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Python Programming: Lists

--- ## Lesson Objectives _After this lesson, you will be able to..._ - Create lists in Python. - Print out specific elements in a list. - Perform common list operations. --- ## Unit 2 Kickoff In Unit 1, we ended by printing the rating for a movie: `print('The rating for', movie_title, 'is', movie_rating)`. In Unit 2, we're going to learn to add logic and make this much more complex. By the end of this: - We'll have a variable that's set to either `1` or `2`. If the variable is a `1`, we'll print the movie title, and if the variable is a `2`, we'll print the rating. - We'll have many movies in a `list` and print them all out with just one `print` statement using a `loop`. - We'll make pieces of our program easy to reuse using `functions`. Ready? Let's go! --- ## What is a List? Variables hold one item. ```python my_color = "red" my_peer = "Brandi" ``` **Lists** hold multiple items - and lists can hold anything. ```python # Declaring lists colors = ["red", "yellow", "green"] my_class = ["Brandi", "Zoe", "Steve", "Aleksander", "Dasha"] # Strings colors = ["red", "yellow", "green"] # Numbers my_nums = [4, 7, 9, 1, 4] # Both! my_nums = ["red", 7, "yellow", 1, 4] ```

**Teaching Tips**: - After explaining what a list is, walk through the syntax (dashes, commas). - Point out anything can be in a list - it's just a variable that holds many things. **Talking Points**: - "Until now we've used a few different types of variables such as numbers and strings. However, what if we wanted to keep track of more than one thing? Instead of just my single favorite color, how can I store the names of all the colors I like? How can I store the numbers of everyone on my baseball team? - "Python has this problem solved with something called a _List_." - "Because a variable is just a box that can hold information, it can also hold lists. Python knows that your variable will hold a list if it begins and ends with square brackets" - "A list is a data structure in Python, which is a fancy way of saying we can put data inside of it. In the same way you recognize strings by the quotation marks surround them, you can recognize lists by square brackets that surround them." - "Notice in the example below, we have a list but there are strings inside of it. A list can store data of other types! In this case, I have a list of strings that stores my classmates names."

--- ## Accessing Elements **List Index** means the location of something (an _element_) in the list. List indexes start counting at 0! | List | "Brandi" | "Zoe" | "Steve" | "Aleksander" | "Dasha" | | :---: | :------: | :---: | :-----: | :----------: | :-----: | | Index | 0 | 1 | 2 | 3 | 4 | ```python 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" ``` --- ## We Do: Lists 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. --- ## List Operations - Length `len()`: - A built in `list` operation. - How long is the list? ```python # length_variable = len(your_list) my_class = ["Brandi", "Zoe", "Steve", "Aleksander", "Dasha"] num_students = len(my_class) print("There are", num_students, "students in the class") # => 5 ``` --- ## Adding Elements: Append `.append()`: - A built in `list` operation. - Adds to the end of the list. - Takes any element. ```python # your_list.append(item) my_class = ["Brandi", "Zoe", "Steve", "Aleksander", "Dasha"] my_class.append("Sonyl") print(my_class) # => ["Brandi", "Zoe", "Steve", "Aleksander", "Dasha", "Sonyl"] ``` --- ## Adding Elements: Insert `.insert()`: - A built in `list` operation. - Adds to any point in the list - Takes any element and an index. ```python # 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"] ``` --- ## Removing elements - Pop `.pop()`: - A built in `list` operation. - Removes an item from the end of the list. ```python # 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"] ``` --- ## Removing elements - Pop(index) `.pop(index)`: - A built in `list` operation. - Removes an item from the list. - Can take an index. ```python # your_list.pop(index) my_class = ["Brandi", "Zoe", "Steve", "Aleksander", "Dasha", "Sonyl"] 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", "Sonyl"] ``` --- ## Partner Exercise: Pop, Insert, and Append Partner up! Choose one person to be the driver and one to be the navigator, and see if you can do the prompts: --- ## Pop, Insert, Append Solution --- ## !! List Mutation: Warning !! This won't work as expected - don't do this! ```python colors = ["red", "yellow", "green"] print(colors.append("blue")) # => None ``` This will work - do this! ```python colors = ["red", "yellow", "green"] colors.append("blue") print(colors) # => ["red", "yellow", "green", "blue"] ``` --- ## Quick Review: Basic List Operations ```python # List Creation my_list = ["red", 7, "yellow", 1] # List Length list_length = len(my_list) # 4 # List Index print(my_list[0]) # red # List Append my_list.append("Yi") # ["red", 7, "yellow", 1, "Yi"] # List Insert at Index my_list.insert(1, "Sanju") # ["red", "Sanju", 7, "yellow", 1, "Yi"] # List Delete student_that_left = my_list.pop() # "Yi"; ["red", "Sanju", 7, "yellow", 1] # List Delete at Index student_that_left = my_list.pop(2) # 7; ["red", "Sanju", "yellow", 1] ``` --- ## Numerical List Operations - Sum Some actions can only be performed on lists with numbers. `sum()`: - A built in `list` operation. - Adds the list together. - Only works on lists with numbers! ```python # sum(your_numeric_list) team_batting_avgs = [.328, .299, .208, .301, .275, .226, .253, .232, .287] sum_avgs = sum(team_batting_avgs) print("The total of all the batting averages is", sum_avgs) # => 2.409 ``` --- ## List Operations - Max/Min `max()` or `min()`: - Built in `list` operations. - Finds highest, or lowest, in the list. ```python # max(your_numeric_list) # min(your_numeric_list) team_batting_avgs = [.328, .299, .208, .301, .275, .226, .253, .232, .287] print("The highest batting average is", max(team_batting_avgs)) # => 0.328 print("The lowest batting average is", min(team_batting_avgs)) # => 0.208 ``` --- ## You Do: Lists 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 (divide the sum of `numbers` by the length). 7. Print `numbers`. --- ## Summary and Q&A We accomplished quite a bit! ```python # List Creation my_list = ["red", 7, "yellow", 1] # List Length list_length = len(my_list) # 4 # List Index print(my_list[0]) # red # List Append my_list.append("Yi") # ["red", 7, "yellow", 1, "Yi"] # List Insert at Index my_list.insert(1, "Sanju") # ["red", "Sanju", 7, "yellow", 1, "Yi"] # List Delete student_that_left = my_list.pop() # "Yi"; ["red", "Sanju", 7, "yellow", 1] # List Delete at Index student_that_left = my_list.pop(2) # 7; ["red", "Sanju", "yellow", 1] ``` --- ## Summary and Q&A And for numerical lists only... ```python # Sum all numbers in list sum_avgs = sum(team_batting_avgs) # Find minimum value of list min(team_batting_avgs) # Find maximum value of list max(team_batting_avgs) ``` --- ## Additional Resources - [Python Lists - Khan Academy Video](https://www.youtube.com/watch?v=zEyEC34MY1A) - [Google For Education: Python Lists](https://developers.google.com/edu/python/lists) - [Python-Lists](https://www.tutorialspoint.com/python/python_lists.htm)