Chapter 6: while Loops — Indefinite Iteration#

6.1 Why We Need Loops#

Consider this problem: print every integer from 1 to 1,000,000.

With only what you learned in Week 1, you would need one million print() statements, each written by hand. That's absurd. What you actually want to say is: "keep doing this same thing, with a small variation, until some condition is met." That is exactly what a loop does.

A loop is a control structure that repeats a block of code, either:

• A known number of times (definite iteration — Chapter 7's for loop), or
• Until some condition becomes false (indefinite iteration — this chapter's while loop)

6.2 The while Statement#

The while loop repeats a block of code as long as a condition remains True. The syntax mirrors the if statement you already know:

while condition:
    # body — runs repeatedly as long as condition is True
    statement1
    statement2

Here is the simplest possible example:

n = 0
while n < 5:
    print(n)
    n = n + 1

Output:

0
1
2
3
4

Tracing This Loop Step by Step#

This is the most important habit to build this week. Trace every loop you encounter by tracking the value of every variable at every step:

Notice: the condition is checked before each pass through the body, not after. If the condition is False the very first time it's checked, the body never runs at all.

n = 10
while n < 5:
    print(n)   # this line NEVER executes
print("after loop")   # this DOES run

6.3 The Three Parts of Every Counting Loop#

Nearly every while loop that counts something has three essential parts. Missing any one of them is the source of almost every loop bug you will write this semester:

n = 0          # 1. INITIALIZE the loop variable
while n < 5:   # 2. TEST the condition
    print(n)
    n = n + 1  # 3. UPDATE the loop variable

1. Initialize: set the loop variable to its starting value, before the loop begins.
2. Test: check the condition at the top of each pass through the loop.
3. Update: change the loop variable inside the loop body so that, eventually, the condition becomes False.

The #1 beginner mistake with while loops: forgetting the update step. If you forget it, the condition never changes, and the loop runs forever. This is called an infinite loop.

n = 0
while n < 5:
    print(n)
    # forgot n = n + 1 !!
    # This prints 0 forever and never stops.

If you accidentally write an infinite loop while experimenting, you can stop it:

• In a terminal: press Ctrl+C
• In Jupyter: click the ■ (stop) button, or go to Kernel → Interrupt

6.4 Infinite Loops — Sometimes On Purpose#

Not all infinite loops are bugs. Sometimes a program is designed to loop forever until something specific happens — like a server that keeps running until manually shut down, or a game loop that keeps asking the player for moves. Python lets you write this directly:

while True:
    # this body would run forever...
    # unless something inside it stops the loop (we'll learn break in Ch. 10)
    pass

For now, treat while True: with caution — make sure you understand how the loop is supposed to end before using it.

6.5 Sentinel-Controlled Loops#

A very common loop pattern asks the user for input repeatedly until they enter a special "stop" value, called a sentinel:

total = 0
num = int(input("Enter a number (or -1 to stop): "))
while num != -1:
    total = total + num
    num = int(input("Enter a number (or -1 to stop): "))
print(f"Total: {total}")

Notice something important here: input() is called twice — once before the loop (to "prime" the loop with a first value) and once again inside the loop body (to get the next value). This is called the priming read pattern. If you only call input() inside the loop, the condition can never be checked before the first pass.

Trace This Example#

If the user enters: 5, 3, 8, -1

Final output: Total: 16

6.6 A Realistic Example: The "Lost Forest" Game#

Here's a classic teaching example that builds intuition:

where = input("You are lost. Go left or right? ")
while where == "right":
    where = input("Still lost! Go left or right? ")
print("You got out!")

Trace: as long as the user keeps typing "right", the loop keeps asking. The moment they type anything else (including "left", or a typo like "Right" — remember, strings are case-sensitive!), the loop condition becomes False and the program moves on.

6.7 Common Mistakes with while Loops#

Mistake 1: Forgetting the Update Step (Infinite Loop)#

# BUG: infinite loop
count = 0
while count < 10:
    print("hello")
    # forgot count += 1

Mistake 2: Updating the Wrong Variable#

# BUG: x never changes, only y does — infinite loop if testing x
x = 0
y = 0
while x < 5:
    print(x)
    y += 1   # wrong variable updated!

Mistake 3: Off-by-One in the Condition#

# Prints 0 1 2 3 4 -- five numbers, NOT including 5
n = 0
while n < 5:
    print(n)
    n += 1

# Prints 0 1 2 3 4 5 -- six numbers, including 5
n = 0
while n <= 5:
    print(n)
    n += 1

Always ask yourself: "do I want this loop to include the boundary value or not?" This single question prevents the majority of loop bugs.

Mistake 4: Initializing Inside the Loop#

# BUG: total is reset to 0 every single iteration!
while some_condition:
    total = 0          # WRONG — this should be BEFORE the loop
    total += next_value

# CORRECT: initialize once, before the loop
total = 0
while some_condition:
    total += next_value

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