# 1: Variables

**Source**: https://cscircles.cemc.uwaterloo.ca/1-variables/
**Parent**: https://cscircles.cemc.uwaterloo.ca/

Variables act as "storage locations" for data in a program. They are a way of naming information for later usage. Each variable has a *name*; an example variable name we will use is `myLuckyNumber`. To store information in a variable, we write a command using an equal sign in the following way:

```
«the variable name» = «the value you want to store»
```

(We use «double angle brackets» in our lessons, like above, to indicate special parts of expressions.) For example, the Python line

```
myLuckyNumber = 13
```

stores the value `13` in the variable `myLuckyNumber`. Then, anywhere you write the variable name `myLuckyNumber` again, Python retrieves the stored value. Below, there is a short example of using variables. It has more than one line of instructions: Python executes the first line, then the second line, and so forth until it reaches the last line. Press the **Run program** button to see what it does. 

Example

Example of using a variable.

myLuckyNumber = 13
print(myLuckyNumber + 1)
print(myLuckyNumber)
myLuckyNumber = 5 + 2
print(myLuckyNumber)

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Look at the 5 lines of the program in order, and how they correspond to the output. As you can see, `myLuckyNumber` keeps its value of `13` for the first two `print` statements, then its value is changed to `7`. We also introduced the *plus* operator (`+`) above, which adds two numbers together. Similarly, there are operators for subtraction (`-`), multiplication (`*`), and division (`/`). We'll return to these in a later lesson. You can simulate the memory storage of a computer with paper and pencil, by keeping track of the values in a table. Here is an example; remember that `*` means multiplication in Python.

**Goal**: determine the final values of all variables at the end of the program.

```
first = 2
second = 3
third = first * second
second = third - first
first = first + second + third
third = second * first
```

**Idea**: We use a table to keep track of the values as they change. Scroll to the bottom to see the final answer.

| Statement | Values after statement executes | | |
| --- | --- | --- | --- |
|  | first | second | third |

|  |  |  |  |
| --- | --- | --- | --- |
| first = 2 | **2** |  |  |
| second = 3 | 2 | **3** |  |
| third = first \* second | 2 | 3 | **6** |
| second = third - first | 2 | 3 **4** | 6 |
| first = first + second + third | 2 **12** | 4 | 6 |
| third = second \* first | 12 | 4 | 6 **48** |

Thus at the end of the program, the value of `first` is `12`, the value of `second` is `4`, and the value of `third` is `48`.

Drawing a table like this on pencil and paper is always a good idea and helpful when understanding or fixing code. We also have an automated [Python3 visualization tool](https://cscircles.cemc.uwaterloo.ca/visualize) to virtually execute programs like this one step at a time (see also the link in the top menu). Here's what it looks like when we run the same program on the visualizer. *Use the **Forward >** button or press the arrow key on your keyboard to step forward (or back).*Note how the variables change as each line executes. Here is a short answer exercise about variables.

Short Answer Exercise: `x` Marks the Spot

What is the value of `x` after these commands execute?

```
x = 10\
x = x + x\
x = x - 5
```

Your answer (enter a number): 

Correct!

## Two Common Errors

If you ask Python about a variable that has not been defined, you get an error. 

Example: An Undefined Variable

print(trouble)

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As you can see, we get an error message saying `NameError: name 'trouble' is not defined`. Sometimes you can get errors like this from simple typos: if you define a variable `address=32`, then try to `print(adress)`, the same type of error occurs. Another error has to do with accidentally swapping the sides of an `=` statement. 

Example: `SyntaxError: can't assign to literal`

x = 4
4 = x

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The first line is fine but the second line causes an error: Python thinks the second line `4 = x` is trying to change the value of `4`, but you are only allowed to change the values of variables, and `4` is not a variable. While `A = B` and `B = A` are the same in mathematics, they are different in programming.

## Exercise

This is a warm-up to get you started with variables. 

Coding Exercise: Heads, Shoulders, Knees and Toes

Write a *code fragment* (a short part of a Python program) to count heads, shoulders, knees, and toes at a party. The grader will automatically define a variable `people` for you, counting the number of people at the party. Your code must define four variables, one called `heads`, one called `shoulders`, one called `knees`, and one called `toes`, equal to the number of heads, shoulders, knees, and toes in total at the party. Your program does not need to print any output. *The grader will select a new random value for*`people`*each time your code runs*. \
 Click here if you want a hint on getting started

You need to create an account and log in to ask a question.

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## Code Scramble

The next item in this lesson is a new type of programming exercise, where you don't have to do any programming. We will provide you with a correct program, but the catch is that its lines have been put into a scrambled-up order. Your job is to drag-and-drop the lines to rearrange them into a correct program. 

Scramble Exercise: Speed Calculator

You are in a bike race which goes up and down a hill. The grader will pre-define four variables for you: `uphillDistance` and `downhillDistance` give the distance (in km) of both parts of the race, and `uphillTime` and `downhillTime` give the time (in minutes) of how long it took you to complete each part of the race. Write a program that will print out your average speed (in km/min) for the entire race.\
*Drag and drop with your mouse to rearrange the lines.\*Click for a hint if you're stuck

- totalDistance = uphillDistance + downhillDistance
- print(averageSpeed)
- averageSpeed = totalDistance / totalTime
- totalTime = uphillTime + downhillTime

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## Exchange Program

Here is the last exercise in this lesson. 

Coding Exercise: Exchange Program

Write a program to **swap** the values of two variables. The grader will automatically define variables `x` and `y` for you, with numerical values. You must write code that exchanges their values: the value of `x` after your code runs must equal the value that `y` had before your code ran, and the value of `y` after your code runs must equal the value that `x` had before your code ran. Your program does not need to print any output. \
*The most common solution is short, but can be hard to find. Click on each hint if you want to read it.*

Hint, part 1

You do not need to use any arithmetic (`+-*/`) to solve this problem. You only need to use variables and `=`. You can define new variables with any name, if you need to.

Hint, part 2

We have two goals: put the original value of `y` into `x`, and put the original value of `x` into `y`. If all we wanted to do was the first goal (in other words, if we only wanted to put the value of `y` into `x`), we could use the one-line program

```
x = y
```

How can we do both goals at once?