title: Convert a column in pandas dataframe from String to Float
body: <p>I've already read about various solutions, and tried the solution stated here: <a href="https://stackoverflow.com/questions/18434208/pandas-converting-to-numeric-creating-nans-when-necessary">Pandas: Converting to numeric, creating NaNs when necessary</a></p>
  
  <p>But it didn't really solve my problem:
  I have a dataframe contains multiple columns, in where a column <code>['PricePerSeat_Outdoor']</code> contains some float values, some empty values, and some <code>'-'</code></p>
  
  <pre><code>    print type(df_raw['PricePerSeat_Outdoor'][99])
      print df_raw['PricePerSeat_Outdoor'][95:101]
      df_raw['PricePerSeat_Outdoor'] = df_raw['PricePerSeat_Outdoor'].apply(pd.to_numeric, errors='coerce')
      print type(df_raw['PricePerSeat_Outdoor'][99]) 
  </code></pre>
  
  <p>Then I got:</p>
  
  <pre><code>&lt;type 'str'&gt;
  95     17.21
  96     17.24
  97         -
  98         -
  99      17.2
  100    17.24
  Name: PricePerSeat_Outdoor, dtype: object
  &lt;type 'str'&gt;
  </code></pre>
  
  <p>Values at row #98 and 99 didn't get converted. Again, I've already tried multiple methods including following but it just didn't work. Much appreciated if someone can give me some hints.</p>
  
  <p><code>df_raw['PricePerSeat_Outdoor'] = df_raw['PricePerSeat_Outdoor'].apply(pd.to_numeric, errors='coerce')</code></p>
  
  <p>Also, how can I convert multiple columns to numeric at once? Thanks.</p>