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.

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

• 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 associative array	Array of indexed key-value pairs
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

>>>
>>> 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