2.1: Variables Part 1: Data Types

By the end of this lesson, you should be able to:

• Define what a variable is in programming
• Explain how variables can be used in a program
• Describe the benefits of using variables in your programs
• List the different types of data in python programs
• Explain why we have different data types and provide examples of when to use the different data types

Steps to Success

1. Why variables?
2. What’s in a name? Variable Naming Best Practices
3. Types of variables
4. Practice Exercises with Variables
5. Going in Squares Again
6. Challenge: The Drone Slide

Why Variables?

If you remember from the last lesson we discussed the concept of abstraction. We said that abstraction was a way of making code short, simpler, easier to use, and easier to understand. Variables are a tool we can use to continue to add abstraction to our code and add an additional feature, reusability! To learn more about variables in Python, watch the video below:

Now that you understand the basics of how variables work, play around with creating variables and changing their values. Try to figure out the answers to the following questions:

1. Are there any numbers, letters, or characters that are not allowed in variable names?
2. Are there any rules that you discovered for variables name?
3. Is there a maximum or minimum length that a variable name can be?
4. If you were going to come up with rules for variable names, what would your rules be?

Now that we know how to create variables, lets look at some of the rules we need to follow when it comes to variables.

So far, we learned to store whole number values like 3 and 2, but these are completely different types of information that we might want to store. For example, how would we store the text of a social media post or keep track of its number of likes? We can use variables! Python supports a number of variable types, but we will focus on four that we will encounter most frequently in our programs. 

• Int: positive and negative whole numbers like 10,-3, or 142
• Float: positive and negative decimal values like 3.14, -1.2, or 54.214
• String: a group of letters and/or numbers such as “Hello World”
• Boolean: Can be set to True or False values

There are a few important rules that you should know regarding data types of variables. First is that strings must be enclosed in single or double quotes ‘’ or “”. For these lessons we will use primarily double quotes. 

Boolean values use what are called reserved words. These are words in a programming language that serve a specific purpose and therefore cannot be used as the names for variables and functions. The two reserved words that Boolean data types use are True and False. They must have the first letter capitalized and the rest lowercase as shown in order to be considered a Boolean value. This also means that you cannot name a variable or procedure True or False:

True = 2 # will result in an error

Going back to our social media post, we might choose to save the number of likes as an integer data type variable while storing the text of the post as a string. We could even create a variable called hasImage which could store the Boolean data type telling us whether or not the post has an image attached. What other information might we store and as what data types?

Now that we know a little about data types, let’s try creating some different types of variables. In the python console, enter the following:

>>>w = 4             
>>>x = 3.2           
>>>phrase = "Hello"  
>>>is_flying = True

Once you have entered these in the console, look at the variables pane on the right. Next to the name of each variable after the equals sign you should see in parenthesis what the data type is as shown in the image below:

Now let’s try changing some of the variable values and see what happens. Enter the following in the console:

>>>x=3

What is the variable type that you see for x now? It was a float before but now it is an int!. Let’s try seeing if we can change the string variable phrase to something else:

>>>phrase=3.14

You should see that phrase is now a float instead of a string. This is because we assigned it a new value that was a decimal. 

Now that we have a better understanding of the different data types, try the following exercises on your own:

1. Change the value of is_flying to False
2. Create 4 variables, each representing one of the four data types we discussed
3. Try to create a variable with a phone number as its value. 
   1. What happens?
   2. How would you fix this issue?
4. Try to create a variable with an email as its value. 
   1. What happens?
   2. How would you fix this issue?
5. What data type do emails and phone numbers have to be stored as?
6. What data type should you store a date as?
7. Create two variables a and b. Enter the command a=b
   1. What happens?
   2. What happens when you change the value of a? b?

Now that we have a better understanding of the purpose of variables in general, let’s look at an example where their use makes a program that we have written simpler. To get set up, complete the following steps:

1. Create a new directory called 3_variables
2. Create a new file called 3_1_square
3. Copy the code from where you rewrote the the square challenge with the go() command

Here is an example of what the code could look like:

from CoDrone_mini import Direction # Using the go() command so need to import Direction
import CoDrone_mini
drone = CoDrone_mini.CoDrone()
drone.pair()
drone.takeoff()
drone.go(Direction.FORWARD,2,20) #first leg of square
drone.go(Direction.LEFT,2,20)
drone.go(Direction.BACKWARD,2,20)
drone.go(Direction.RIGHT,2,20) # last leg of square
drone.land()
drone.close()

Now, we will add two variables called duration and power:

duration = 2 # integer value for how long to fly in a direction
power = 50 # integer value for the power setting of the drone

We will now substitute the variables for the parameters in our go() commands as shown below:

drone.go(Direction.FORWARD,duration,power)
drone.go(Direction.LEFT,duration,power)
drone.go(Direction.BACKWARD,duration,power)
drone.go(Direction.RIGHT,duration,power)

You may be thinking, “That didn’t really save me any time and my code is actually a little bit longer. So why would I use variables here?” The answer is that since we are flying in a square, all we have to do is change the values of the duration and power variables to make any size square we want. This code is now easier to read and understand since we used names that describe what the variables are and the code is much more useful as it can create a wide variety of squares with minimal change. 

Now that you know how to create variables, let’s use them in a program with CoDrone Mini! For this challenge, you need to have your drone fly a zig-zag pattern like the one shown below:

Rules: 

1. Name the file 3_1_challenge
2. You must use at least 2 integer or float variables
3. The forward motions in the zig-zag must be the same length
4. The side to side motions in the zig-zag must be the same length