1: Variables

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»

For example, the Python line

myLuckyNumber = 13

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.

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.

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

Try the Python3 visualization tool to virtually execute programs like this one step at a time. We recommend it as a useful resource whenever you are confused about what Python is doing, or when you are stuck debugging! You can find this link again later in the Resources page.

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!

Undefined Variables

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

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.

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. Click here if you want a hint

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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

Switcheroo

Here is the last exercise in this lesson.

Coding Exercise: Switcheroo

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 x into y), we could use the one-line program

x = y

How can we do both goals at once?

Hint, part 3

The following program might seem promising,

x = y
y = x

But there is a problem. For example, let's say that the grader starts by defining x equal to 10 and y equal to 99. If we run the two-line program above, after the first line both x and y will equal 99. The second line has no effect. You can try submitting this solution to the grader to check for yourself what happens. How can we keep the original value of x somewhere, in order to put it into y later?

Hint, part 4

Your first line should be something like xOriginal = x, to store the original value of x for later. Then you can set x = y. Finally, set y equal to the original value of x.

Hint, part 5

Click to read a wikipedia page giving pseudocode (in a different language) for swapping. Here is a link to a second method that uses arithmetic in a sneaky way, but it will only work with numbers and not with text.

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Once you get this exercise correct, you are ready to move on to the next lesson. Click the Next button below.