INTRO O

PROBLEM SOLVING AND

PROGRAMMING IN PYTHON

(use the Space key to navigate through all slides)

Prof. Andrea Gallegati
tuj81353@temple.edu

Prof. Dario Abbondanza
tuk96119@temple.edu

CIS 1051 - WHILE LOOP

Understanding the

statement

REPEATING Actions in Code

while

Repeating Until a Condition is Met

Keep asking for a password until the correct one.
Keep rolling a dice until you get a 6.
Count down from 10 to 1 before a rocket launch.

`while` loops
repeat actions
as long as a condition remains true.

Syntax:

write Python programs that
repeat the same code execution

while condition:
    # Code to repeat (indented block)
	pass

The condition is checked before each iteration.
If it's True, the loop executes.
If it's False, the loop stops.

Example: Counting Up

count = 1  # Start at 1

while count <= 5:  # Keep going while count is ≤ 5
    print("Count:", count)
    count += 1  # Increase count by 1

count starts at 1.
Each time, count increases.
The loop stops at 6.

Example: Launch Countdown!

countdown = 5

while countdown > 0:
    print("T-minus", countdown)
    countdown -= 1  # Reduce countdown

print("🚀 Liftoff!")

Runs while countdown > 0.
Decreasing countdown.
Stops, "Liftoff!" prints.

Example: Using it for User Input

password = ""

while password != "python123":  # Keep asking
    password = input("Enter the password: ")

print("Access granted! ✅")

Useful when we don't know how many times we'll repeat an action.

entering "python123", the loop stops.
for anything else, the loop keeps asksing.

Using `break` to Exit a Loop Early

number = 1

while number <= 10:
    print("Checking number:", number)
    if number == 7:  # Stop when number is 7
        print("Number 7 found! Stopping loop.")
        break
    number += 1

Sometimes, we need to exit a loop immediately.

break exits the loop even if the condition is still True.

Using `continue` to Skip an Iteration

number = 0

while number < 10:
    number += 1
    if number % 2 == 0:
        continue  # Skip even numbers
    print(number)

It skips the rest of the loop's code for the current iteration.

if even, continue skips
the print() statement.

Using `else` within a `while` Loop

number = 1

while number <= 5:
    print(number)
    number += 1
else:
    print("Loop finished successfully!")

else block runs if the loop
completes normally (without break).

no break, then else block runs.

⚠️ Danger ZONE

If the loop condition never becomes `False`, it runs forever.

✅ To avoid infinite loops, make sure:

condition eventually becomes False.
modify variables inside the loop (like increasing count).

Example: Infinite Loop

while True:
    print("This loop runs forever!")

If the loop condition never becomes False, it runs forever.

If always True, it never stops.
Ctrl+C to STOP the program.

Example: Simulating a Dice Roll `🎲`

import random

roll = 0

while roll != 6:
    roll = random.randint(1, 6)  # Roll a die (1 to 6)
    print("Rolled:", roll)

print("🎲 Got a 6! Loop stopped.")

we don’t know how many times
it will run!

it keeps rolling until we get 6
the number is random, thus we don’t know how many rolls it takes

Recap

while loop repeats actions as long as condition is True.
Use break to exit a loop immediately.
Use continue to skip an iteration.
while loop can have an else block
(runs if there’s no break).

Be careful of infinite loops—always make sure the condition eventually becomes False.

Try It Yourself!

write a program that keeps asking the user for a number (greater than 10) until they enter it!

number = 0  # Start with a number less than 10

while number <= 10:
    number = int(input("Enter a number greater than 10: "))

print("Thanks! You entered", number)

Great Job! 🎉

Now you understand while loops and how to repeat actions until a condition is met!

Try experimenting with your
own conditions! 🚀

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