title: Taking logarithm of column
body: <p>Im quite new to programming (in python) and I would like to create a new variable that is the logarithm of a column (from an imported excel file). I have tried different solutions from this site, but I keep getting an error. My latest error is <code>AttributeError: 'str' object has no attribute 'log'.</code> 
  I have already dropped all the values that are not "numbers', but I still don't know how to convert the values from strings to integers (if this is the case, because 'int(neighborhood)' doesn't work).</p>
  
  <p>This is the code I have now:</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  
  df=pd.read_excel("kwb-2016_del_col_del_row.xls")
  df = df[df.m_woz != "."] # drop rows with values "."
  neighborhood=df[df.recs=="Neighborhood"]
  neighborhood=neighborhood["m_woz"]
  print(neighborhood)
  
  np.log(neighborhood)
  </code></pre>
  
  <p>and this is the error I'm getting:</p>
  
  <pre><code>AttributeError                            Traceback (most recent call last)
  &lt;ipython-input-66-46698de51811&gt; in &lt;module&gt;()
       12 print(neighborhood)
       13 
  ---&gt; 14 np.log(neighborhood)
  
  
  AttributeError: 'str' object has no attribute 'log'
  </code></pre>
  
  <p>Could someone help me please?</p>