1.7: For Loops

Goals:

• Identify the purpose of loops
• Write and execute different types of For loops
• Identify when to use a For loop in a program
• Identify the components of the range command
• Use the range command to count up, down, and in unique increments
• Describe other ways to count and print using a for loop
• Utilize loop knowledge to create flight patterns for different types of polygons
• Explain what is a private or local variable and how they are used in loops

Step for Success:

• What are Loops?
• Anatomy of a FOR Loop
• First FOR Loops
• A More Efficient Square
• Challenge: Fly a Polygon

To get started, make sure you have a directory for your files in pycharm called Lesson_2.

Once you have the directory created, create a file called 2_3_for_loops.

When we want our drone to repeat tasks, we use loops. There are two types of loops that we will learn in this lesson and the next:

• FOR loops: These loops are generally used to count up or down a set number of times. They are great for doing activities where we know how many times we want the drone to do that activity.
• WHILE loops: These loops are used to perform an activity until a CONDITION is met. They are great for activities where we DON’T know a set number of times that a loop needs to run.

For example:

FOR loop: Think back to the box mission you flew in Unit 1 (Where you had to fly in a square). This is a perfect opportunity to use a for loop because we know a square has 4 sides so we can write a loop that executes exactly 4 times.

WHILE loop: Imagine that we wanted our drone to fly up to a wall without hitting it. We might use a while loop and the ultrasonic sensor to fly until it got to the distance from the wall that we want. We could place the drone anywhere and it would always stop this distance from the wall. Since we don’t know the exact distance, we use a WHILE loop.

Loops are what we call CONTROL STRUCTURES. They control the order that steps are performed in a program by repeating steps multiple times. Normally, our program would just follow the steps in order without repeating.

Loops have two primary parts:

Loop condition: The rule that keeps the loop running. The loop will continue to run as long as the condition is True.

Loop body: Where the commands to execute are placed. These are the indented lines after the loop condition.

Loops also contain what is called a private or local variable. This variable is used in the loop condition to keep count or track of how many times our loop has executed. Since this variable is only used for the loop, good style is to give it a very simple name like i, j, k, x, etc…

In the video below, we see the parts of a FOR loop and how they are written in a Python program.

Now that we understand the parts of a FOR loop, let’s practice some examples!

To recap, in the video we saw that the range command has 3 inputs:

• The minimum (start) is the value that your variable will start at.
• The maximum (end) is the value that your variable will end at.
• Increment is how much will be added to your variable every time, and decrement is how much will be subtracted from your variable every time. A positive increment counts up, a negative counts down.

Try the practice exercises below to see if you can create some more advanced loops:

1. Print the values 5-20 by fives
2. Print the all the even numbers between 100 and 200
3. Print the count down from 20 to 10 (including 10)
4. Add up all the numbers from 50 to 100 (not including 100) and print the total value
5. Print CoDrone EDU letter by letter
6. Count and then print the number of characters (including the space) in “Hello World”
7. Print “I like to fly” 5 times

Program your CoDrone EDU to fly in a square. Remember that computers start counting at 0 and 4 is the maximum.

from codrone_edu.drone import *

drone = Drone()

drone.pair()


drone.takeoff()

for x in range(0, 4, 1):

    drone.set_pitch(50)   

    drone.move(2) 

    drone.set_pitch(0)    

    drone.set_yaw(-50)   

    drone.move(2)

    drone.set_yaw(0)      

drone.land()

drone.close()

Next, try other shapes, like triangles, pentagons, and hexagons!

For this challenge, we want your drone to fly in a polygon shape of your choosing. You can choose:

• Pentagon
• Hexagon
• Octagon

To fly these properly, you will need to calculate how much you will have to turn to create the appropriate angles for each shape. The chart below demonstrates how to calculate the angles. You will need to be able to choose your polygon, then make the turns so that they turn the correct amount to make the shape (approximately). For example, a square has 360 total interior degrees.

360/4 = 90 degrees per corner

This means that at each turn, the drone would need to make a 90 degree turn. You would then need to determine how much yaw for how long will equate to a 90 degree turn. For the polygon you choose, you will need to do the same.

(Hint: figure out 90 and then use it as a ratio for your turn, for example 60 degrees is 2/3 of 90 degrees so take 2/3 of the time from the Yaw movement that got you a 90 degree turn and you will have a 60 degree turn. )

In this lesson, you learned about the concept of loops. You learned about the different kinds of loops as well as when to use those different kinds. Loops are very useful for reducing long and repetitive code, so make sure to use them as much as you can!