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DOC: Add %in% operator into compare w r (GH3850) #5875

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DOC: Add %in% operator into compare w r (GH3850) #5875

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DOC: r match function
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Update comparison_with_r.rst
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41 changes: 41 additions & 0 deletions doc/source/comparison_with_r.rst
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Expand Up @@ -66,6 +66,44 @@ function.
For more details and examples see :ref:`the groupby documentation
<groupby.split>`.

|match|_
~~~~~~~~~~~~

A common way to select data in R is using ``%in%`` which is defined using the
function ``match``. The operator ``%in%`` is used to return a logical vector
indicating if there is a match or not:

.. code-block:: r

s <- 0:4
s %in% c(2,4)

The :meth:`~pandas.DataFrame.isin` method is similar to R ``%in%`` operator:

.. ipython:: python

s = pd.Series(np.arange(5),dtype=np.float32)
s.isin([2, 4])

The ``match`` function returns a vector of the positions of matches
of its first argument in its second:

.. code-block:: r

s <- 0:4
match(s, c(2,4))

The :meth:`~pandas.core.groupby.GroupBy.apply` method can be used to replicate
this:

.. ipython:: python

s = pd.Series(np.arange(5),dtype=np.float32)
s.apply(lambda x: [2, 4].index(x) if x in [2,4] else np.nan)
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There is actually a pd.match(s, [2, 4]) function which does exactly this. Although, I don't know to which extent it should be advertised, as it is nowhere in the docs and is maybe also not in the best shape? @y-p @jreback ?

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You're right. pd.match() ignores it's na_sentinal argument, but otherwise it's a 1:1 match.

The _hashtable_algo function it uses seems to be missing at least the int32 case, btw.

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hmm...looks kind of like Index.get_indexer (and the non_unique version). Never even knew this existed. Maybe should open an issue to see use case / doc or deprecate? I don't see it being used anywhere internally

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@chappers

replace this with

Series(pd.match(s,[2,4],np.nan)) (which now works), see #5943


For more details and examples see :ref:`the reshaping documentation
<indexing.basics.indexing_isin>`.

|tapply|_
~~~~~~~~~

Expand Down Expand Up @@ -372,6 +410,9 @@ For more details and examples see :ref:`the reshaping documentation
.. |aggregate| replace:: ``aggregate``
.. _aggregate: http://finzi.psych.upenn.edu/R/library/stats/html/aggregate.html

.. |match| replace:: ``match`` / ``%in%``
.. _match: http://finzi.psych.upenn.edu/R/library/base/html/match.html

.. |tapply| replace:: ``tapply``
.. _tapply: http://finzi.psych.upenn.edu/R/library/base/html/tapply.html

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