.. _groupby:

Group by: split-apply-combine
#############################

..
    Go through and add the nuances (i.e. currently doesn't work with strings as data type, mention using filtering for mean, etc.)

Using the :meth:`.CDataFrame.groupby` functionality, we can partition the rows of a table based on the value of one of the columns, and perform subsequent operations on the resulting parts.
The grouping functionality is similar to the Pandas library, and it is strongly related to the SQL ``GROUP BY`` clause.
The process involves splitting the data into groups, applying an aggregation function to each group, and then combining the results.

.. note::

   Currently, only *integer* columns can be the base of a grouping operation. See :ref:`tips` to see how to work with this when you are working with string columns.

To group data in crandas, you can use the :meth:`.CDataFrame.groupby` method. This will create a :class:`crandas.groupby` object, which we call a *grouping*. This object determines the way the original table must be transformed to group based on a column.

Aggregation operations
------------------------

Here is a list of aggregation operations that can be performed on grouped data, through a :class:`crandas.groupby` object.

- :data:`.crandas.groupby.sum`: Compute the sum of each group.
- :data:`.crandas.groupby.max`: Find the maximum value of each group.
- :data:`.crandas.groupby.min`: Find the minimum value of each group.

Below is an example that demonstrates how to use these operations with crandas:

.. code:: python


    import numpy as np
    import pandas as pd

    import crandas as cd

    # Create a dataframe using pandas
    ipl_data = {"Team": ["Riders", "Riders", "Devils", "Devils", "Kings", "kings", "Kings", "Kings", "Riders", "Royals", "Royals", "Riders"],
    "Rank": [1, 2, 2, 3, 3, 4, 1, 1, 2, 4, 1, 2],
    "Year": [2014, 2015, 2014, 2015, 2014, 2015, 2016, 2017, 2016, 2014, 2015, 2017],
    "Points": [876, 789, 863, 673, 741, 812, 756, 788, 694, 701, 804, 690]}

    df = pd.DataFrame(ipl_data)

After creating the dataframe, we are able to use :func:`.crandas.upload_pandas_dataframe` and then group by ``Year`` to execute different operations. 

.. code:: python 
    
    # Upload the pandas dataframe to the VDL (CDataFrame), then group by year and 
    # perform aggregation operations
    cdf = cd.upload_pandas_dataframe(df)

    # This is our grouping object
    cdgrouped = cdf.groupby("Year")
    cdsum = cdgrouped["Points"].agg(cd.groupby.sum)
    cdmax = cdgrouped["Points"].agg(cd.groupby.max)
    cdmin = cdgrouped["Points"].agg(cd.groupby.min)

.. note:: 
    
    For reference, this is slightly different to how one would use the ``GroupBy`` method in pandas. The difference can be seen below:
    
    .. code:: python
       
        df = pd.DataFrame(ipl_data)

        pdgrouped = df.groupby("Year")
        pdsum = pdgrouped["Points"].agg(np.sum)
        pdmax = pdgrouped["Points"].agg(np.max)
        pdmin = pdgrouped["Points"].agg(np.min)

We can also look at the size of each group that we create using the :meth:`.CDataFrame.groupby` method. 
Below we create a pandas dataframe which we upload to the VDL. We then group on column ``a`` (leading to 3 groups - 1, 2 and 3).
We can then use the :meth:`.CDataFrameGroupBy.size` to compute the size of each group. 

.. code:: python

    pa = pd.DataFrame({"a": [1, 1, 1, 2, 2, 2, 2, 3, 3]})
    ca = cd.upload_pandas_dataframe(pa)
    cgrouping = ca.groupby("a")
    csizes = cgrouping.size()

>>> csizes
         a 
    1    3
    2    4
    3    2

Many-to-One Joins
-------------------
One of the primary uses of the :class:`crandas.groupby` objects in crandas is to allow :ref:`many-to-one joins<left join>`. That is, joining two tables on a column whose values are repeated in the first table and unique in the second. Grouping the first table allows us to connect both tables, which is why in that case, we input the grouping instead of a column as a key.

Reusing a grouping
-------------------

.. versionadded:: 1.5

    Re-using a grouping.
    
Occassionally, after performing a grouping you will modify the table that such a grouping belonged to, e.g. computing new columns or performing a join. After doing that, the old :class:`crandas.groupby` will no longer work.

.. code:: python

    tab = cd.DataFrame({"a": [1, 2, 2, 4, 5, 5]}, auto_bounds=True)
    new_data = cd.DataFrame({"a": [2, 3, 4, 5], "b": [6, 7, 8, 9]}, auto_bounds=True)

    # make grouping
    grouping = tab.groupby("a")

    # edit table tab
    tab = cd.merge(tab, new_data, how="left", left_on=grouping, right_on="a")

    # this does not work, since the grouping was made for the old tab
    grouping["b"].agg(cd.groupby.sum)

However, instead of fully recomputing the grouping, which might take time. We can simply convert the previous one:

.. code:: python

    new_grouping = tab.groupby(grouping)

    # Now this will work
    new_grouping["b"].agg(cd.groupby.sum)

While the steps to create this new grouping might seem the same, doing it this way is more efficient than computing it from scratch.

For this strategy to work, it is important that the order of the rows in the original table is not modified. If all of therows are not in their original place, we would need to create a new grouping from the table.

.. code:: python

    tab =  tab.shuffle()

    # This will not work
    newer_grouping = tab.groupby(new_grouping)

    # But this will
    newest_grouping = tab.groupby("a")