6.1d Iteration: for & while Loops — Edexcel 1CP2

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What is Iteration?

Iteration means repeating a block of code. Instead of writing the same code multiple times, we use a loop to execute it repeatedly. Python has two main loop types: for loops (count-controlled) and while loops (condition-controlled).

for Loops — Count-Controlled Iteration

A for loop repeats a set number of times. It iterates over a sequence (like a range of numbers or items in a list).

# Print 1 to 5 using range()
for i in range(1, 6):
    print(i)
# Output: 1  2  3  4  5

range() function:

Syntax	What it produces	Example
`range(n)`	0, 1, 2, ... n-1	`range(5)` → 0, 1, 2, 3, 4
`range(start, stop)`	start, start+1, ... stop-1	`range(1, 6)` → 1, 2, 3, 4, 5
`range(start, stop, step)`	start, start+step, ... up to stop	`range(0, 10, 2)` → 0, 2, 4, 6, 8

# Iterating over a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)
# Output: apple  banana  cherry

# Summing numbers with a for loop
total = 0
for i in range(1, 6):
    total += i
print(total)  # Output: 15

while Loops — Condition-Controlled Iteration

A while loop repeats as long as a condition remains True. The number of iterations is not fixed — it depends on when the condition becomes False.

# Basic while loop
count = 1
while count <= 5:
    print(count)
    count += 1
# Output: 1  2  3  4  5

# Input validation with while loop
password = input("Enter password: ")
while password != "secret":
    print("Wrong password, try again")
    password = input("Enter password: ")
print("Access granted!")

for vs while — When to Use Each

for loop	while loop
Use when you know how many times to repeat	Use when you repeat until a condition is met
Iterating over a list or range	Input validation (repeat until valid input)
Processing each element of a collection	Game loops (repeat until player loses)
Counting (1 to 10)	Searching (repeat until found)
Always terminates (given finite range/list)	Risk of infinite loop if condition never becomes False

Nested Loops

A loop inside another loop is called a nested loop. The inner loop completes all its iterations for every single iteration of the outer loop.

# Multiplication table using nested loops
for i in range(1, 4):
    for j in range(1, 4):
        print(i * j, end="  ")
    print()
# Output:
# 1  2  3
# 2  4  6
# 3  6  9

break and continue

Keyword	Effect	Example
break	Immediately exits the loop entirely	Search a list and stop when item is found
continue	Skips the rest of the current iteration and goes to the next	Skip even numbers, only process odd

# break example — stop at 5
for i in range(1, 10):
    if i == 5:
        break
    print(i)
# Output: 1  2  3  4

# continue example — skip even numbers
for i in range(1, 8):
    if i % 2 == 0:
        continue
    print(i)
# Output: 1  3  5  7

Exam tip: Paper 2 often asks you to trace loops (show the value of variables after each iteration). Always track the loop variable AND any accumulators. Remember range(1,6) gives 1,2,3,4,5 — NOT 6. The stop value is excluded.

⚠️ Common Mistakes

Infinite while loop — forgetting to update the variable inside the loop so the condition never becomes False
Off-by-one errors — range(1, 10) produces 1–9, not 1–10. To include 10, use range(1, 11)
Wrong indentation — Python uses indentation to define loop bodies; inconsistent indentation causes errors
Modifying a list while iterating over it — can cause unexpected behaviour

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✍️ Worksheet — 6.1d Iteration: for & while Loops

8 questions

Q1What is the difference between a for loop and a while loop in Python? When would you use each?[4]

✅ Mark scheme

A for loop is count-controlled — it repeats a fixed number of times, iterating over a sequence or range [1]; use a for loop when you know in advance how many times to repeat, e.g. processing each item in a list [1]; a while loop is condition-controlled — it repeats as long as a condition is True; the number of iterations is not predetermined [1]; use a while loop when repeating until a condition is met, e.g. input validation, game loops, or searching [1].

Q2What values does range(2, 10, 3) produce? Explain each part of the range() call.[4]

✅ Mark scheme

range(2, 10, 3) produces: 2, 5, 8 [1]; start=2 — the sequence begins at 2 [1]; stop=10 — the sequence goes up to but NOT including 10 [1]; step=3 — each value increases by 3 (so 2 → 5 → 8; next would be 11 which exceeds 10, so it stops) [1].

Q3Trace the following code. Show the value of total after each iteration of the loop.

total = 0
for i in range(1, 5):
total = total + i
print(total)[4]

✅ Mark scheme

i=1: total = 0+1 = 1 [1]; i=2: total = 1+2 = 3 [1]; i=3: total = 3+3 = 6 [1]; i=4: total = 6+4 = 10 [1]; print(total) outputs: 10. (range(1,5) gives 1,2,3,4 — NOT 5.)

Q4Write Python code using a while loop that asks the user to enter a number between 1 and 10 and keeps asking until a valid number is entered.[4]

✅ Mark scheme

num = int(input("Enter a number (1-10): "))
while num < 1 or num > 10:
    print("Invalid! Try again.")
    num = int(input("Enter a number (1-10): "))
print("Valid number:", num)

Mark: getting input before the loop [1]; while condition checks both boundaries (num<1 or num>10) [1]; print error message inside loop [1]; get input again inside loop (essential — otherwise infinite loop) [1].

Q5Explain what an infinite loop is and give an example of how one could occur in Python with a while loop.[3]

✅ Mark scheme

An infinite loop is a loop that never terminates — the condition controlling it never becomes False [1]; example: forgetting to update the loop variable inside a while loop [1]; e.g. count = 1; while count < 10: print(count) — count is never incremented so the condition (count < 10) is always True and the loop runs forever [1].

Q6What does the break keyword do in a loop? Write a short Python example using break.[3]

✅ Mark scheme

break immediately terminates the current loop, regardless of whether the loop condition is still True or there are more iterations remaining [1]; example: for i in range(1, 10): [1] if i == 5: break; print(i) — this prints 1, 2, 3, 4 and then exits the loop when i reaches 5 [1].

Q7Write Python code using a for loop to print the multiplication table for 7 (7×1 to 7×10).[4]

✅ Mark scheme

for i in range(1, 11):
    print("7 x", i, "=", 7 * i)

Mark: correct for loop syntax with range [1]; range(1, 11) to include 10 [1]; correct multiplication 7*i [1]; print statement showing the multiplication [1]. Output: 7 x 1 = 7, 7 x 2 = 14, ..., 7 x 10 = 70.

Q8Describe and explain the output of the following nested loop code:

for i in range(3):
for j in range(3):
print(i, j)[4]

✅ Mark scheme

The outer loop runs for i = 0, 1, 2 (three times) [1]; for each value of i, the inner loop runs completely for j = 0, 1, 2 (three times) [1]; total of 9 outputs [1]; output: 0 0 / 0 1 / 0 2 / 1 0 / 1 1 / 1 2 / 2 0 / 2 1 / 2 2 [1]. (Accept trace table format showing all pairs.)

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Term	Definition

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What is Iteration?

for Loops — Count-Controlled Iteration

while Loops — Condition-Controlled Iteration

for vs while — When to Use Each

Nested Loops

break and continue

✍️ Worksheet — 6.1d Iteration: for & while Loops

Mini Test — Iteration