Map, Filter and Reduce
------------

These are three functions which facilitate a functional approach to
programming. We will discuss them one by one and understand their use
cases.

Map
^^^^^^

``Map`` applies a function to all the items in an input\_list. Here is
the blueprint:

**Blueprint**

.. code:: python

    map(function_to_apply, list_of_inputs)

Most of the times we want to pass all the list elements to a function
one-by-one and then collect the output. For instance:

.. code:: python

    items = [1, 2, 3, 4, 5]
    squared = []
    for i in items:
        squared.append(i**2)

``Map`` allows us to implement this in a much simpler and nicer way.
Here you go:

.. code:: python

    items = [1, 2, 3, 4, 5]
    squared = list(map(lambda x: x**2, items))

Most of the times we use lambdas with ``map`` so I did the same. Instead
of a list of inputs we can even have a list of functions!

.. code:: python

    def multiply(x):
        return (x*x)
    def add(x):
        return (x+x)

    funcs = [multiply, add]
    for i in range(5):
        value = list(map(lambda x: x(i), funcs))
        print(value)

    # Output:
    # [0, 0]
    # [1, 2]
    # [4, 4]
    # [9, 6]
    # [16, 8]

Filter
^^^^^^^^^

As the name suggests, ``filter`` creates a list of elements for which a
function returns true. Here is a short and concise example:

.. code:: python

    number_list = range(-5, 5)
    less_than_zero = list(filter(lambda x: x < 0, number_list))
    print(less_than_zero)

    # Output: [-5, -4, -3, -2, -1]

The filter resembles a for loop but it is a builtin function and faster.

**Note:** If map & filter do not appear beautiful to you then you can
read about ``list/dict/tuple`` comprehensions.

Reduce
^^^^^^^^^

``Reduce`` is a really useful function for performing some computation on 
a list and returning the result. It applies a rolling computation to sequential 
pairs of values in a list. For example, if you wanted to compute the product 
of a list of integers. 

So the normal way you might go about doing this task in python is using
a basic for loop:

.. code:: python

    product = 1
    list = [1, 2, 3, 4]
    for num in list: 
        product = product * num
    
    # product = 24


Now let's try it with reduce:

.. code:: python

    from functools import reduce
    product = reduce((lambda x, y: x * y), [1, 2, 3, 4])

    # Output: 24