Python While Loops

Learn how to use while loops in Python to repeat code execution based on conditions.

Python Loops

Python has two primitive loop commands:

while loops
for loops

The while Loop

With the while loop we can execute a set of statements as long as a condition is true.

Example - Print i as long as i is less than 6:

i = 1
while i < 6:
    print(i)
    i += 1

Note: remember to increment i, or else the loop will continue forever.

The while loop requires relevant variables to be ready, in this example we need to define an indexing variable, i, which we set to 1.

The break Statement

With the break statement we can stop the loop even if the while condition is true:

Example - Exit the loop when i is 3:

i = 1
while i < 6:
    print(i)
    if i == 3:
        break
    i += 1

The continue Statement

With the continue statement we can stop the current iteration, and continue with the next:

Example - Continue to the next iteration if i is 3:

i = 0
while i < 6:
    i += 1
    if i == 3:
        continue
    print(i)

The else Statement

With the else statement we can run a block of code once when the condition no longer is true:

Example - Print a message once the condition is false:

i = 1
while i < 6:
    print(i)
    i += 1
else:
    print("i is no longer less than 6")

Practical Examples

Countdown Timer

countdown = 5
while countdown > 0:
    print(f"Countdown: {countdown}")
    countdown -= 1
print("Blast off!")

User Input Validation

password = ""
while password != "secret":
    password = input("Enter password: ")
    if password != "secret":
        print("Incorrect password, try again.")
print("Access granted!")

Sum of Numbers

total = 0
number = 1
while number <= 10:
    total += number
    number += 1
print(f"Sum of numbers 1 to 10: {total}")

Finding Factors

num = 12
factor = 1
factors = []

while factor <= num:
    if num % factor == 0:
        factors.append(factor)
    factor += 1

print(f"Factors of {num}: {factors}")

Infinite Loops

Be careful with while loops! If the condition never becomes false, the loop will run forever:

Example - This will run forever:

i = 1
while i < 6:
    print(i)
    # i is never incremented, so it will always be 1

To avoid infinite loops:

Always ensure the loop variable is modified inside the loop
Make sure the condition can eventually become false
Use a counter or maximum iterations as a safety measure

Example - Safe loop with maximum iterations:

i = 1
max_iterations = 100
iterations = 0

while i < 6 and iterations < max_iterations:
    print(i)
    i += 1
    iterations += 1

if iterations >= max_iterations:
    print("Loop stopped due to maximum iterations")

Nested While Loops

You can have while loops inside while loops:

Example - Multiplication table:

i = 1
while i <= 3:
    j = 1
    while j <= 3:
        print(f"{i} x {j} = {i * j}")
        j += 1
    print("---")
    i += 1

While Loop vs For Loop

When to use while loops vs for loops:

Use While Loop When:

You don't know how many iterations you need
The loop depends on a condition that may change unpredictably
You need to loop until user input meets criteria
You're waiting for an external condition

Use For Loop When:

You know the number of iterations in advance
You're iterating over a collection (list, string, etc.)
You need a counter variable
You're working with ranges of numbers