title: Where Column item is equal to specific value insert new item
body: <p>I am trying to insert a new <code>Column</code> based off values in a separate <code>Column</code>. For the <code>df</code> below I have a <code>Column</code> of days. Where the value in this <code>Column</code> is equal to <code>Monday</code>, I want to insert a <code>0</code> into a new <code>Column</code> called <code>Group</code>.</p>
  
  <p>Here is my attempt:</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  
  d = ({             
     'Day' : ['Monday','Tuesday','Wednesday','Thursday','Friday','Saturday','Sunday'],                                                                                                                                                 
      })
  
  df = pd.DataFrame(data=d)
  
  df['Group'] = np.where(df['Day'] == 'Monday', 0)
  print(df)
  </code></pre>
  
  <p>Error:</p>
  
  <blockquote>
    <p>ValueError: either both or neither of x and y should be given</p>
  </blockquote>