title: zoo object aggregation
body: <p>Dear Community,                     </p>
  
  <p>the data I receive will be in a data frame:                         </p>
  
  <pre><code>Var_1      Var_2         Date        VaR_3  VaR_4   VaR_5   Var_6
  1           4       2010-01-18         7    apple    10    sweet
  2           5       2010-07-19         8    orange   11    sour
  3           6       2010-01-18         9    kiwi     12    juicy
  ...        ...      ...               ...   ...     ...    ... 
  </code></pre>
  
  <p>I would like to use zoo, since it seems to be a flexible object class. I'm just starting with R and I tried to read the description (vignettes) for the package.                        </p>
  
  <p>Questions:</p>
  
  <ol>
  <li>Given the above data as a data frame, which method is recommended to convert the complete df into a zoo object, telling zoo that it shall use the third column as date column (dates can occur multiple times in the data)?                      </li>
  <li>How do I aggregate all other columns monthly, except columns 4 and 6 using zoo built-in functions? Is zoo able to automatically discard categorical variables and just use those columns that are suited for aggregation?                        </li>
  <li>How do I aggregate all numeric columns monthly, for each category in column 4 (column 6 shall not be included, since it is non-numeric).</li>
  </ol>
  
  <p>Thanks for your support.</p>