1.8: While Loops

Lesson Goals

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

• Explain the difference between FOR and WHILE loops
• Identify when to use a WHILE loop
• Identify the parts of a WHILE loop condition
• Create custom loop conditions based on provided values and variables
• Create and use random integers in a program
• Use the Python console to test conditions
• Identify the different logical operators and how they are used in Python programs

Steps to Success:

1. What are loops?
2. While loops
3. While Loop Practice
4. Making Comparisons
5. Checking our Logic
6. Drone Sways with While Loops
7. Well that was Random!
8. Challenge 1: Tower of Terror
9. Challenge 2: Boomerang

In the previous lesson, we learned about loops, and more specifically, FOR loops. In this lesson, we will learn about WHILE loops. Both FOR and WHILE loops have the same basic parts:

Loop Condition: What causes the loop to continue or to stop. Typically in the form of an inequality (for example i < 5).

Loop Body: What the loop will do or execute. The steps that the loop will repeat until it is done.

The most important difference between FOR and WHILE loops is their purpose:

FOR LOOPS: Use when we have a set number of times we want the loop to run.

WHILE LOOPS: Use when we DON’T know how many times the loop will run.

See the video below for how to write a WHILE loop and some examples of when we might use WHILE loops:

As you can see, WHILE loops are often used to make our drones seem more intelligent by having them respond to their environments.

Now that we understand what a WHILE loop is and how it works, let’s look at one of the more important parts, the condition. The condition typically has 3 parts as shown in the diagram below:

As we see above, the condition compares two values that will result in a True or False (A Boolean value). The WHILE loop checks this condition each time it runs. If the condition evaluates to True, the loop will continue. If it evaluates to False, the loop will stop.  Let’s practice writing some conditions that we might set up for a WHILE loop.

For the questions below, write out the LOOP CONDITION only. The first question is given as an example:

1. The loop will run as long as the variable day is “Wednesday”
   Answer: while day = “Wednesday”:
2. The loop will run as long as the variable home_score is greater than the variable away_score
3. The loop will run as long as the variable stop_date is not true
4. The loop will run as long as the variable battery_power is greater than 0%
5. The loop will run as long as the variable progress_percent is less than 100%
6. The loop will run as long as the variable error is False (There are two ways to write this answer. See if you can figure out both!)

When creating conditions, we will often want to make different comparisons based on mathematical values. Python uses many of the logical operators that you will see in math class and they function in the same way.

There are a number of different logical operators:

• Greater Than (>) : Is the value on the left  bigger than the value on the right?
• Less Than (<) : Is the value on the left  smaller than the value on the right?
• Equal To (==) : Is the value on the left  equal to the value on the right?
• Not (!) : Is the value on the left not the same as the value on the right?

You may have noticed that Equal To is a double equals sign. This is not a typo! Remember that a single equals sign is used to ASSIGN values where a double equals is used to TEST whether or not two values are the same.

We can even combine these operators to make more complex comparisons:

• Greater Than or Equal To (>=) : Is the value on the left  bigger  or equal to the value on the right?
• Less Than or Equal To (<=) : Is the value on the left  smaller  or equal to the value on the right?
• Not Equal To (!=) : Is the value on the left not the same as the value on the right?

Let’s practice some examples of how these logical tests work! Test each of these examples with a print() function. For example, the first one should be:

print(1>2)

• 1>2
• 3>2
• 5>9
• 8<10
• 10 <= 10
• 5 >= 6
• 3==3
• 2==3
• “Hello” == “Hello”
• “hello” == “Hello”
• 5 != 4
• 3 != 3
• !False
• !True

Now that we have a better understanding of how these types of comparisons work, let’s try some exercises on your own. For each question, do you get True, False, or an Error?

• 382 > 438
• 32<56
• True < True
• False == False
• True !=False
• “World” =”world”
• “Codrone” == “Codrone”

Program your CoDrone EDU to do pushups using a while loop. Have the condition be true only until a variable passes 10.

from codrone_edu.drone import *
drone = Drone()
drone.pair()

sways = 0

drone.takeoff()

while sways < 3:

    drone.set_roll(40)

    drone.move(1)

    drone.set_roll(-40)

    drone.move(1)

    sways += 1   # increment sways by 1

drone.land()

P.S.: While loops can go on forever if you set the condition in the while loop to be “1” or “True”.

An example of running a while loop until it finishes without knowing the end point can be seen using random numbers. To generate random numbers, we use the Random Library. If you remember libraries from our first lesson, they are just collections of tools that add on to the Python programming language. In this case, the random library has multiple different functions that generate all sorts of random numbers. The function that we will use is randint(a,b). In this function, a is the starting integer and b is the ending integer (very similar to the range function we learned in the FOR loop lesson. When you call this function, it will generate a random integer that falls between and/or including a and b.

Create a file in your lesson directory called thats_random.py.

Type the following code and run the file:

from random import randint
myRand = 0
while myRand < 10:
    print(myRand)
    myRand = randint(1,10)

Respond to the following questions:

1. When you run the program the first time, how many times did the loop execute?
2. When you run the program again, how many times did the loop execute?
3. Do the numbers change when you run the program multiple times?

As we see with this example, the same loop ran a different number of times each time we ran the program. However, they always stopped when we reached the value 10. This is why WHILE loops are so valuable, no matter how many times the loop executes, all we need to know is what should make the loop stop!

If you want to learn more about the random library in python, check out the docs HERE

Challenge: Tower of Terror

The Tower of Terror at a few of the Disney parks is a drop tower ride. These rides typically take riders up very high and very slowly, and then drop down very quickly. With a lot of drop tower rides, the riders go down, pause, go up a little bit but not that high, pause, go down, pause…you get it.

Your challenge is to use loops to program your CoDrone EDU to fly in the same pattern as the Tower of Terror!

Rules:

• You must use either a for or while loop.
• Your drone, after going up, must go down, pause, go up, and then pause at least three times.
• You must use the randomint(a,b) function at least 2 times to vary the speed and timing of the drops.

Below is an example (NOTE it DOES NOT include the random component, you would need to add that to your code):

from codrone_edu.drone import *

drone = Drone()

drone.pair()


drone.takeoff() 

drone.set_throttle(50)

drone.move(2)

drone.hover(1)


for x in range(0, 5, 1):

    drone.set_throttle(50)

    drone.move(2)

    drone.hover(1)

    drone.set_throttle(-40)

    drone.move(2)

    drone.hover(1)

    
drone.land()

drone.close()

Challenge: Boomerang

For this challenge you will create a program where your drone will fly in 3 random directions and then return back to its starting position. For this challenge, you need to meet the following requirements:

1. Your drone must move in 3 random directions
2. Your drone must store the values of its movements
3. Your drone must use the stored values to return to its original starting point

You may use the go function or individual movement functions (pitch, yaw, roll, throttle) to make the drone move.

In this lesson, you learned about WHILE loops and how they differ from FOR loops. You also learned more about conditions used in loops and how to create conditions based on variables and values. Make sure you know which type of loop to use in each situation!