Unit 3 · Lesson 2

for Loops & range()

When you know how many times to repeat, for is your go-to. Paired with range(), it gives you precise control over counting.

Write for loops with range()

Use range(start, stop, step)

Know when to use for vs while

Build multiplication tables and patterns

The for Loop

A for loop iterates over a sequence. The simplest way to create a sequence of numbers is range():

for i in range(5): print(i) # Output: 0, 1, 2, 3, 4 (each on its own line)

range(5) generates numbers 0 through 4 — that's 5 numbers, starting at 0. The variable i takes each value in turn.

The Three Forms of range()

Form	Produces	Use When
range(5)	0, 1, 2, 3, 4	You just need N repetitions
range(1, 6)	1, 2, 3, 4, 5	You need a specific start and end
range(0, 10, 2)	0, 2, 4, 6, 8	You need to skip/step by N
range(10, 0, -1)	10, 9, 8, …, 1	You need to count backwards

Key rule: range(start, stop) includes start but excludes stop. So range(1, 6) gives you 1–5, not 1–6. This is consistent throughout Python!

for vs while

Use for when you know how many iterations you need (or you're iterating over a sequence). Use while when you're waiting for a condition to change and don't know how many iterations it'll take.

# for — known count for i in range(1, 11): print(f"{i} × 7 = {i * 7}") # while — unknown count (wait for condition) # guess = input("Guess: ") # keep asking until correct

⚠️ Common mistake: Off-by-one errors! If you want numbers 1–10, use range(1, 11) not range(1, 10). The stop value is always excluded.

Practice Time

Challenges

✖️

Challenge 1: Times Table

Guided

Print a multiplication table for a given number.

Instructions: Using a for loop and range, print the times table for num from 1 to 10. Each line should look like: 3 x 1 = 3
Use an f-string: f"{num} x {i} = {num * i}"

Hint 1: for i in range(1, 11): gives you 1 through 10.
Hint 2: Inside: print(f"{num} x {i} = {num * i}")

➕

Challenge 2: Sum of N

Guided

Calculate the sum of integers from 1 to N using a for loop.

Instructions: Using a for loop, calculate the sum of all integers from 1 to n (inclusive) and store it in total.

Example: n=5 → 1+2+3+4+5 = 15

Hint 1: for i in range(1, n + 1): — remember, range excludes the stop value!
Hint 2: Inside: total += i
Hint 3: Fun fact: the answer for n=100 is 5050 (Gauss figured this out as a kid!).

📐

Challenge 3: Staircase

Solo

Print a right-aligned staircase of # symbols.

Instructions: Given height, print a right-aligned staircase. Each line has spaces followed by # characters.

Example (height=5):
    #
   ##
  ###
####
#####

Hint: On row i (starting from 1), there are (height - i) spaces and i hashes.

Hint 1: Use for i in range(1, height + 1): — i goes from 1 to height.
Hint 2: Each line = spaces + hashes. Spaces = " " * (height - i), hashes = "#" * i
Hint 3: print(" " * (height - i) + "#" * i)

🍾

Challenge 4: FizzBuzz

Stretch

The classic programming challenge! Print numbers 1 to n, but replace multiples of 3 with "Fizz", multiples of 5 with "Buzz", and multiples of both with "FizzBuzz".

Instructions: For each number from 1 to n:
• Divisible by 3 AND 5 → print "FizzBuzz"
• Divisible by 3 only → print "Fizz"
• Divisible by 5 only → print "Buzz"
• Otherwise → print the number

Example: 1, 2, Fizz, 4, Buzz, Fizz, 7, 8, Fizz, Buzz, 11, Fizz, 13, 14, FizzBuzz, 16...

Hint 1: Check divisible by BOTH (3 and 5) first! Otherwise "Fizz" catches it before "FizzBuzz" can.
Hint 2: if i % 3 == 0 and i % 5 == 0: → "FizzBuzz", elif i % 3 == 0: → "Fizz", etc.
Hint 3: Alternative: if i % 15 == 0: (since 15 = 3 × 5)