Map Filter and Reduce
=====================

No real consensus about what that means.

But there are some "classic" methods available in Python.

map
---

``map``  "maps" a function onto a sequence of objects -- It applies the function to each item in the list, returning another list


.. code-block:: ipython

    In [23]: l = [2, 5, 7, 12, 6, 4]
    In [24]: def fun(x):
                 return x*2 + 10
    In [25]: map(fun, l)
    Out[25]: [14, 20, 24, 34, 22, 18]


But if it's a small function, and you only need it once:

.. code-block:: ipython

    In [26]: map(lambda x: x*2 + 10, l)
    Out[26]: [14, 20, 24, 34, 22, 18]

filter
------

``filter``  "filters" a sequence of objects with a boolean function --
It keeps only those for which the function is True -- filtering our the rest.

To get only the even numbers:

.. code-block:: ipython

    In [27]: l = [2, 5, 7, 12, 6, 4]
    In [28]: filter(lambda x: not x%2, l)
    Out[28]: [2, 12, 6, 4]

If you pass ``None`` to ``filter()``, you get only items that evaluate to true:

.. code-block:: ipython

    In [1]: l = [1, 0, 2.3, 0.0, 'text', '', [1,2], [], False, True, None ]

    In [2]: filter(None, l)
    Out[2]: [1, 2.3, 'text', [1, 2], True]

reduce
------

``reduce``  "reduces" a sequence of objects to a single object with a function that combines two arguments

To get the sum:

.. code-block:: ipython

    In [30]: l = [2, 5, 7, 12, 6, 4]
    In [31]: reduce(lambda x,y: x+y, l)
    Out[31]: 36


To get the product:

.. code-block:: ipython

    In [32]: reduce(lambda x,y: x*y, l)
    Out[32]: 20160

or

.. code-block:: ipython

    In [13]: import operator
    In [14]: reduce(operator.mul, l)
    Out[14]: 20160

Comprehensions
--------------

Couldn't you do all this with comprehensions?

Yes:

.. code-block:: ipython

    In [33]: [x+2 + 10 for x in l]
    Out[33]: [14, 17, 19, 24, 18, 16]

    In [34]: [x for x in l if not x%2]
    Out[34]: [2, 12, 6, 4]

    In [6]: l
    Out[6]: [1, 0, 2.3, 0.0, 'text', '', [1, 2], [], False, True, None]
    In [7]: [i for i in l if i]
    Out[7]: [1, 2.3, 'text', [1, 2], True]

(Except Reduce)

But Guido thinks almost all uses of reduce are really ``sum()``

Functional Programming
----------------------

Comprehensions and map, filter, reduce are all "functional programming" approaches}

``map, filter``  and ``reduce``  pre-date comprehensions in Python's history

Some people like that syntax better

And "map-reduce" is a big concept these days for parallel processing of "Big Data" in NoSQL databases.

(Hadoop, MongoDB, etc.)