Comprehension
=============

* defining comprehension
* conditional statements and comprehension
* dictionary comprehension

Comprehension is a nice way to build lists, sets, and dictionaries.   First let's build a list from a string using a loop.

.. code::
 
 alphabet = 'abcdefghijklmnopqrstuvwxyz'

 >>> list = []
 >>> for letter in alphabet:
 ...     list.append(letter)
 >>> list
 ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']

Every list comprehension has three parts:  (1) a member, (2) an expression, and (3) an iterable.  A conditional is optional and can be used to control membership. Here are the forms of list comprehension:

.. code::

 [expression for member in iterable]
 [expression for member in iterable (if conditional)]
 [expression (if conditional) for member in iterable]

Now we will construct the same list we construct with a loop using a comprehension.  

.. code::

>>> [letter for letter in alphabet]
['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z]
 
Note that we by using conprehension we reduced the construction to one easy-to-read line.
 
Here are some examples using conditionals along with comprehension.
 
.. code::

 new_list = [expression for member in iterable (if conditional)]
 vowels1 = {i for i in alphabet if i in 'aeiou'}
 print('The vowels are', vowels1)
 
 
 new_list = [expression for member in iterable (if conditional)]
 
Here we generate a set to show that comprehension can be used to generate sets.

.. code::

 consonants =  {i for i in alphabet if i not in 'aeiou'}
 print('The consonants are', consonants)
 
 new_list = [expression (if conditional) for member in iterable]
 delete_vowels = [letter if letter not in 'aeiou' else '*' for letter in alphabet]
 print('Letters with vowels marked', delete_vowels)
 
 # vowels2 = [i (if i in 'aeiou') for i in alphabet]
 # print('The vowels are', vowels2)
 
Comprehension can also be used to generate a dictionary.  
Create a dictionary using comprehension.

.. code::

 dict1 = {i: alphabet[i] for i in range(26)}
 print('The last letter of the alphabet is', dict1[25], '.')