for Loops

A for loop repeats a block of code for each item in a sequence (like a list, string, or range). It is used when you know how many times you want to loop.

# Loop through a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(f"I like {fruit}")

# Loop with range()
for i in range(5):          # 0, 1, 2, 3, 4
    print(f"Count: {i}")

for i in range(2, 10, 2):   # 2, 4, 6, 8 (start, stop, step)
    print(f"Even: {i}")

# Loop through a string
for char in "Python":
    print(char, end=" ")    # P y t h o n

range(5) generates: 0, 1, 2, 3, 4 | range(2, 10, 2) generates: 2, 4, 6, 8

while Loops

A while loop repeats a block of code as long as a condition remains True. Be careful to ensure the condition eventually becomes False, or the loop will run forever (an infinite loop).

# Count down from 5
count = 5
while count > 0:
    print(f"Countdown: {count}")
    count -= 1      # Same as count = count - 1
print("Blast off!")

# Accumulator pattern
total = 0
number = 1
while number <= 100:
    total += number  # Add each number to total
    number += 1
print(f"Sum of 1 to 100 = {total}")

Output: Countdown: 5, 4, 3, 2, 1 then Blast off! | Sum of 1 to 100 = 5050

break, continue, and Nested Loops

break exits the loop immediately. continue skips the rest of the current iteration and moves to the next. Nested loops are loops inside loops.

# break - exit early when we find what we need
names = ["Alice", "Bob", "Charlie", "Diana"]
for name in names:
    if name == "Charlie":
        print(f"Found {name}!")
        break
    print(f"Checking {name}...")

# continue - skip odd numbers
for i in range(1, 11):
    if i % 2 != 0:
        continue        # Skip odd numbers
    print(f"Even: {i}")  # Only prints 2, 4, 6, 8, 10

# Nested loops - multiplication table
for row in range(1, 4):
    for col in range(1, 4):
        print(f"{row}x{col}={row*col}", end="\t")
    print()  # New line after each row

Key Vocabulary

Iteration

One complete execution of the loop body. A loop with 5 items has 5 iterations.

range()

A function that generates a sequence of numbers: range(start, stop, step).

Infinite Loop

A loop that never stops because its condition never becomes False.

Accumulator

A variable that builds up a result across iterations, like summing numbers.

Worked Examples

1

Calculate the average of student scores using a for loop

scores = [85, 92, 78, 95, 88, 76, 90]
total = 0

for score in scores:
    total += score

average = total / len(scores)
print(f"Total: {total}")
print(f"Average: {average:.1f}")

Step 1: We initialise total = 0 as an accumulator.

Step 2: Each iteration adds the current score to total.

Output: Total: 604 | Average: 86.3

2

Use a while loop to guess a number

secret = 7
guess = 0
attempts = 0

while guess != secret:
    guess = int(input("Guess the number (1-10): "))
    attempts += 1
    if guess < secret:
        print("Too low!")
    elif guess > secret:
        print("Too high!")

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

Step 1: The while loop continues until the guess matches the secret number.

Step 2: We track attempts and give hints (too low/high) each round.

3

Print a times table using nested loops

# Print multiplication table for 1-5
for i in range(1, 6):
    for j in range(1, 6):
        result = i * j
        print(f"{result:4}", end="")
    print()  # New line after each row

Step 1: The outer loop controls the row (1 to 5).

Step 2: The inner loop controls the column (1 to 5). {result:4} pads the number to 4 characters.

Output: A 5x5 multiplication table grid.

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

How many times does for i in range(3): print(i) print?

2 times 1 time 4 times 3 times

Question 2

What does break do inside a loop?

Exits the loop immediately Skips to the next iteration Pauses the loop Restarts the loop from the beginning

Question 3

What values does range(1, 6) generate?

1, 2, 3, 4, 5, 6 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5, 6

Question 4

Which loop type is best when you do NOT know how many times to repeat?

for loop if statement while loop range()

Question 5

What does continue do inside a loop?

Exits the loop immediately Restarts the loop Does nothing Skips the rest of the current iteration and moves to the next

Python Loops