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Module 014: Loops: while

Phase: 2. Control Flow & Data | Estimated time: 2 hours | Milestone Project: No

Prerequisites

Learning Objectives

  • Write while loops with proper conditions
  • Avoid and escape infinite loops
  • Use while loops for input validation
  • Implement sentinel-controlled loops
  • Apply the accumulator pattern (summing, counting)

Why This Matters

Loops let you repeat work without writing the same code over and over. The while loop is the most flexible loop — it keeps going as long as a condition holds true. You'll use it for input validation (keep asking until you get valid input), game loops, and any situation where you don't know in advance how many iterations you need.

Concept Explanation

The while Loop

A while loop repeats a block of code as long as its condition is truthy:

count = 0
while count < 3:
    print(f"Count is {count}")
    count = count + 1
# Output:
# Count is 0
# Count is 1
# Count is 2

Flow:

  1. Check the condition (count < 3)
  2. If truthy, run the indented block
  3. Go back to step 1
  4. If falsy, skip the block and continue

The Loop Variable

The variable in the condition must change inside the loop, or the loop will run forever:

# This loop will terminate because `i` changes each time
i = 1
while i <= 5:
    print(i)
    i = i + 1
# Output: 1 2 3 4 5

Infinite Loops

An infinite loop occurs when the condition never becomes falsy:

# DANGER: Infinite loop!
# i = 1
# while i > 0:
#     print(i)
#     i = i + 1  # i keeps growing, always > 0

If you run an infinite loop in a terminal, press Ctrl+C to stop it.

Ways to avoid infinite loops:

  • Ensure the condition variable changes toward the exit condition
  • Add a maximum iteration counter as a safety net
  • Use break (covered in Module 016)

Input Validation with while

A common pattern: keep asking for input until you get something valid:

age = None
while age is None:
    user_input = input("Enter your age: ")
    if user_input.isdigit():
        age = int(user_input)
    else:
        print("Invalid input. Please enter a number.")

print(f"You are {age} years old.")

Sentinel Values

A sentinel is a special value that signals "stop":

total = 0
print("Enter numbers to sum. Enter -1 to stop.")

num = int(input("Enter a number: "))
while num != -1:
    total = total + num
    num = int(input("Enter a number: "))

print(f"Total: {total}")

The Accumulator Pattern

Build up a result inside a loop:

Summing:

total = 0      # accumulator starts at 0 for sums
i = 1
while i <= 100:
    total = total + i
    i = i + 1
print(f"Sum of 1 to 100: {total}")  # Sum of 1 to 100: 5050

Counting:

count = 0      # accumulator starts at 0 for counts
i = 1
while i <= 100:
    if i % 2 == 0:
        count = count + 1
    i = i + 1
print(f"Count of even numbers 1-100: {count}")  # Count of even numbers 1-100: 50

Product (multiplication):

product = 1    # accumulator starts at 1 for products
i = 1
while i <= 5:
    product = product * i
    i = i + 1
print(f"5! = {product}")  # 5! = 120

Common Pitfalls

  1. Forgetting to update the loop variable — This causes an infinite loop. Always ensure the condition will eventually become falsy.

  2. Off-by-one errors — If you need exactly N iterations, double-check your condition. while i < N runs N times (i = 0, 1, ..., N-1). while i <= N runs N+1 times.

  3. Using = instead of == in the condition — while x = 5 is an assignment (which Python 3.12 disallows in this context), not a comparison.

  4. Not handling invalid input — If int() receives non-numeric input, the program crashes with a ValueError. Always validate before converting.

  5. Starting accumulator with wrong value — Sums start at 0, products start at 1. Starting a product at 0 gives 0 every time.

Hands-On Walkthrough

Let's build a guessing game with input validation:

import random

secret = random.randint(1, 100)
guess = None
attempts = 0

while guess != secret:
    user_input = input("Guess the number (1-100): ")
    if user_input.isdigit():
        guess = int(user_input)
        attempts = attempts + 1
        if guess < secret:
            print("Too low!")
        elif guess > secret:
            print("Too high!")
    else:
        print("Please enter a valid number.")

print(f"Correct! You got it in {attempts} attempts.")

Now a calculator loop using sentinel:

print("Simple Calculator")
print("Enter 'q' to quit.")

while True:
    user_input = input("Enter a number (or 'q' to quit): ")
    if user_input == 'q':
        break
    if user_input.isdigit():
        num = int(user_input)
        total = total + num if 'total' in dir() else num
        print(f"Running total: {total}")
    else:
        print("Invalid input.")

Key Takeaways

  • while condition: repeats the indented block as long as the condition is truthy
  • Update the loop variable inside the body to avoid infinite loops
  • Press Ctrl+C to interrupt a runaway loop
  • Use sentinel values to signal loop termination
  • The accumulator pattern: initialize before the loop, update inside the loop
  • Sum accumulators start at 0; product accumulators start at 1
  • Always validate user input before converting with int() or float()
  • while is ideal when you don't know the number of iterations in advance

Further Reading

Next Module

Module 015: Loops: for — Learn the for loop for iterating over sequences, and master the range() function.