title: Replacing values in a pandas dataframe based on multiple conditions
body: <p>I have a fairly simple question based on this sample code:</p>
  
  <pre><code>x1 = 10*np.random.randn(10,3)
  df1 = pd.DataFrame(x1)
  </code></pre>
  
  <p>I am looking for a single DataFrame derived from <code>df1</code> where positive values are replaced with <code>"up"</code>, negative values are replaced with <code>"down"</code>, and <code>0</code> values, if any, are replaced with <code>"zero"</code>. I have tried using the <code>.where()</code> and <code>.mask()</code> methods but could not obtain the desired result.</p>
  
  <p>I have seen other posts which filter according to multiple conditions at once, but they do not show how to replace values according to different conditions.</p>