Iteration

A for loop is count-controlled — it runs a fixed number of times, known before the loop starts. It maps directly from pseudocode FOR i ← 1 TO n ... NEXT i.

# Basic syntax — loops over a sequence
for variable in sequence:
    # code runs once per item

# range(5) → 0 to 4
for i in range(5):
    print(i)
# Output: 0 1 2 3 4

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

# range(0, 10, 2) → even numbers
for i in range(0, 10, 2):
    print(i)
# Output: 0 2 4 6 8

A for loop can step through every item in a list or every character in a string directly — no index needed.

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)
# apple
# banana
# cherry

# Summing a list
numbers = [10, 20, 30, 40, 50]
total = 0
for num in numbers:
    total += num
print(total)  # 150

for char in "Python":
    print(char)
# P y t h o n (each on a new line)

# Count vowels
word = "programming"
vowels = "aeiou"
count = 0
for char in word:
    if char in vowels:
        count += 1
print(count)  # 3

A nested loop is a loop inside another loop. For every single iteration of the outer loop, the inner loop runs its full sequence. Used for multiplication tables, 2D grids and processing 2D arrays.

# Nested loop — i is outer, j is inner
for i in range(1, 4):
    for j in range(1, 4):
        print(f"i={i}, j={j}")
# i=1,j=1  i=1,j=2  i=1,j=3
# i=2,j=1  i=2,j=2  i=2,j=3
# i=3,j=1  i=3,j=2  i=3,j=3

# Multiplication table 1×1 through 3×3
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} x {j} = {i*j}")

A while loop is condition-controlled — it keeps running as long as its condition is True, and stops the moment it becomes False. The number of iterations is not known in advance. It maps from pseudocode WHILE condition DO ... ENDWHILE.

# Pseudocode: WHILE x <= 5 DO ... ENDWHILE
x = 1
while x <= 5:
    print(x)
    x += 1
# Output: 1 2 3 4 5

Countdown & sentinel loop

# Countdown timer
count = 10
while count > 0:
    print(count)
    count -= 1
print("Blast off!")

# Sentinel loop — stop on -1
total = 0
number = int(input("Enter number (-1 to stop): "))
while number != -1:
    total += number
    number = int(input("Enter number (-1 to stop): "))
print(f"Total: {total}")

break and continue give fine-grained control over a loop without needing extra Boolean flags. break is often used in search loops (stop when found); continue in filtering loops (skip items that do not match).

# break in a for loop
for i in range(1, 11):
    if i == 7:
        break
    print(i)
# Output: 1 2 3 4 5 6

# Search list — stop when found
numbers = [1, 2, 3, 5, 8, 13]
for num in numbers:
    if num == 5:
        print("Found!")
        break

# Skip number 5
for i in range(1, 11):
    if i == 5:
        continue
    print(i)
# Output: 1 2 3 4 6 7 8 9 10

# Print only odd numbers
for i in range(1, 11):
    if i % 2 == 0:
        continue
    print(i)
# Output: 1 3 5 7 9

Cambridge Paper 4 questions describe a scenario and expect you to pick the correct loop type. Choosing the wrong loop does not always crash, but it produces incorrect structure — and the mark scheme specifies which loop is expected based on the pseudocode construct.

Common mistakes that lose marks

Exam-style Lab Tasks

Fifteen exam-style tasks. Click Hint for bullet-point guidance, then Help to reveal a worked Python solution.