Data Structures and Looping
###########################

.. note::
    Learning objectives:
    Differentiate lists, tuples, dictionaries, and sets.
    Iterate over lists, sets, dicts, tuples, and strings.
    Iterate with a comprehension.


.. code-block:: python

    >>> list_of_rodents = [ 'rabbit', 'rat', 'squirrel', 'capybara' ]
    >>> for rodent in list_of_rodents:
    ...     print rodent
    ...
    rabbit
    rat
    squirrel
    capybara

.. graphviz::

    digraph looping {
        for_statement [ label="for the case when\n rodent equals..." ]
        print_statement [ label="print the rodent" ]
	for_statement -> rabbit
	for_statement -> rat
	for_statement -> squirrel
	for_statement -> capybara
        rabbit -> print_statement
        rat -> print_statement
        squirrel -> print_statement
        capybara -> print_statement
    }

* Iterable data structures

+------------+------------------------+-------------------------------------------------+
| Class Name | Common Name            | Meaning                                         |
+============+========================+=================================================+
| list       | Array or list          | A mutable array of indexed data                 |
+------------+------------------------+-------------------------------------------------+
| tuple      | Tuple or ordered set   | Immutable array of indexed data                 |
+------------+------------------------+-------------------------------------------------+
| dict       | Dictionary or          | Array of indexed key-value pairs                |
|            | associative array      |                                                 |
+------------+------------------------+-------------------------------------------------+
| set        | Set                    | Like a set in real math with unique values      |
+------------+------------------------+-------------------------------------------------+
| str        | String                 | It's still a string, but it's like an array of  |
|            |                        | characters                                      |
+------------+------------------------+-------------------------------------------------+

.. code-block:: python

    >>>
    >>> mylist = [] # empty list
    >>> myotherlist = [1, 2, 3] # list of numbers
    >>> mylist
    []
    >>> myotherlist
    [1, 2, 3]
    >>> mylist + myotherlist
    [1, 2, 3]
    >>> mylist.append('squirrel')
    >>> mylist + myotherlist          # You can add them!
    ['squirrel', 1, 2, 3]
    >>> mylist.extend(myotherlist)
    >>> mylist
    ['squirrel', 1, 2, 3]
    >>> print(mylist[0])
    squirrel
    >>> mylist.append('squirrel')
    >>> mylist                        # notice how squirrel now appears twice
    ['squirrel', 1, 2, 3, 'squirrel']
    >>> mylist[4]
    'squirrel'
    >>> mytuple = tuple(mylist)
    >>> mytuple                       # Notice the parentheses showing that this is a tuple
    ('squirrel', 1, 2, 3, 'squirrel')
    >>> mytuple[4]                    # You can still grab members of it like you could from a list
    'squirrel'
    >>> mylist[4] = 'rat'
    >>> mylist
    ['squirrel', 1, 2, 3, 'rat']
    >>> mytuple[4] = 'rat'            # A different outcome!  Tuples don't change.
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: 'tuple' object does not support item assignment
    >>> mylist = [1, 2, 3, 4, 5, 6, 1, 2, 3, 4] # Notice the duplicates.
    >>> mylist
    [1, 2, 3, 4, 5, 6, 1, 2, 3, 4]
    >>> myset = set(mylist)
    >>> myset                         # Notice how the duplicates are gone.
    set([1, 2, 3, 4, 5, 6])
    >>> myset.add(7)                  # Adds seven to the set.
    >>> myset
    set([1, 2, 3, 4, 5, 6, 7])
    >>> myset.add(6)                  # Does nothing.  Why?
    >>> myset
    set([1, 2, 3, 4, 5, 6, 7])
    >>> mydict = {}                   # Empty dictionary
    >>> mydict['squirrel'] = 'nuts'
    >>> mydict['rat'] = 'cheese'
    >>> mydict['rabbit'] = 'carrots'
    >>> mydict
    {'rat': 'cheese', 'squirrel': 'nuts', 'rabbit': 'carrots'}
    >>> mydict['squirrel']
    'nut'
    >>> mydict.keys()                 # keys() is a special function that returns the keys of your dict
    ['rat', 'squirrel', 'rabbit']
    >>> mydict.values()
    ['cheese', 'nuts', 'carrots']     # values() is a special function that returns the values of your dict
    >>> for myrodent in mydict.keys():
    ...     print(myrodent + ' prefers ' + mydict[myrodent])
    ...
    rat prefers cheese
    squirrel prefers nuts
    rabbit prefers carrots