The while Loop in Python

Introduction

The while loop executes a code block as long as a condition is True. It’s useful when we don’t know in advance how many times to repeat.

Basic Syntax

while condition:
    # code block
    # executes while condition is True

Simple example:

counter = 1

while counter <= 5:
    print(counter)
    counter += 1  # Important: update the condition!

# Displays: 1 2 3 4 5

Anatomy of a while Loop

# 1. Initialization
i = 0

# 2. Continuation condition
while i < 5:
    # 3. Loop body
    print(i)

    # 4. Update (avoid infinite loop!)
    i += 1

Infinite Loops

An infinite loop occurs when the condition never becomes False:

# WRONG - infinite loop!
i = 0
while i < 5:
    print(i)
    # Missing: i += 1

# CORRECT
i = 0
while i < 5:
    print(i)
    i += 1

Intentional infinite loop:

while True:
    command = input("Command: ")
    if command == "exit":
        break  # Exits the loop
    print(f"You wrote: {command}")

The break Statement

break immediately exits the loop:

i = 0
while i < 10:
    if i == 5:
        break  # Exits when i == 5
    print(i)
    i += 1

# Displays: 0 1 2 3 4

Searching for an element:

numbers = [3, 7, 2, 9, 5]
target = 9
found = False
i = 0

while i < len(numbers):
    if numbers[i] == target:
        found = True
        break
    i += 1

print(f"Found: {found}")  # Found: True

The continue Statement

continue skips to the next iteration:

i = 0
while i < 5:
    i += 1
    if i == 3:
        continue  # Skips print when i == 3
    print(i)

# Displays: 1 2 4 5 (without 3)

Filtering numbers:

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

# Displays: 1 3 5 7 9

The else Clause with while

The else block executes when condition becomes False (not on break):

i = 0
while i < 3:
    print(i)
    i += 1
else:
    print("Loop completed normally")

# Displays: 0 1 2 Loop completed normally

With break:

i = 0
while i < 5:
    if i == 3:
        break
    print(i)
    i += 1
else:
    print("This message won't appear")  # Doesn't execute!

# Displays: 0 1 2

Practical application - search:

numbers = [1, 3, 5, 7, 9]
target = 4

i = 0
while i < len(numbers):
    if numbers[i] == target:
        print(f"Found at index {i}")
        break
    i += 1
else:
    print("Not found")  # Displays

# Displays: Not found

Common Patterns

Reading until special value:

total = 0
while True:
    number = int(input("Number (0 = stop): "))
    if number == 0:
        break
    total += number
print(f"Total: {total}")

Input validation:

while True:
    age = input("Age (1-120): ")
    if age.isdigit():
        age = int(age)
        if 1 <= age <= 120:
            break
    print("Invalid age!")
print(f"Age: {age}")

Countdown:

n = 5
while n > 0:
    print(n)
    n -= 1
print("Start!")

# 5 4 3 2 1 Start!

while vs for Comparison

# for - when you know iteration count
for i in range(5):
    print(i)

# while - when you don't know iteration count
answer = ""
while answer != "yes":
    answer = input("Continue? ")

Common Mistakes

1. Accidental infinite loop

i = 0
while i < 5:
    print(i)
    # Missing: i += 1

2. Wrong condition

i = 10
while i > 5:  # Never False if i increases
    print(i)
    i += 1    # Should be i -= 1

3. Off-by-one errors

# Want 5 iterations but get 4
i = 1
while i < 5:  # 1,2,3,4 (4 iterations)
    print(i)
    i += 1

# Correct for 5 iterations
i = 1
while i <= 5:  # 1,2,3,4,5 (5 iterations)
    print(i)
    i += 1

Key Points for Exam

• while repeats as long as condition is True
• break exits the loop immediately
• continue skips to next iteration
• else with while only executes if no break
• Ensure condition becomes False at some point
• Use while True + break for conditionally exited loops

Review Questions

1. When does the else block execute in a while loop?
2. What’s the difference between break and continue?
3. How do you avoid an infinite loop?
4. When do you use while instead of for?