11-doing-things-to-lists.md

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11. Doing Things to Lists

Okay. Let's make our little book application a little more robust. We are going to create a list of books (remember lists?) that we can then manipulate in all sorts of ways.

First, go back to your terminal and enter the REPL, or Python's interactive mode. When you see the >>>, create a list with at least three books that are important to your research right now. Shorten the titles to one or two words if need be. Let's call this list our library. Remember the proper syntax for creating a list includes square brackets with commas separating the list items. Because the items are strings, they should also be inside quotes.

library = ["Orlando", "Confessions of the Fox", "These Waves of Girls"]

Next, let's sort our library in alphabetical order. There's a handy method called sort() for doing just this kind of thing. What's a method, you might ask? Well, methods are very similar to functions, and you'll remember that functions are ways of doing things, like print() and type(). Methods are also ways of doing things, but these things are attached to what we call objects in Python. Objects are part of object-oriented-programming, and that's definitely not necessary to learn right now. Suffice it to say that methods are just like functions, that is, they are ways of doing things to your data.

To sort the list, use the sort() method on your list. It should look like this:

library = ["Orlando", "Confessions of the Fox", "These Waves of Girls"]
library.sort()
print(library)

What happened here? Let's take it line by line. First, we created a list library with three items attached to it. Then, we applied the sort() method to the library list. Finally, we printed the library, which is now sorted in alphabetical order.

You'll see that we have a couple of new things happening with symbols.

• First, the period . which is an operator in Python. The period operator is another part of object-oriented-programming, and it basically means that we are applying a task to whatever precedes the period. In this case, we are applying the sort() method to our library list. It's kind of like attaching a function to our library.
• Second, we have the parenthesis () after sort. When you get more comfortable with programming, you'll often use the parentheses to include what we call arguments that allows us to do more complex things to data. Let's see how an argument works with the append() method.

What if we want to add items to the list? We can use the append() method for that. Try:

library = ["Orlando", "Confessions of the Fox", "These Waves of Girls"]
library.append("La Frontera")
print(library)

Here, we added "La Frontera" as an argument to the append() method by putting the string between the parenthesis. It basically means that we will be appending this specific title to the library list.

When you print library, you should see your new book appear at the end of the list. Pretty cool, right? Go ahead and add a couple more books to your list.

What if you wanted to take out some of the books? We can use pop() to remove the last item, or "pop" it off, from our list.

library = ["Orlando", "Confessions of the Fox", "These Waves of Girls", "La Frontera", "Dawn"]
library.pop()
print(library)

The last item that you added to your list should be missing from the library when you print the list.

Challenge

Remember the input() function from the last lesson? This challenge uses that function in combination with what you know about list methods to create a little library app. You will play around with the input button, asking the user what kinds of things they want to do with their library, and writing some code that does those things and prints out the results.

First, create a new file called library.py. Save it to your current working folder.

Second, create a list of library books, with at least three books (you can use the same ones as before).

library = ["Orlando", "Confessions of the Fox", "These Waves of Girls"]

Then, add an input statement that will save the user's response to a variable, like response.

response = input("What do you want to do with your books today? ")

Now, create a conditional statement that matches the user's response to series of options for doing things to the library list. You can include sort(), append(), and pop(). I'll do the first one, sort(), for you:

library = ["Orlando", "Confessions of the Fox", "These Waves of Girls"]
response = input("What do you want to do with your books today? ")
if response == "sort":
    library.sort()
    print(library)
else:
    print("I don't know what you want me to do!")

See how the order of statements build on each other toward the final product? First, we create a library of books. Then, we set the user's response about what to do with those books. Then, we create a conditional statement that matches the response to specific tasks. The first condition checks to see if the user wants to "sort" the books, then sorts them, then prints the final result.

After adding a few more conditions, test out your code! You should have a little library app that sorts, adds, and removes books from your list.

Solution

library = ["Orlando", "Confessions of the Fox", "These Waves of Girls"]
response = input("What do you want to do with your books today? ")
if response == "sort":
    library.sort()
    print(library)
elif response == "add":
    library.append("La Frontera")
    print(library)
elif response == "remove":
    library.pop()
    print(library)
else:
    print("I don't know what you want me to do!")

Evaluation

Select the following statements that truly describe sort(), append(), and pop().

• methods are like functions which are attached to objects.*
• sort(), append(), and pop() are functions.
• append() always takes an argument.*
• pop() can be applied to a string.

Advanced question: If you sort() the library in between adding and popping a book, you'll end up with a different list than if you didn't run sort() in between append() and pop(). Can you guess why?

Keywords

Do you remember the glossary terms from this section?

• append
• sort()
• pop()

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