Iteration (for, while, do-while loops)

Iteration means repeating a block of code. Loops are one of the three fundamental programming constructs and are tested extensively in GCSE Computer Science (AQA 3.2 / OCR J277 2.2). This lesson covers the three types of loop you need to know.

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Why Do We Need Iteration?

Without loops, repeating an action 100 times would require writing the same code 100 times. Iteration lets you write the code once and repeat it as many times as needed — making programs shorter, more efficient, and easier to maintain.

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Count-Controlled Loops (FOR Loops)

A FOR loop repeats a block of code a known number of times. You use it when you know in advance how many repetitions are needed.

Pseudocode

FOR i ← 1 TO 10
    OUTPUT i
ENDFOR

This outputs the numbers 1 through 10.

Python

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

Important: In Python, range(1, 11) generates numbers from 1 to 10 (the upper bound is exclusive). In AQA pseudocode, FOR i ← 1 TO 10 is inclusive of 10.

Using a Step Value

FOR i ← 0 TO 20 STEP 5
    OUTPUT i
ENDFOR

Output: 0, 5, 10, 15, 20

Python equivalent:

for i in range(0, 21, 5):
    print(i)

Counting Backwards

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

This counts from 10 down to 1.

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Condition-Controlled Loops (WHILE Loops)

A WHILE loop repeats a block of code as long as a condition remains True. You use it when you do not know in advance how many repetitions are needed.

Pseudocode

password ← ""
WHILE password ≠ "secret123"
    OUTPUT "Enter password:"
    password ← USERINPUT
ENDWHILE
OUTPUT "Access granted"

Python

password = ""
while password != "secret123":
    password = input("Enter password: ")
print("Access granted")

Key Characteristics of WHILE Loops

• The condition is checked before each iteration.
• If the condition is False at the start, the loop body never executes.
• You must ensure the condition will eventually become False, otherwise you create an infinite loop.

Exam Tip: An infinite loop is where the condition never becomes False. This is almost always a bug. Always check that something inside the loop changes the condition variable.

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DO...UNTIL Loops (Post-Condition Loops)

A DO...UNTIL loop (also called a REPEAT...UNTIL loop) checks the condition after each iteration, guaranteeing the loop body runs at least once.

Pseudocode

REPEAT
    OUTPUT "Enter a number between 1 and 10:"
    number ← INT(USERINPUT)
UNTIL number >= 1 AND number <= 10

Python

Python does not have a built-in do-while loop, but you can simulate one:

while True:
    number = int(input("Enter a number between 1 and 10: "))
    if 1 <= number <= 10:
        break

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Iteration Flow

flowchart TD
    A[Start loop] --> B[Initialise counter or condition variable]
    B --> C{"Check condition<br/>WHILE/FOR"}
    C -->|False| D[Exit loop]
    C -->|True| E[Execute loop body]
    E --> F[Update counter/variable]
    F --> C
    D --> G[Continue with next statement]

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Comparing the Three Loop Types

Feature	FOR	WHILE	DO...UNTIL
When to use	Known number of iterations	Unknown iterations, may be zero	Unknown iterations, at least one
Condition check	Automatic (counter-based)	Before each iteration	After each iteration
Minimum executions	The specified count (could be 0)	0 (may never execute)	1 (always runs at least once)
Risk of infinite loop	Very low	Yes, if condition never becomes False	Yes, if condition never becomes True

Exam Tip: If a question says "repeat until a valid input is entered," consider a DO...UNTIL loop because the user must enter at least one value before it can be checked.

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Nested Loops

Loops can be placed inside other loops. This is called nesting and is useful for working with 2D structures (such as grids or tables).

for row in range(1, 4):
    for col in range(1, 4):
        print(row * col, end="\t")
    print()

Output:

1   2   3
2   4   6
3   6   9

The outer loop runs 3 times. For each outer iteration, the inner loop also runs 3 times, giving 3 x 3 = 9 iterations in total.

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Using Loops with Totals and Counters

A common pattern is to use a loop to accumulate a total or count items:

total = 0
count = 0
for i in range(5):
    mark = int(input("Enter a mark: "))
    total = total + mark
    count = count + 1
average = total / count
print(f"Average: {average}")

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Common Exam Mistakes

1. Off-by-one errors — check whether your loop runs one too many or one too few times.
2. Infinite loops — forgetting to update the variable used in the while condition.
3. Using the wrong loop type — a FOR loop when the number of iterations is unknown, or a WHILE loop when a DO...UNTIL is more appropriate.
4. Not initialising the counter or total before the loop.

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Summary

• FOR loops repeat a known number of times.
• WHILE loops repeat while a condition is True (may execute zero times).
• DO...UNTIL loops run at least once, then check the condition.
• Loops can be nested for working with multi-dimensional data.
• Always initialise totals and counters before a loop.

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Extended Worked Examples

Example 1: Sum the first N integers with a FOR loop

n = int(input("Enter N: "))
total = 0
for i in range(1, n + 1):
    total = total + i
print(f"Sum of 1..{n} = {total}")

Trace for n = 5:

Iteration	i	total before	total after
1	1	0	1
2	2	1	3
3	3	3	6
4	4	6	10
5	5	10	15