Introduction to Variables

Make sure you are using the robot simulator for these steps.

Step 1: Understanding Python Variables

Python uses an equal sign, =, as an assignment operator. Don't worry, you don't have to remember that = is called an assignment operator. You do have to think about what = means, however.
In math, 4 + 3 = 7 MEANS that four things in one group plus three things in another group is the SAME as seven things. = basically means "the same." That's true in Python, too.
Let's say I wanted to make the Spongebob Squarepants theme song lyrics appear on the screen many times. That means I would have to type:
print("Who lives in a pineapple under the sea? Absorbent and yellow and porous is he.") Over and over and over and over again. That's too much typing!
Instead, I could create a variable using =. That means I am saying that two things, although they may look different are really the same. So, before I give my print command with all the Spongebob lyrics, I could create a shortcut to those lyrics. All I have to do is think up a variable name (it can be whatever you want) and set it equal = to that long text. I'll choose Gary as my variable. So now, I can have a script that looks like this:
Gary = "Who lives in a pineapple under the sea? Absorbent and yellow and porous is he." print(Gary)
The code above is two lines long. Here's what it is doing: In line 1, I said Gary is the same (=) as the message "Who lives in a pineapple under the sea? Absorbent and yellow and porous is he." On the second line I used print, which will put something on the robot's screen, and I told it to print Gary, which is really the lyrics to the song. Try copying the code below into the robot simulator and running it to see what happens:
from time import sleep Gary = "Who lives in a pineapple under the sea? Absorbent and yellow and porous is he." print(Gary) sleep(2) print(Gary) sleep(2) print(Gary) sleep(2) print(Gary) sleep(2)

Step 2: Tips on variables to use with your robot.

Every time I want my robot to move, I have to use a MoveTank command. If I wanted my robot to move in a square, my program will look something like this:
from ev3dev2.motor import MoveTank, OUTPUT_B, OUTPUT_C MoveTank(OUTPUT_B, OUTPUT_C).on_for_rotations(50,50,4) MoveTank(OUTPUT_B, OUTPUT_C).on_for_rotations(50,0,1.6) MoveTank(OUTPUT_B, OUTPUT_C).on_for_rotations(50,50,4) MoveTank(OUTPUT_B, OUTPUT_C).on_for_rotations(50,0,1.6) MoveTank(OUTPUT_B, OUTPUT_C).on_for_rotations(50,50,4) MoveTank(OUTPUT_B, OUTPUT_C).on_for_rotations(50,0,1.6) MoveTank(OUTPUT_B, OUTPUT_C).on_for_rotations(50,50,4) MoveTank(OUTPUT_B, OUTPUT_C).on_for_rotations(50,0,1.6)
That is way too much typing. That's even too much to copy and paste for me. But, I do want you to notice something. What is EXACTLY the same in every single line of MoveTank? Well, this is the same in every single line:
MoveTank(OUTPUT_B, OUTPUT_C).on_for_rotations
The tuple (the three numbers at the end of the command that control speed and rotations) is what changes every time. So every time I need to move, I'm typing MoveTank(OUTPUT_B, OUTPUT_C).on_for_rotations - well, let's use a variable to make that easier!
It is important to remember that a variable is a shortcut. I have to CREATE the shortcut before I USE the shortcut. Here's how I can use a variable to make my move around in a square program above much more simple:
from ev3dev2.motor import MoveTank, OUTPUT_B, OUTPUT_C #I will use the word "move" as my variable name below. move = MoveTank(OUTPUT_B, OUTPUT_C).on_for_rotations #Now the word "move" is the same as typing MoveTank(OUTPUT_B, OUTPUT_C).on_for_rotations move(50,50,4) move(50,0,1.6) move(50,50,4) move(50,0,1.6) move(50,50,4) move(50,0,1.6) move(50,50,4) move(50,0,1.6)
Set your robot in the robot simulator to X: 500 Y: 1000. Copy the code above into the simulator. Click run and see what happens.
HINT: Feel free to copy and paste from this page.