2A: Variables

Before we get into variables, you need to understand a situation where they would be useful. let’s review how to write a program for a square.

First, write a program that flies CoDrone Mini in a square. For each side, CoDrone Mini should fly half a second (0.5 seconds).

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

import CoDrone_mini

drone = CoDrone_mini.CoDrone()

drone.pair()


drone.takeoff()           
drone.hover(3)            


drone.set_pitch(30)       
drone.move(0.5)            


drone.set_pitch(0)       
drone.set_roll(-30)      
drone.move(0.5)       


drone.set_roll(0)         
drone.set_pitch(-30)      
drone.move(0.5)            


drone.set_pitch(0)        
drone.set_roll(30)        
drone.move(0.5)            

drone.land()

Now, make your square bigger by increasing the duration of all of the move commands from 0.5 to 1.

Now, make it even bigger by increasing the seconds of all of the move commands again.

Phew! That’s a lot of work to change each line of code every time. How do you think we can make this code more efficient?

Variables are important in all programming languages and in most programs! Just like in math, variables in programming are placeholders and data storage for numbers. Typically, you can create, or declare, a variable and then save, or initialize, its value at the beginning of your program. You can then call that value later on in the program whenever you use the variable.

x = 2     # Declaring variable x and initializing the value to 2

print(x)  # Calling the variable in a print statement

You can declare multiple variables to hold many values. Just be sure to give them different names, because a variable can change values when running a program. If you make a new variable with the same name as a previous one, the new value will override the old value.

x = 2    # Declaring a variable x and initializing it to 2

print(“Old Value: ”,x) # Printing initial value of x

x = 3    # Reassigning variable x to 3

print(“New Value: “,x) # Printing new value of x

You can also assign variables to other types of data, like  int ,  float ,  string , and  boolean .

w = 4             # this is an int

x = 3.2           # this is a float

phrase = "Hello"  # this is a string

is_flying = True  # this is a boolean

• Int: a negative or positive whole number
• Float: decimal values
• String: a sequence of characters enclosed in quotation marks
• Boolean: can be set to True or False

The great thing about Python is that you don’t need to include the data type in the variable initialization. In a language like Arduino, you might need to write  int w = 4 , but with Python, you can just write  w = 4 .

Now that you know how to create variables, let’s use them in a program with CoDrone Mini! Create a variable called  duration and assign it to an integer value that will represent the number of seconds that you want the drone to perform an action. Next, call that variable inside functions in your program.

import CoDrone_mini

drone = CoDrone_mini.CoDrone()

drone.pair()

duration = 3                # initialize variable 'duration' to 3

drone.takeoff()

drone.set_pitch(50)           # Set positive pitch to 50% power

drone.move(duration)          # Pitch forward for 3 seconds

drone.land()

drone.close()

You can call a variable’s value multiple times in a program. For example, take a look at this code for flying in a square.

import CoDrone_mini

drone = CoDrone_mini.CoDrone()

drone.pair()

drone.takeoff()

drone.set_pitch(50)

drone.move(1)

drone.set_pitch(0)

drone.set_roll(50)

drone.move(1)

drone.set_pitch(-50)

drone.set_roll(0)

drone.move(1)

drone.set_pitch(0)

drone.set_roll(-50)

drone.move(1)

drone.land()

Now what if you wanted to make the square bigger? You would have to individually change each  move() command’s duration. Instead, you can create a variable to represent duration and only change it once!

import CoDrone_mini

drone = CoDrone_mini.CoDrone()

drone.pair()

duration = 1

drone.takeoff()

drone.set_pitch(50)

drone.move(duration)

drone.set_pitch(0)

drone.set_roll(50)

drone.move(duration)

drone.set_pitch(-50)

drone.set_roll(0)

drone.move(duration)

drone.set_pitch(0)

drone.set_roll(-50)

drone.move(duration)

drone.land()

Change the value of duration to make the square bigger or smaller. To make even more precise movements, you can assign  duration to a different data type. Try assigning  duration to a float, string, or boolean. Do they work? Why or why not?

duration = 2.5

duration = “4”

duration = True

You can also change the value of duration in the middle of your program! Take a look at this program and guess what will happen to the hovering behavior throughout the program.

import CoDrone_mini

drone = CoDrone_mini.CoDrone()

drone.pair()


duration = 1

drone.takeoff()

drone.hover(duration)

drone.land()

duration = 2

drone.takeoff()

drone.hover(duration)

drone.land()

duration = 3

drone.takeoff()

drone.hover(duration)

drone.land()

Did you notice that everytime the variable is reassigned, it takes on the new value?

Just like in your math class, you can do math with your variables. These are some of the operations you can use:

• + : addition 
• – : subtraction 
• * : multiplication 
• / : division 
• %: modulus 

What do you think the following code will output? Try it out! 

y = 4

z = 3

x = y + z

print(x)

Instead of creating a new variable to hold the sum, you can also add values directly to the variable. For example,

power = 5 # initialize a variable to 5

print(power)

power = power + 5 # add 5 to power and reassign the value

print(power)

You can also subtract, multiply, and divide in the same way.

If you have done some algebra in your math class, this might look a little strange. In programming, always evaluate the right side first of the equal sign and then assign the value to the variable on the left side.

You can create as many variables to a program as you like! Just follow the same process for each one: create a variable, initialize its value at the top of your program, and then call it in your program when you need it.

In your last programs, you set a variable for  duration . Include a variable for power, set its value to 25, and then run your program. Next, change its value to 75. What happens when the power variable’s value increases?

import CoDrone_mini

drone = CoDrone_mini.CoDrone()

drone.pair()


duration = 5             # initialize variable 'duration' to 5

power = 25 # initialize variable ‘power’ to 25

drone.takeoff()

drone.hover(duration)         # in this case, hover for 5 seconds

drone.set_pitch(power)        # Set positive pitch to 50% power

drone.move(duration)          # forward for 5 seconds

drone.land()

drone.close()

Challenge: Variable 

Create at least 5 variables and use math operations on them to make them equal to power values. 

Before you run your program, don’t forget to import your libraries, create your drone object, and pair your CoDrone Mini!

Rules:

1. You can’t use any numbers after you initialize the variables!
2. You must use one float value.