In order to begin to truly write dynamic programs, we need to be able to work with dynamic data where we do not know how much of a certain type of variable we have.

The problem, essentially is, variables hold only one item.

my_color = "red"
my_peer = "Brandi"

Lists hold multiple items - and lists can hold any datatype.

Here are some different ways to declare a list variable:

colors = ['red', 'yellow', 'green'] #strings
grades = [100, 99, 65, 54, 19] #numbers
bools = [True, False, True, True] #booleans

To create a new blank list, simply use python blank_list = list().

The list index means the location of something (an element) in the list.

List indexes start counting at 0!

my_class = ['Brandi', 'Zoe', 'Steve', 'Aleksander', 'Dasha']
print(my_class[0]) # Prints "Brandi"
print(my_class[1]) # Prints "Zoe"
print(my_class[4]) # Prints "Dasha"

If you want to extend the content of a single list, you can use .append(), .extend() .insert() to add elements of any data type.

.append() & .extend(): These methods both add items to the end of the list. The difference here is that .append() will add whatever value or group of values you pass it in one chunk. In contrast, if you pass a group of values into .extend(), it will add each element of the group individually. Here are a few examples to show you the difference in outcomes.

# passing direct argument
x = ['a', 'b', 'c', 'd']
x.append(['e', 'f', 'g'])
print(x) # ['a', 'b', 'c', 'd', ['e', 'f', 'g']]

x = ['a', 'b', 'c', 'd']
x.extend(['e', 'f', 'g'])
print(x) # ['a', 'b', 'c', 'd', 'e', 'f', 'g']

# passing argument within a var
x = ['a', 'b', 'c', 'd']
y = ['e', ('f', 'g'), ['h', 'i'], 'j']
x.append(y)
print(y) # ['a', 'b', 'c', 'd', ['e', ('f', 'g'), ['h', 'i'], 'j']]

x = ['a', 'b', 'c', 'd']
y = ['e', ('f', 'g'), ['h', 'i'], 'j']
x.extend(y)
print(x) # ['a', 'b', 'c', 'd', 'e', ('f', 'g'), ['h', 'i'], 'j']

Notice that .extend() only considers individual values of the parent list. It still added the tuple and list - ('f', 'g') and ['h', 'i'] - to our list x as their own items.

.insert(index, value): If you want to add an item to a specific point in your list, you can pass the desired index and value into .insert() as follows.

# your_list.insert(index, item)

my_class = ['Brandi', 'Zoe', 'Steve', 'Aleksander', 'Dasha', 'Sonyl']
my_class.insert(1, 'Sanju')
print(my_class)
# => ['Brandi', 'Sanju', 'Zoe', 'Steve', 'Aleksander', 'Dasha', 'Sonyl']

l[index:index]=: To replace items in a list by their index position, you can use the same syntax for adding a single new value. You simply reference which indeces you want to replace and specify the new values.

x = ['Brandi', 'Sanju', 'Zoe', 'Steve', 'Aleksander', 'Dasha', 'Sonyl']
x[1] = 'Raju'
x[6:] = ['Chloe', 'Phoebe']
print(x) # ['Brandi', 'Raju', 'Zoe', 'Steve', 'Aleksander', 'Dasha', 'Chloe', 'Phoebe']

.join(): If you need to, you can compile your list items into a single string.

letters = ['j', 'u', 'l', 'i', 'a', 'n', 'n', 'a']
name = ''.join(letters)
print(name) # 'julianna'

words = ['this', 'is', 'fun']
sentence = ' '.join(words)
print(f'{sentence}.') # 'this is fun.'

.split('by_char'): You can also do the opposite - split values out of a string and turn each value into a list item. This one doesn't work for single words you might want to split into individual characters. That said, you can specify what character should convey to the method when to split out a new item. By default, .split() will use a space character to split the string.

x = 'this is fun'
sentence = x.split() # note - using default split char at space
print(sentence) # ['this', 'is', 'fun']

y = 'Sandra,hi@email.com,646-212-1234,8 Cherry Lane,Splitsville,FL,58028'
data = y.split(',')
print(data) # ['Sandra', 'hi@email.com', '646-212-1234', '8 Cherry Lane', 'Splitsville', 'FL', '58028']

Likewise, you can use .pop() or .pop(index) to remove any type of element from a list.

.pop():

• Removes an item from the end of the list.

# your_list.pop()

my_class = ['Brandi', 'Zoe', 'Steve', 'Aleksander', 'Dasha', 'Sonyl']
student_that_left = my_class.pop()
print("The student", student_that_left, "has left the class.")
# Sonyl
print(my_class)
# => ['Brandi', 'Zoe', 'Steve', 'Aleksander', 'Dasha']

.pop(index):

• Removes an item from the list.
• Can take an index.

# your_list.pop(index)

my_class = ['Brandi', 'Zoe', 'Steve', 'Aleksander', 'Dasha']
student_that_left = my_class.pop(2) # Remember to count from 0!
print("The student", student_that_left, "has left the class.")
# => "Steve"
print(my_class)
# => ['Brandi', 'Zoe', 'Aleksander', 'Dasha']

Python has some built-in operations that allow you to analyze the content of a list. Some basic ones include:

len(): This tells you how many items are in the list; can be used for lists composed of any data type (i.e. strings, numbers, booleans)

# length_variable = len(your_list)

my_class = ['Brandi', 'Zoe', 'Aleksander', 'Dasha']
num_students = len(my_class)
print("There are", num_students, "students in the class")
# => 5

sum(): This returns the sum of all items in numerical lists.

# sum_variable = sum(your_numeric_list)

team_batting_avgs = [.328, .299, .208, .301, .275, .226, .253, .232, .287]
sum_avgs = sum(team_batting_avgs)
print(f"The total of all the batting averages is {sum_avgs}")
# => 2.409

min() & max():

These return the smallest and largest numbers in a numerical list respectively.

# max(your_numeric_list)
# min(your_numeric_list)

team_batting_avgs = [.328, .299, .208, .301, .275, .226, .253, .232, .287]
print(f"The highest batting average is {max(team_batting_avgs}")
# => 0.328
print("The lowest batting average is", min(team_batting_avgs))
# => 0.208

If you want to organize your lists better, you can sort them with the sorted() operator. At the some basic level, you can sort both numerically and alphabetically.

Numbers - Ascending & Descending

numbers = [1, 3, 7, 5, 6, 4, 2]

ascending = sorted(numbers)
print(ascending) # [1, 2, 3, 4, 5, 6, 7]

To do this in descending order, simply add reverse=True as an argument in sorted() like this:

descending = sorted(numbers, reverse=True)
print(descending) # [7, 6, 5, 4, 3, 2, 1]

Letters - Alphabetically & Reverse

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

ascending = sorted(letters)
print(ascending) # ['a', 'b', 'c', 'd', 'e']

descending = sorted(letters, reverse=True)
print(descending) # ['e', 'd', 'c', 'b', 'a']

NOTE! You cannot sort a list that includes different data types.

Tuples are a special subset of lists - they are immutable - in that they cannot be changed after creation.

We write tuples as:

score_1 = ('Taq', 100)

# OR

score_2 = 'Sue', 101

Tuples are denoted with the ().

We read tuples just like we would read a list:

print(score_1[0]) # 'Taq'

Sets are special lists in that they can only have unique elements

set_1 = {1,2,3,4,5} # this is a set, notice the {}
set_2 = {1,1,1,2,2,3,4,5,5,5} # this is still a set
print(set_2) # {1,2,3,4,5}

print(set_1 == set_2) # True

Sets are not indexed, so you cannot access say the 3rd element in a set. Instead, you can:

print(2 in set_1) # True
print(9 in set_1) # False

Here's a helpful list of set operations.

1. Create a list with the names "Holly", "Juan", and "Ming".
2. Print the third name.
3. Create a list with the numbers 2,4, 6, and 8.
4. Print the first number.

1. Declare a list with the names of your classmates
2. Print out the length of that list
3. Print the 3rd name on the list
4. Delete the first name on the list
5. Re-add the name you deleted to the end of the list
6. You work for Spotify and are creating a feature for users to alphabetize their playlists by song title. Below are is a list of titles from one user's playlist. Alphabetize these songs. playlist_titles = ["Rollin' Stone", "At Last", "Tiny Dancer", "Hey Jude", "Movin' Out"]
7. Create a list with 6 numbers and sort it in descending order.

On your local computer, create a .py file named list_practice.py. In it:

1. Save a list with the numbers 2, 4, 6, and 8 into a variable called numbers.
2. Print the max of numbers.
3. Pop the last element in numbers off; re-insert it at index 2.
4. Pop the second number in numbers off.
5. Append 3 to numbers.
6. Print out the average number.
7. Print numbers.

• Python Lists - Khan Academy Video
• Google For Education: Python Lists
• Python-Lists
• Python List Methods
• Python Data Structures: Lists, Tuples, Sets, and Dictionaries Video