title: Randomly selecting rows from dataframe column
body: <p>For a given dataframe column, I would like to randomly select roughly 60% and add to a new column, add the remaining 40% to another column, multiply the 40% column by (-1), and create a new column that merges these back together like so:</p>
  
  <pre><code>dict0 = {'x1': [1,2,3,4,5,6]}
  data = pd.DataFrame(dict0)### 
  
  dict1 = {'x1': [1,2,3,4,5,6],'x2': [1,'nan',3,'nan',5,6],'x3': ['nan',2,'nan',4,'nan','nan']}
  data = pd.DataFrame(dict1)### 
  
  
  dict2 = {'x1': [1,2,3,4,5,6],'x2': [1,'nan',3,'nan',5,6],'x3': ['nan',-2,'nan',-4,'nan','nan']}
  data = pd.DataFrame(dict2)### 
  
  dict3 = {'x1': [1,2,3,4,5,6],'x2': [1,'nan',3,'nan',5,6],'x3': ['nan',-2,'nan',-   4,'nan','nan'],,'x4': [1,-2,3,-4,5,6]}
  data = pd.DataFrame(dict3)### 
  </code></pre>