While Loops Review

Thursday, September 12th, 2024 | CSCI100: Intro to Computer Science

What are while loops and why do we use them?

• Remember that while loops allow us to run a piece of code repeatedly as long as a condition is met
• Some ways that this shows up in our day-to-day are:
• Driving a car (while you are pressing the gas pedal, the car accelerates)
• Playing a game of UNO (while everyone has cards, we keep playing the game)
• Participating in spelling bee (while you continue to spell words correctly, you can still compete)

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• If we were to visualize while loops as a decision tree: it would look something like this:

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While loop syntax

• Generally while loops are written as follows:

while boolean_expression:

# statements to run

# if boolean expression evaluates to True

• Python uses indentation to identify code that is and is not inside a while loop. This is known as scope

i = 10

while i < 15:

print(i, “is less than 15”)

i += 1

print(“I’m not in the while loop”)

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Code Reading Exercise

Code Reading Exercise

num1 is greater than 0 so go into the loop

Code Reading Exercise

num2 equals num2 * 10 (0) + num1 % 10 (3)

So num2 = 3

Code Reading Exercise

Divide the current value of num1 by 10 and update num1 to that quotient. Num1 is now 524

Code Reading Exercise

Num1 (524) is greater than 0 so go into the loop

Code Reading Exercise

Num2 (3) equals num2 * 10 (30) + num1 % 10 (4)

So num2 = 34

Code Reading Exercise

Divide the current value of num1 by 10 and update num1 to that quotient. Num1 is now 52

Code Reading Exercise

Num1 (52) is greater than 0 so go into the loop

Code Reading Exercise

Num2 (34) equals num2 * 10 (340) + num1 % 10 (2)

So num2 = 342

Code Reading Exercise

Divide the current value of num1 by 10 and update num1 to that quotient. Num1 is now 5

Code Reading Exercise

Num1 (5) is greater than 0 so go into the loop

Code Reading Exercise

Num2 (342) equals num2 * 10 (3450) + num1 % 10 (5)

So num2 = 3425

Code Reading Exercise

Divide the current value of num1 by 10 and update num1 to that quotient. Num1 is now 0

Code Reading Exercise

Num1 (0) is not greater than 0 so don’t go into the loop

Code Reading Exercise

Print the values of num 1 and num2

num1: 0

num2: 3425

Code Writing Exercise

• Write a program to ask the user to enter their password (the password is “M!llIon@!r1”). They get 3 attempts to guess their password, and if they don’t get it in 3 tries, tell them to try again later
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While loop Tips

• Remember that while loops run as long as a condition is true, so make sure that the boolean expression in your while statement reflects that
• Make sure that you prevent infinite loops by changing the value of the variable in your while statement
• If you do get an infinite loop, you can end the program by pressing CTRL+C