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Lists

  • a lot like strings!
  • a string is a sequence of characters
  • a list is a sequence of — whatever you want!

Creating a list

>>> letters = ['a', 'b', 'c', 'd']
>>> letters
['a', 'b', 'c', 'd']

list indexing

lists can hold a variety of things

letters = ['a', 'b', 'c', 'd']

ways_to_say_goodbye = ['goodbye', 'later', 'sayonara', 'peace out', 'ciao']

scores = [100, 97, 83, 94, 99]

# empty list
nobody = []

# you can do this but it's not a good idea
mixed_list = ['goodbye', 83, 'p', 43.79, 'This is not a good idea!']

Length of a list

ways_to_say_goodbye = ['goodbye', 'later', 'sayonara', 'peace out', 'ciao']

print(len(ways_to_say_goodbye))

nobody = []
print(len(nobody))
    5
    0

Indexing a list

ways_to_say_goodbye = ['goodbye', 'later', 'sayonara', 'peace out', 'ciao']

print(ways_to_say_goodbye[3])
    peace out
print(ways_to_say_goodbye[-1])
    ciao
  • finding the index
ways_to_say_goodbye = ['goodbye', 'later', 'sayonara', 'peace out', 'ciao']
i = ways_to_say_goodbye.index('sayonara')
print(i)
j = ways_to_say_goodbye.index('whatever')
print(j)
    2



    ---------------------------------------------------------------------------

    ValueError                                Traceback (most recent call last)

    /var/folders/9x/cb134v3d2nb22_rksynbspqm0000gn/T/ipykernel_45336/2451658592.py in <module>
          2 i = ways_to_say_goodbye.index('sayonara')
          3 print(i)
    ----> 4 j = ways_to_say_goodbye.index('whatever')
          5 print(j)


    ValueError: 'whatever' is not in list

Looping over a list

for item in ways_to_say_goodbye:
    upper_item = item.upper()
    print(upper_item)
    GOODBYE
    LATER
    SAYONARA
    PEACE OUT
    CIAO
for i in range(len(ways_to_say_goodbye)):
    upper_item = ways_to_say_goodbye[i].upper()
    print(upper_item)
    GOODBYE
    LATER
    SAYONARA
    PEACE OUT
    CIAO

Lists are Mutable

  • you can change them
  • remember, strings are immutable (not changeable)
ways_to_say_goodbye = ['goodbye', 'later', 'sayonara', 'peace out', 'ciao']

ways_to_say_goodbye[0] = 'see ya'
print(ways_to_say_goodbye)
    ['see ya', 'later', 'sayonara', 'peace out', 'ciao']
for i in range(len(ways_to_say_goodbye)):
    ways_to_say_goodbye[i] = ways_to_say_goodbye[i].upper()
print(ways_to_say_goodbye)
    ['SEE YA', 'LATER', 'SAYONARA', 'PEACE OUT', 'CIAO']

Appending to a list

  • one way to change a list
ways_to_say_goodbye = ['goodbye', 'later', 'sayonara', 'peace out', 'ciao']

ways_to_say_goodbye.append('see ya')
print(ways_to_say_goodbye)
    ['goodbye', 'later', 'sayonara', 'peace out', 'ciao', 'see ya']
  • common to start with an empty list and then append
ways_to_say_goodbye = []

ways_to_say_goodbye.append('goodbye')
ways_to_say_goodbye.append('later')
ways_to_say_goodbye.append('sayonara')

print(ways_to_say_goodbye)
    ['goodbye', 'later', 'sayonara']
  • can append a lot in a loop
numbers = []
for i in range(10):
    numbers.append(i * 10)
print(numbers)
    [0, 10, 20, 30, 40, 50, 60, 70, 80, 90]
  • the first example listed below does not work
  • append() returns None
numbers = []
numbers = numbers.append(1)
print(numbers)

numbers = []
numbers.append(1)
print(numbers)
    None
    [1]
  • because lists are mutable, you change it directly
  • different from strings
numbers = []
numbers.append(1)
print(numbers)

name = "Emma"
name = name + "!"
print(name)

Example

  • function that returns all the numbers from 0 to n - 1
def list_n(maximum):
    numbers = []
    for i in range(maximum):
        numbers.append(i)
    return numbers

some_numbers = list_n(10)
print(some_numbers)
    [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

in and not in tests

letters = ['a', 'b', 'c', 'd']
if 'c' in letters:
    print('c is in the list')
    c is in the list
if 'x' not in letters:
    print('x is not in the list')
    x is not in the list

Example

  • create a new list that has all the items that are in both list a and list b
def intersect(a, b):
    # accumulator pattern
    result = []
    # loop through all items in list a
    for item in a:
        # check if the item is in list b
        if item in b:
            result.append(item)
    return result


intersection = intersect([1, 2, 3, 4, 5], [2, 3, 5])
print(intersection)
# order of lists doesn't matter
intersection = intersect([2, 3, 5], [1, 2, 3, 4, 5])
print(intersection)

    [2, 3, 5]
    [2, 3, 5]

Example

  • remember index()?
ways_to_say_goodbye = ['goodbye', 'later', 'sayonara', 'peace out', 'ciao']
# this creates an error
ways_to_say_goodbye.index('whatever')
  • write a function called find_index_of_item()
  • return the index for an item if it is in the list, otherwise return -1
def find_index_of_item(item, shopping_list):
    if item in shopping_list:
        return shopping_list.index(item)
    return -1

my_shopping_list = ['sausage', 'peppers', 'tomatoes']
index = find_index_of_item('peppers', my_shopping_list)
print(index)
index2 = find_index_of_item('pasta', my_shopping_list)
print(index2)
    1
    -1

Constants

STATES = [ 'AK', 'AL', 'AR', 'AZ', 'CA', 'CO', 'CT', 'DC', 'DE', 'FL', 'GA',
           'HI', 'IA', 'ID', 'IL', 'IN', 'KS', 'KY', 'LA', 'MA', 'MD', 'ME',
           'MI', 'MN', 'MO', 'MS', 'MT', 'NC', 'ND', 'NE', 'NH', 'NJ', 'NM',
           'NV', 'NY', 'OH', 'OK', 'OR', 'PA', 'RI', 'SC', 'SD', 'TN', 'TX',
           'UT', 'VA', 'VT', 'WA', 'WI', 'WV', 'WY']

def find_the_best_state():
    for state in STATES:
        if state == 'OR':
            print("Oregon is the best state")

find_the_best_state()

Oregon is the best state

Important notes on constants

  • a type of “global” variable since it is not inside a function
  • any function in your program can see it
  • Python conventions
    • upper case means its a constant
    • it’s intended to be a read-only value, code should not modify it