>import numpy as np
  
  # empty as default
  positions = None
  forces = None
  
  # append
  np.append(positions, int(input("Please input position: ")))
  np.append(forces, int(input("Please input position: ")))
  
  # print positions and forces
  print(positions)
  print(forces)
  </code></pre>
  
  <p>but the variables <code>positions</code> and <code>forces</code> stay empty.</p>