Iteration: for Loops, while Loops, do-until Loops

This lesson covers iteration (loops) as required by OCR J277 Section 2.3. Iteration means repeating a block of code. OCR requires you to know three types of loop: for loops (count-controlled), while loops (condition-controlled), and do...until loops (condition-controlled, post-test).

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What Is Iteration?

Iteration is the process of repeating a set of instructions either a fixed number of times or until a condition is met. Without iteration, you would have to write the same code many times over.

There are two main categories:

Type	Description	OCR Loop Types
Count-controlled	Repeats a fixed number of times	for loop
Condition-controlled	Repeats until a condition changes	while loop, do...until loop

The following flowchart shows the logic of a while loop (condition checked before each iteration):

flowchart TD
    A([Start]) --> B{Condition\ntrue?}
    B -- No --> E([End loop])
    B -- Yes --> C["Execute loop\nbody"]
    C --> D["Update\nvariable"]
    D --> B

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for Loops (Count-Controlled)

A for loop repeats a block of code a specific number of times. You know in advance how many iterations are needed.

OCR Pseudocode:

for i = 0 to 4
    print("Hello " + str(i))
next i

This prints:

Hello 0
Hello 1
Hello 2
Hello 3
Hello 4

Python:

for i in range(5):  # range(5) gives 0, 1, 2, 3, 4
    print("Hello", i)

OCR for Loop Syntax

for variable = start to end
    // code to repeat
next variable

• The loop runs from start to end inclusive.
• for i = 1 to 10 runs 10 times (i = 1, 2, 3, ..., 10).
• for i = 0 to 9 also runs 10 times (i = 0, 1, 2, ..., 9).

OCR Exam Tip: In OCR pseudocode, for i = 0 to 4 iterates 5 times (0, 1, 2, 3, 4) because both endpoints are inclusive. In Python, range(5) also gives 5 values (0 to 4) but the upper bound is exclusive. Be careful with this difference.

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while Loops (Pre-Test Condition-Controlled)

A while loop repeats as long as a condition is true. The condition is checked before each iteration, so the loop body might not execute at all.

OCR Pseudocode:

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

Python:

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

Key Points About while Loops

• The condition is checked before each iteration.
• If the condition is false initially, the loop body never executes.
• You must ensure the condition eventually becomes false, or you create an infinite loop.
• Use a while loop when you do not know how many iterations are needed.

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do...until Loops (Post-Test Condition-Controlled)

A do...until loop is similar to a while loop, but the condition is checked after each iteration. This means the loop body always executes at least once.

OCR Pseudocode:

do
    password = input("Enter password: ")
until password == "secret"
print("Access granted")

Python equivalent (Python does not have a built-in do...until, so we simulate it):

while True:
    password = input("Enter password: ")
    if password == "secret":
        break
print("Access granted")

Comparing while and do...until

Feature	while loop	do...until loop
Condition check	Before each iteration	After each iteration
Minimum executions	0 (might not run at all)	1 (always runs at least once)
OCR syntax	while...endwhile	do...until
Use when	Loop might not need to run	Loop must run at least once

OCR Exam Tip: The do...until loop is a distinctive OCR feature — not all exam boards include it. Remember that it runs the code first, THEN checks the condition. The loop stops when the condition becomes TRUE (unlike while, which stops when the condition becomes FALSE).

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Choosing the Right Loop

Situation	Best Loop	Reason
Repeat exactly 10 times	for	Known number of iterations
Repeat until user enters "quit"	while or do...until	Unknown number of iterations
Validate input (must ask at least once)	do...until	Must execute at least once
Process each item in a list	for	Known number of items
Keep running until a condition is met	while	Condition-controlled

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Worked Example: Input Validation with do...until

OCR Pseudocode:

do
    mark = int(input("Enter a mark (0-100): "))
    if mark < 0 OR mark > 100 then
        print("Invalid! Must be between 0 and 100")
    endif
until mark >= 0 AND mark <= 100
print("Valid mark entered: " + str(mark))

Python:

while True:
    mark = int(input("Enter a mark (0-100): "))
    if 0 <= mark <= 100:
        break
    print("Invalid! Must be between 0 and 100")
print("Valid mark entered:", mark)

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Nested Loops

Loops can be placed inside other loops. This is called nesting.

OCR Pseudocode — printing a multiplication table:

for i = 1 to 5
    for j = 1 to 5
        print(str(i) + " x " + str(j) + " = " + str(i * j))
    next j
next i

Python:

for i in range(1, 6):
    for j in range(1, 6):
        print(f"{i} x {j} = {i * j}")

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Summary

• for loops: count-controlled, repeat a fixed number of times.
• while loops: condition-controlled (pre-test), repeat while condition is true.
• do...until loops: condition-controlled (post-test), repeat until condition becomes true, always execute at least once.
• Choose the loop type based on whether you know the number of iterations and whether the loop must run at least once.

OCR Exam Tip: If a question says "the user must enter at least one value", this is a hint to use a do...until loop. If the question says "repeat 10 times", use a for loop. Choosing the correct loop type demonstrates understanding of iteration.

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Worked Example: Number Guessing Game