Data structures

Any code you write in the workbook should be re-usable where possible, i.e. it should work if the values assigned to any variables are modified.

List indexing

Remember that we can have: - any number of items in a list, separated by commas - different data types and structures within our list

… and that the syntax for accessing elements is:

• [position]

• [start:stop]

• [start:stop:step]

presidents = ['Ronald Reagan', 'George H. W. Bush', 'William J. Clinton', \
         'George W. Bush', 'Barack Obama', 'Donald J. Trump'] #U.S. Presidents 1981-2020

Assign to a variable called ``number_presidents`` the number of entries in (i.e. ``len``gth of) ``presidents``:

number_presidents = len(presidents)
number_presidents

6

Assign to a variable called ``last_president`` the final president in ``presidents``:

last_president = presidents[-1]
last_president

'Donald J. Trump'

n = 4

Assign to a variable called ``nth_president``, the name of that president in the list:

• the first entry in presidents would be returned when n = 1 (remember that Python is zero-indexed)

• you can assume that n is always <= len(presidents)

nth_president = presidents[n-1]
nth_president

'George W. Bush'

String indexing

Remember that we can use indexing syntax in the same way on strings.

Assign to a variable called ``space_index``, the (zero-indexed) position of the first space character (``’ ‘``) in ``nth_president``: - use the .index() method - you can assume all entries contain a space

space_index = nth_president.index(' ')
space_index

6

Using ``nth_president`` and ``space_index``, assign to a variable called ``name_nth_president`` a string of the characters in ``nth_president`` up to (but not including) the first space:

name_nth_president = nth_president[:space_index]
name_nth_president

'George'

List methods

ratings = [7, 4, 8, 2, 9, 4, 3, 6, 8, 6, 7, 9, 2, 0, 3, 4, 1, 0, 2]
today = [5, 8, 2]

Use ``.extend()`` to add the ``today`` list to ``ratings``:

ratings.extend(today)
ratings

[7, 4, 8, 2, 9, 4, 3, 6, 8, 6, 7, 9, 2, 0, 3, 4, 1, 0, 2, 5, 8, 2]

Use ``.sort()`` and then ``.reverse()`` to order the elements in ``ratings`` with the highest numbers coming first:

ratings.sort()
ratings.reverse()

ratings

[9, 9, 8, 8, 8, 7, 7, 6, 6, 5, 4, 4, 4, 3, 3, 2, 2, 2, 2, 1, 0, 0]

Use ``.count()`` to assign the number of entries which equal ``0`` to ``zero_ratings``:

• Information on the .count() method and other list methods can be found in the documentation

zero_ratings = ratings.count(0)
zero_ratings

2

Use ``.index()`` to assign to ``high_ratings`` how many ratings are ``> 7``:

• Your list must have been sorted correctly in the earlier question

high_ratings = ratings.index(7)
high_ratings

5

Dictionaries

guest = {'name': 'Boris Johnson',
          'country': 'United Kingdom',
          'partner': {'name': 'Carrie Symonds',
                      'relationship': 'Engaged',
                      },
          'age': 55,
          'job': {'title': 'Prime Minister',
                  'years': 1,
                 },
          }

Assign to a variable called ``guest_country`` the value in the ``guest`` dictionary associated with the ``country`` key:

guest_country = guest['country']
guest_country

'United Kingdom'

Assign to a variable called ``partner_name`` the value for ``name`` from the dictionary which is itself the entry for ``partner`` in the ``guest`` dictionary:

partner_name = guest['partner']['name']
partner_name

'Carrie Symonds'

Add a new key:value pair to ``guest``, with a key of ``’order’`` and a value of ``’Roast Beef’``:

guest['order'] = 'Roast Beef'
guest['order']

'Roast Beef'

Change the value for ``relationship`` in ``partner`` to ``’Married’``:

guest['partner']['relationship'] = 'Married'
guest['partner']

{'name': 'Carrie Symonds', 'relationship': 'Married'}

Use the ``.get()`` method to assign to a variable called ``guest_notes`` the value for the key ``’notes’`` in ``guest``, using ``’None found’`` as a fallback value if the key isn’t found:

guest_notes = guest.get('notes', 'None found')
guest_notes

'None found'

Tuples

Assign to ``guest_tag`` a tuple containing three elements in the following order, with values from ``guest``: - name - title (from inside the nested job dictionary) - country

guest_tag = (guest['name'], guest['job']['title'], guest['country'])
print(guest_tag[0] + '\n' + guest_tag[1] + '\n' + guest_tag[2])

Boris Johnson
Prime Minister
United Kingdom

Sets

hobbies = {'drumming', 'cycling', 'chess', 'travelling'}

Use the ``.discard()`` method to discard ``’travelling’`` from ``hobbies`` and then the ``.add()`` method to add ``’swimming’``:

hobbies.discard('travelling')
hobbies.add('swimming')
hobbies

{'chess', 'cycling', 'drumming', 'swimming'}

Built-in functions

Use the built-in function ``list()`` to create a list from ``hobbies``, assigning it to ``hobbies_list``:

hobbies_list = list(hobbies)
hobbies_list

['cycling', 'drumming', 'swimming', 'chess']

Use the built-in function ``sorted()``, and list indexing, to assign to ``first_hobby`` the item from ``hobbies_list`` which is first alphabetically:

first_hobby = sorted(hobbies_list)[0]
first_hobby

'chess'