What is a list?
List: A list is a data structure.
Remember how we said that a variable stores data? In a similar way, a list also stores data; however, it’s even more interesting because it can store more than one piece of data and even more than one type of data at once! Building on our box analogy about variables, you can imagine that a list is a row of boxes, each of which holds a different data point that you could otherwise store into separate variables.
To access what a specific box is holding, the list cleverly has an address/name for each box in the row, like a house address in a neighborhood.
To initialize a variable, we often create a new variable name and assign it to the value 0 or the empty string like so:
my_variable = 0 your_variable = ""
In the following parts, we’ll be using code snippets to show examples of how you can use different list methods. If you want to follow along, feel free to open up a new file in IDLE and run the code as you go.
How do I create a list?
To create a list, we similarly give it a name and assign it to equal the empty list with the bracket characters, like so:
favorites = []
How do I alter a list?
From previous lessons, you’ve learned a method is a built in function. There’s an entire set of built-in methods in Python you can call on a list.
Add Elements to a List
To add elements to the list, we use a method called append(). Using the favorites list created above, we can add elements to be stored in the “boxes” in the list. I’m going to add my favorite city, my favorite coffee order and my favorite number:
favorites.append("Chicago")
favorites.append("cold brew")
favorites.append(24)
Alternatively, you could have added all of these elements while initializing the list (skipping the appending), like so:
#You will typically use this option less in Intro to Hack B #Don't type this in your file if you're following along with the coding snippets favorites = ["Chicago", "cold brew", 24]
Notice how in either case, you can store both numbers and text types (strings) in a single list.
Accessing Elements in a List
To access elements, let’s return to the address analogy. Let’s pretend the name of your list is the street and the row of boxes holding data have house addresses counting up from 0, going from first to last, where the order depends on the order in which you added the element to the list. We’ll call the “address” of each box an index.
Keep in mind that similar to loops, lists begin indexing from 0, NOT 1.
In our favorites list example, since we added my favorite city first, favorites[0] stores "Chicago". Since we added my favorite coffee order next, favorites[1] stores "cold brew". Since we added my favorite number last, favorite[2] stores 24.
In memory, the list actually looks a lot like the boxes in a row we described.
| Label | favorites[0] | favorites[1] | favorites[2] |
|---|---|---|---|
| Data | “Chicago” | “cold brew” | 24 |
You can reference the data stores in the list using indexing and do anything you would usually do with a variable with that data in your code.
For example, assuming our list favorites still exists from above, the following two lines will print out equivalent output. If you’ve been following along and typing the example code snippets from above in your own file, you can run it using IDLE to check that this is true!
fav_city = "Chicago"
#Option 1: Using concatenation with a string variable (which you're familiar with from previous lessons)
print("My favorite city is " + fav_city + " .")
#Option 2: Using concatenation with a string stored in the favorites list
print("My favorite city is " + favorites[0] + " .")
You can also re-assign values to a given address in the list, just like you can re-assign values to variables.
favorites[0] = "Seattle"
#What do you expect this to print now?
print("My favorite city is " + favorites[0] + " .")
Now, in memory, the favorites list look like this.
| Label | favorites[0] | favorites[1] | favorites[2] |
|---|---|---|---|
| Data | “Seattle” | “cold brew” | 24 |
Printing a List
Sometimes, if you just want to see all of the elements in a list, rather than printing each element out through accessing, you can simply call print(), passing in the name of your list as an argument. For example, print(favorites).
Length of a List
At times, I’m even too lazy to print out the entire list. When I just want to know how many elements are in the list, I’ll use the len() method, which gives me the count of the elements in a list when I pass the list name as an argument. What do you expect print(len(favorites)) to print? Try it out and see if the result matches what you predicted!
Deleting Elements in a List
To delete elements from a list, you can use the method clear() to remove all of the elements in a list. You’ll have an empty list if you call this method on a list.
Another option is to use pop(). You can remove a certain element from a list by passing an argument into the parentheses. Returning to the row of boxes analogy, pop() removes a box from the row at the specified index. Once it is removed, all the boxes are places back into one contiguous row, with the next door box filling the space and index of the deleted box. pop() also returns the value stored at the deleted index, which can come in handy.
temp = favorites.pop(1)
The above line of code removes "cold brew" stored at "favorites[1]. As a result, 24 stored in the favorites[2] box scoots up to the favorites[1] position. Additionally, the temp variable holds the removed string, "cold brew". To test that, add the next two lines:
print(favorites[1]) #this should now print 24 instead of cold brew print(temp) #this should now print cold brew print(len(favorites)) #this should now print 2, instead of 3
Another look at memory:
| Label | favorites[0] | favorites[1] |
|---|---|---|
| Data | “Seattle” | 24 |
Inserting Elements
You may have noticed that when you add elements with append(), the element is added to the end of the list. To insert elements in other places in the list, use insert(). Similar to the pop() method, the insert() method will scoot down boxes after the specified index to “make room” for the next box. Say I want to insert the name of my favorite author into the favorites[0] position in my list:
favorites.insert(0, "Zadie Smith")
What’s the order of elements now? Why don’t you try thinking about what you’d expect? If you’ve been following along with all the code above, you should see: ["Zadie Smith", "Seattle", 24] when you print out the list. Notice that "Seattle", which was originally at the favorites[0] address has been moved down to the favorites[1] address.
One last look at memory:
| Label | favorites[0] | favorites[1] | favorites[2] |
|---|---|---|---|
| Data | “Zadie Smith” | “Seattle” | 24 |
More Methods
If you’re interested in learning more about built-in Python methods for lists, see: https://docs.python.org/3/tutorial/datastructures.html. This link will take you to the official Python documentation on lists. This is a super helpful skill to learn, as most programmers read documentation regularly in their day to day jobs.