Unit 3 · Lesson 3
Loop Patterns
Most loops follow a handful of common patterns. Learn them once and you'll recognize them everywhere — they're the building blocks of real programs.
Master the accumulator pattern
Use counter and max/min patterns
Build strings inside loops
Combine loops with conditionals
Four Essential Patterns
Almost every loop you write will use one (or more) of these patterns:
📊 Accumulator
Build up a running total
total = 0
for x in data:
total += x🔢 Counter
Count how many times something happens
count = 0
for x in data:
if x > 10:
count += 1🏆 Max / Min
Track the biggest or smallest value
biggest = data[0]
for x in data:
if x > biggest:
biggest = x📝 String Builder
Construct a string piece by piece
result = ""
for i in range(5):
result += "*"The += Shorthand Family
You've seen += for addition. The whole family works the same way:
x += 5 # same as x = x + 5
x -= 3 # same as x = x - 3
x *= 2 # same as x = x * 2
x //= 10 # same as x = x // 10
s += "!" # string concatenation works too!
Pattern recognition tip: When you see a problem, ask yourself: "Am I summing? Counting? Finding a max? Building a string?" The answer tells you which pattern to reach for.
Practice Time
Challenges
Challenge 1: Average Calculator
Guided
Calculate the average of a series of numbers using the accumulator pattern.
Instructions: Given a list of scores (we'll use a simple approach — the scores are in a range), calculate the sum of all numbers from 1 to n and then compute the average (as a float). Store the running sum in total.
Hint 1:
Hint 2: For n=10: total=55, count=10, average=5.5
for i in range(1, n + 1): then total += i and count += 1Hint 2: For n=10: total=55, count=10, average=5.5
Challenge 2: Digit Counter
Guided
Count how many digits a number has using the counter pattern.
Instructions: Given number, count its digits by repeatedly dividing by 10. Store the count in digits. Use a while loop: divide temp by 10 until it reaches 0, incrementing digits each time.
Hint 1:
Hint 2: 34567 → 3456 → 345 → 34 → 3 → 0 (5 steps = 5 digits)
while temp > 0: then digits += 1 and temp //= 10Hint 2: 34567 → 3456 → 345 → 34 → 3 → 0 (5 steps = 5 digits)
Challenge 3: Star Triangle
Solo
Print a left-aligned right triangle made of * characters using the string builder pattern.
Instructions: Given rows, print a triangle where row 1 has 1 star, row 2 has 2 stars, etc.
Example (rows=4):
*
**
***
****
Example (rows=4):
*
**
***
****
Hint 1:
Hint 2: Each line:
for i in range(1, rows + 1):Hint 2: Each line:
print("*" * i)Challenge 4: Prime Checker
Stretch
Determine if a number is prime by checking for divisors.
Instructions: Set is_prime to True or False. A number is prime if it's greater than 1 and not divisible by any integer from 2 to num-1. Use a for loop to check for divisors. If you find one, set is_prime to False.
Optimization: you only need to check up to the square root, but checking up to num-1 is fine too!
Optimization: you only need to check up to the square root, but checking up to num-1 is fine too!
Hint 1: First handle edge case:
Hint 2:
Hint 3: Inside the loop:
if num < 2: is_prime = FalseHint 2:
for i in range(2, num): — if num % i == 0, it's not prime.Hint 3: Inside the loop:
if num % i == 0: is_prime = False. You can add break to stop early.