Lesson 4 – For Loops (Iteration)

In this lesson you will:

• Explain what a for loop does (repetition / iteration).
• Use range() to control how many times a loop runs.
• Write for loops with correct indentation.
• Predict the output of simple loops.
• Use a loop to repeat input (e.g. "ask 10 times") and do simple processing (sum / max).

By the end you should be able to:

• Write a for loop to repeat code a fixed number of times.
• Use range(start, stop) and range(start, stop, step) correctly.
• Avoid common range() mistakes (off-by-one errors).
• Use a loop to build a result (e.g. total, average, highest).

1. Quick recap

From the previous lessons, remember:

• Variables store values.
• We can display output using print().
• We can read input using input() and convert to int() or float().
• Indentation matters: Python uses indentation to show which lines belong together.
• if statements run code only when a condition is True.

Mini-check (predict):

What will this print?

x = 3
x = x + 1
print(x)

2. What is a for loop?

A for loop lets Python repeat a block of code a set number of times.

Why this matters: instead of writing lots of similar lines…

print("Hello")
print("Hello")
print("Hello")
print("Hello")
print("Hello")

…you can write one loop:

for i in range(5):
    print("Hello")

How it works:

• Python creates a sequence of values using range().
• The loop variable (here i) takes each value in turn.
• The indented block runs once for each value.

3. Example 1 – Loop from 1 to 10

Predict first: What will the first number be? What will the last number be?

for i in range(1, 11):
    print(i)

What happens:

• When i is 1 → prints 1
• When i is 2 → prints 2
• …
• When i is 10 → prints 10

Then it stops because range(1, 11) stops before 11.

Try it yourself

Run it, then:

• Change it to print 1 to 5
• Change it to print 1 to 20

# Print 1 to 5
for i in range(1, 6):
    print(i)

# Print 1 to 20
for i in range(1, 21):
    print(i)

4. Understanding range()

The range() function controls how many times your loop runs and what values the loop variable takes. There are three ways to use it:

Example A: range(stop) – Start at 0

When you use just one number, the loop starts at 0 and stops before that number.

for i in range(5):
    print(i)

This prints: 0, 1, 2, 3, 4 (5 numbers total, starting from 0)

Try it yourself

Run the code below. Then change it to print numbers going up in steps of 3 (hint: you'll need to add a third number to range).

# To go up in steps of 3, use range(0, 15, 3)
for i in range(0, 15, 3):
    print(i)  # Prints: 0, 3, 6, 9, 12

Example B: range(start, stop) – Choose your start

When you use two numbers, the first is where to start, and the second is where to stop (but not include).

for i in range(3, 8):
    print(i)

This prints: 3, 4, 5, 6, 7 (starts at 3, stops before 8)

Try it yourself

Run the code below. Then change it to print numbers going up in steps of 3.

# To go up in steps of 3, use range(3, 15, 3)
for i in range(3, 15, 3):
    print(i)  # Prints: 3, 6, 9, 12

Example C: range(start, stop, step) – Control the step size

When you use three numbers, you control where to start, where to stop, and how big each step is.

for i in range(0, 10, 2):
    print(i)

This prints: 0, 2, 4, 6, 8 (starts at 0, stops before 10, steps by 2 each time)

Try it yourself

Run the code below. Then change it to go up in steps of 3 instead of 2.

# Change step from 2 to 3
for i in range(0, 10, 3):
    print(i)  # Prints: 0, 3, 6, 9

Key points to remember:

• range(5) → starts at 0, goes up to (but not including) 5
• range(3, 8) → starts at 3, goes up to (but not including) 8
• range(0, 10, 2) → starts at 0, goes up to (but not including) 10, steps by 2
• The stop value is never included in the range!

5. Example 2 – Loop from 0 to 9

Predict first: Does it include 10?

for i in range(10):
    print(i)

What happens:

range(10) starts at 0 and stops before 10. So it prints 0 to 9.

Try it yourself

Change it to print 0 to 4
Change it to print 0 to 19

# Print 0 to 4
for i in range(5):
    print(i)

# Print 0 to 19
for i in range(20):
    print(i)

6. Example 3 – A different step value

Predict first: How many numbers will print?

for i in range(0, 11, 2):
    print(i)

What happens:

• Start 0
• Step 2 each time
• Stop before 11

Prints: 0, 2, 4, 6, 8, 10

Try it yourself

Change the step to 3
Change the start to 1 (keep step 2) and predict what happens

# Step 3
for i in range(0, 11, 3):
    print(i)  # Prints: 0, 3, 6, 9

# Start 1, step 2
for i in range(1, 11, 2):
    print(i)  # Prints: 1, 3, 5, 7, 9

7. Example 4 – Printing even numbers

Predict first: Will any odd numbers appear?

for i in range(2, 21, 2):
    print(i)

Try it yourself

Make it print even numbers from 2 to 50
Make it print even numbers from 0 to 20

# Even numbers from 2 to 50
for i in range(2, 51, 2):
    print(i)

# Even numbers from 0 to 20
for i in range(0, 21, 2):
    print(i)

8. Example 5 – Printing odd numbers

Predict first: What is the last number printed?

for i in range(1, 20, 2):
    print(i)

Try it yourself

Make it print odd numbers from 1 to 99
Make it print odd numbers from 5 to 25

# Odd numbers from 1 to 99
for i in range(1, 100, 2):
    print(i)

# Odd numbers from 5 to 25
for i in range(5, 26, 2):
    print(i)

9. Repeating input with a loop (this is where loops become powerful)

A very common pattern is: "Ask the user something 10 times."

Example – ask for 3 numbers and print them

Predict first: How many times will it ask?

for i in range(3):
    num = int(input("Enter a number: "))
    print("You entered:", num)

Try it yourself

Change it to ask 5 times
Change the message to include the attempt number (extension): Attempt 1, Attempt 2, …

# Ask 5 times
for i in range(5):
    num = int(input("Enter a number: "))
    print("You entered:", num)

# With attempt number
for i in range(3):
    num = int(input("Attempt " + str(i+1) + ": Enter a number: "))
    print("You entered:", num)

Video – For loops with calculations

Watch this video to see how to use for loops for common patterns: calculating sums and averages.

10. Indentation and loop blocks (important)

A for loop line ends with a colon :. The code inside the loop must be indented.

Example – one line inside the loop:

for i in range(5):
    print(i)

Example – two lines inside the loop:

for i in range(5):
    print("Number:", i)
    print("-----")
print("Done")

What happens:

• The indented lines repeat.
• print("Done") runs once at the end.

Try it yourself

Move one of the indented lines back to the left. Predict what changes, then run it.

# If you move print("-----") to the left (no indentation):
for i in range(5):
    print("Number:", i)
print("-----")  # This runs once, not 5 times!
print("Done")

11. Printing star patterns (triangle)

You can use loops to create patterns with stars or other characters. A common pattern is a triangle that grows from 1 star to many stars.

Example – Print a triangle of stars

Predict first: How many lines will print? How many stars on the first line? How many on the last line?

for i in range(1, 6):
    print("*" * i)

What happens:

• When i is 1 → prints "*" (1 star)
• When i is 2 → prints "**" (2 stars)
• When i is 3 → prints "***" (3 stars)
• When i is 4 → prints "****" (4 stars)
• When i is 5 → prints "*****" (5 stars)

Key idea: The expression "*" * i repeats the star character i times. This is called string multiplication.

Try it yourself

Change it to print a triangle with 10 lines
Change it to print a triangle starting from 3 stars (so the first line has 3 stars, second has 4, etc.)

# Triangle with 10 lines
for i in range(1, 11):
    print("*" * i)

# Triangle starting from 3 stars
for i in range(3, 8):
    print("*" * i)  # First line: 3 stars, second: 4, etc.

12. Common misconceptions

Read these carefully — they are very common in tests.

Mistake 1: Expecting the stop value to be included

for i in range(1, 10):
    print(i)

This prints 1 to 9, not 10.

Fix:

for i in range(1, 11):
    print(i)

Mistake 2: Forgetting indentation

Wrong:

for i in range(5):
print(i)

Correct:

for i in range(5):
    print(i)

Mistake 3: Thinking range(10) starts at 1

It starts at 0.

range(10) → 0 to 9

Mistake 4: Counting backwards but forgetting the step is negative

To go down, you need a negative step:

for i in range(10, 0, -1):
    print(i)

(Notice it stops before 0, so 1 is the last value.)

14. Understanding Quiz

Answer all questions below, then click "Check Answers" at the end to see your score.

for i in range(4):
    print(i)

for i in range(5):
print(i)

for i in range(10, 0, -1):
    print(i)

for i in range(1, 10):
    print(i)

for i in range(1, 6):
    print(i * 2)

total = 0
for i in range(5):
    total = total + i
print(total)

13. Coding tasks – For loops practice

Complete all the tasks below. Do one task at a time, run your code, and check the output.

• Task 1 – Count up
  Print the numbers 1 to 10.
• Task 2 – Count down
  Print the numbers 10 down to 1.
• Task 3 – Evens
  Print the even numbers from 2 to 20.
• Task 4 – Step challenge
  Print 5, 10, 15, 20, 25, 30 using range() with a step.
• Task 5 – Squares
  Print the square of each number from 1 to 8 (e.g., 1² = 1, 2² = 4, …).
• Task 6 – Times table (formatting)
  Print the 7 times table from 7 × 1 to 7 × 12 like:

  7 x 1 = 7
  7 x 2 = 14
  7 x 3 = 21
  ...

• Task 7 – Running total
  A shop counts customers each hour for 5 hours. Ask for 5 numbers (customers each hour) and print the total customers.
• Task 8 – Highest score
  A class has 10 quiz scores. Ask for 10 scores and print the highest score.
• Task 9 – Pass counter
  A teacher enters 10 test scores (0–100). Count how many are 50 or more and print: Passes: ___
• Task 10 – Summary stats
  A sports coach enters 8 lap times (seconds). Ask for 8 times and then print the average (total ÷ 8).

Answer all the above tasks here

Use the editor below to complete the coding tasks above.