Learning to Code with Python: Lesson 1.3 - Variables

Key Takeaways

Hi, welcome to learning to code with Python. Lesson three, variables. Variables are a really important concept in programming. A variable's like a label. It's a shortcut for pointing at something that we want to save. Here are some examples. So, I've set apples equal to 10. So, now if I say print apples, I see that apples now is equal to the number 10. If I want to change what apples points to, I can say, now make apples equal to 15. You can also store other things than numbers in a variable. For example, variables can be named just about anything you can think of. It's usually a good idea to pick a name that makes it clear what the value means. For example, if we were making a game where your goal was to collect flowers, that might be a good name for the variable, like this. If you name it bubbles, then later when you want to print out the score, you're going to be confused by why the number of flowers you've collected is called bubbles. So, now that we know what variables are, let's talk about what kinds of things we can store in them. Information inside the computer is called data. And there are lots of different types of data that the computer can remember. Let's start with numbers. We've used those a few times already. Did you know you can use the Python shell like a calculator? Try this out. We can add numbers. We can subtract numbers. We can multiply. And that asterisk there is the symbol for multiplication. It's the number above the eight on your keyboard. So, press shift and eight to get it. And last, you can, of course, divide. You can also calculate with variables. You remember we have our apples variable? Well, apples * 2 is 30. Another type of data we can work with is text. Remember our first program? We printed Hello, world to the screen. The stuff inside the quotation marks is called a string. A string just means a bunch of characters. Those characters can be letters, numbers, whatever we can type. We put them inside quotes so the computer knows that they're not commands. Here's an example. See, I put the word orange in quotes. So, now fruit is equal to orange. What if I left the quotes out? I get an error message, right? Because it thinks orange is a variable. So, orange should be pointing at something, but we never told orange to point at anything. So, the computer doesn't know what we mean. Let's see a little more about how strings work. Strings can be added together. See how adding those two strings together just mushed them together? I didn't include a space, so the computer didn't put one there, either. In fact, every string is just a bunch of individual characters added together. That's why they're called strings. We'll work more with strings later in the class. All right, I've reset my window just so it won't be so cluttered. You don't have to do that if you don't want to. So, the last type of data I'd like to talk about is a list. Lists are really useful if you need to keep track of a bunch of things. This is what a list looks like. We use a square bracket and then any number of items can go inside it separated by commas. You can also make lists that are lists of strings. So, for example, my friends are Bob, Sue, and Alex. And that's a list. Now, how would we use these? Let's imagine that we're building a game about wizards and the player has to collect a bunch of items to make a magic potion. With a list, we can keep track of what's in their backpack. And let's say they've collected a bat eye, some slug slime, and a lizard gizzard. Okay? And that's their list of items inside their backpack. Now, when the computer has items in a list, it keeps track of them by numbering them. However, when a computer counts, it counts a little differently from how you or I might do it. It always starts counting at the number zero. These numbers are how it keeps track of what place in the list each item is, and that's called an index. Let's see how we would use indexes. Backpack number zero is the first item in the list. Backpack number one, item number one is the slug slime. But what if we say we want backpack item number three? Well, we get an error message, right? It says the index is out of range. That's because the list only has an item number zero, an item number one, and an item number two. There is no item number three. It takes a little getting used to to remember to start counting with zero, but with practice, you'll get used to it in no time. Now, lists are useful because if we want to change something, we don't have to retype the whole list. Let's say that instead of slug slime, the wizard actually has red slug slime. Well, we can just change that one item. I want to change backpack item number one to be red slug slime. Now, if I print out the whole backpack, only that item has changed. We didn't have to worry about changing any of the other items in the list.