Loops & Iteration

Making Python do the repetitive work for you

← Lab 2: Conditionals Lab 3 of 10 Lab 4: Functions →

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📖 Concept Recap

for loop — iterates over a sequence (list, string, range)
while loop — runs until a condition becomes False
range(start, stop, step) — generates a sequence of numbers
break — exits the loop immediately
continue — skips to the next iteration
enumerate() — gives you both the index and value: for i, val in enumerate(list)
zip() — pairs two lists together: for a, b in zip(list1, list2)

👀 Worked Example

Three different loop patterns with the same list:

fruits = ["apple", "banana", "cherry", "date"] # Basic for loop for fruit in fruits: print(fruit) print("---") # enumerate — get index AND value for i, fruit in enumerate(fruits): print(f"{i+1}. {fruit}") print("---") # zip — pair two lists together prices = [1.20, 0.50, 2.00, 3.50] for fruit, price in zip(fruits, prices): print(f"{fruit}: ${price}")

✏️ Guided

Exercise 1 — Multiplication Table Generator

Complete the multiplication table generator by filling in the blanks. The table should print lines like 7 × 1 = 7 through 7 × 12 = 84.

number = 7
print(f"Multiplication Table for {number}")
print("-" * 25)

for i in range(1, ___):    # 1 through 12 (inclusive)
    result = number * ___  # calculate the result
    print(f"{number} × {i:2d} = {result:3d}")

Output will appear here...

💡 Hint: range(1, 13) gives 1 through 12. The result is number * i.

💪 Independent

Exercise 2 — Divisibility Finder

Write a loop that finds all numbers between 1 and 100 that are divisible by both 3 and 7. Print each one, and at the end print how many there are total.

# Your code here

Output will appear here...

💡 Hint: A number divisible by both 3 and 7 must be divisible by 21. Use if n % 21 == 0 or if n % 3 == 0 and n % 7 == 0. Keep a counter variable.

🔥 Challenge

Exercise 3 — Number Guessing Game Simulation

Write a number guessing game simulation (no user input — use a preset list of guesses). Rules:

Secret number is 42
Guesses: [10, 55, 30, 42, 67]
For each guess, print "Too low!", "Too high!", or "Correct! Found in X guesses!"
Stop when the correct guess is found (use break)
If not found, print "Not found in the list!"

secret = 42
guesses = [10, 55, 30, 42, 67]

# Your code here

Output will appear here...

💡 Hint: Use enumerate(guesses, 1) to track the attempt number. Use break when you find the secret. Use a for...else clause to handle the not-found case.

🏆 Mini Project

Mini Project — Weekly Sales Summary

Analyze 8 weeks of sales data. Your loop should calculate and print:

Total sales across all weeks
The best week (highest sales) and which week number it was
The worst week (lowest sales) and which week number it was
The weekly average
How many weeks beat the average (use a counter)
A formatted week-by-week breakdown showing each week vs. average

weekly_sales = [12500, 9800, 15200, 11000, 18700, 8900, 13400, 16200]
# Your full analysis here

Output will appear here...

💡 Hint: Calculate total and average first. Then loop through with enumerate(weekly_sales, 1) to track week numbers. Use max()/min() or track them manually in the loop.

✅ Lab 3 Complete!

You’ve practiced for loops, enumerate, zip, break, continue, and real data analysis with loops. Loops are the engine of data processing.

Continue to Lab 4: Functions →

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