title: substring of an entire column in pandas dataframe
body: <p>I have a pandas dataframe "df". In this dataframe I have multiple columns, one of which I have to substring.
  Lets say the column name is "col".
  I can run a "for" loop like below and substring the column:</p>
  
  <pre><code>for i in range(0,len(df)):
    df.iloc[i].col = df.iloc[i].col[:9]
  </code></pre>
  
  <p>But I wanted to know, if there is an option where I don't have to use a "for" loop, and do it directly using an attribute.I have huge amount of data, and if I do this, the data will take a very long time process.</p>