INTRO   O

PROBLEM SOLVING AND

PROGRAMMING IN PYTHON

 

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

Example: Launch Countdown! 

countdown = 5

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

print("🚀 Liftoff!")
  1. Runs while countdown > 0.
  2. Decreasing countdown.
  3. 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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By Andrea Gallegati

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