OCR J277 · Component 2 · Topic 2.2.1b

Programming Fundamentals
Sequence, Selection & Iteration

The three building blocks of every program — in OCR ERL and Python

CSZone OCR GCSE Computer Science J277

Learning Objectives

By the end of this video you will be able to...

Define sequence and explain why the order of statements in a program matters

Write and interpret selection using IF / ELSEIF / ELSE / ENDIF and SWITCH / CASE in OCR ERL — and know the Python equivalents

Write and interpret iteration using FOR, WHILE and DO...UNTIL — knowing when to choose each one

Identify the differences between pre-test (WHILE) and post-test (DO...UNTIL) loops — including the minimum number of iterations

Avoid classic exam mistakes: wrong loop type, Python range() off-by-one, DO...UNTIL exit condition direction, and infinite loops

⚡ Every program ever written uses a combination of these three constructs. Master them and half the programming exam questions become straightforward.

Sequence

Sequence — statements execute top to bottom, in order

DEFINITION

Sequence means that statements are executed one after another, in the exact order they appear. No statement is skipped or repeated — every line runs exactly once.

ORDER MATTERS — WRONG ORDER EXAMPLE

❌ Wrong order — prints before total is calculated print(total) ← total doesn't exist yet total ← a + b

CORRECT SEQUENCE

a ← int(input("Enter a: ")) ← 1st b ← int(input("Enter b: ")) ← 2nd total ← a + b ← 3rd print(total) ← 4th ✓

⚡ In a program with only sequence, every line runs from top to bottom without deviation. Sequence is the simplest of the three constructs — but getting the order wrong causes incorrect results or runtime errors.

Selection

Selection — IF / ELSEIF / ELSE

DEFINITION

Selection allows the program to choose between different paths based on a condition. Only the branch whose condition is True will execute.

USE IF WHEN...

Conditions involve ranges or complex expressions, or when different variables are being compared. If testing one variable against several fixed values, SWITCH / CASE is better.

OCR ERL STRUCTURE

IF score >= 70 THEN print("Distinction") ELSEIF score >= 40 THEN print("Pass") ELSE print("Fail") ENDIF

PYTHON EQUIVALENT

if score >= 70: print("Distinction") elif score >= 40: print("Pass") else: print("Fail")

⚠ KEY DIFFERENCE

OCR ERL uses ELSEIF (one word, no colon) and ends with ENDIF. Python uses elif with a colon and indentation — never write ELSEIF in Python.

Selection

Selection — SWITCH / CASE

WHEN TO USE SWITCH / CASE

Use SWITCH / CASE when you are testing one variable against several specific fixed values. It is cleaner than a long chain of ELSEIF statements for this situation.

OCR ERL STRUCTURE

SWITCH grade: CASE "A": print("Excellent") CASE "B": print("Good") CASE "C": print("Satisfactory") DEFAULT: print("Below C") ENDSWITCH

PYTHON EQUIVALENT

Python uses if / elif chains — there is no SWITCH keyword. In Python 3.10+ match/case exists, but the OCR exam only expects if/elif.

if grade == "A": print("Excellent") elif grade == "B": print("Good") elif grade == "C": print("Satisfactory") else: print("Below C")

⚡ Always include a DEFAULT case in SWITCH to handle unexpected values — just like else in Python. The OCR mark scheme will look for it.

Iteration

FOR loop — count-controlled iteration

WHEN TO USE

Use a FOR loop when you know exactly how many iterations are needed before the loop starts. The loop counter is incremented automatically.

OCR ERL SYNTAX

FOR i = 1 TO 5 print(i) NEXT i ← must write NEXT i

PYTHON — range() IS EXCLUSIVE AT THE END

for i in range(1, 6): # 1,2,3,4,5 — 6 NOT included print(i)

SUMMING 5 NUMBERS — ERL EXAMPLE

total ← 0 FOR i = 1 TO 5 num ← int(input("Enter number: ")) total ← total + num NEXT i print(total)

⚠ PYTHON RANGE() OFF-BY-ONE

ERL FOR i = 1 TO 5 gives 1,2,3,4,5. Python range(1, 5) gives 1,2,3,4 only — the end is excluded. You need range(1, 6).

Iteration

WHILE loop — condition-controlled, pre-test

WHEN TO USE

Use WHILE when the number of iterations is unknown and the loop body might never run at all (condition could be False from the start). Condition is tested before each iteration — this is a pre-test loop.

OCR ERL SYNTAX

WHILE condition DO <statements> ENDWHILE

COUNTDOWN EXAMPLE — ERL

count ← 5 WHILE count > 0 DO print(count) count ← count - 1 ENDWHILE

INPUT VALIDATION EXAMPLE

num ← int(input("Enter 1–10: ")) WHILE num < 1 OR num > 10 DO print("Invalid!") num ← int(input("Enter 1–10: ")) ENDWHILE

⚡ If the condition is False at the start, the body never executes. This means WHILE can run zero times — you must always initialise the variable used in the condition before the WHILE statement.

Iteration

DO...UNTIL — post-test, always runs at least once

WHEN TO USE

Use DO...UNTIL when the loop body must run at least once — e.g. getting user input that you need before you can validate. The condition is tested after each iteration — this is a post-test loop. Exits when condition becomes True.

OCR ERL SYNTAX & EXAMPLE

DO password ← input("Enter password: ") UNTIL password == "secret123"

PYTHON SIMULATION (no DO...UNTIL keyword)

while True: password = input("Enter password: ") if password == "secret123": break

WHILE vs DO...UNTIL — COMPARISON TABLE

Feature	WHILE	DO...UNTIL
Test position	Pre-test (before body)	Post-test (after body)
Min iterations	0	1
Exits when...	Condition False	Condition True

Choosing Constructs

Choosing the right construct

Sequence

Use when every step must always happen, in a fixed order. No branching or repetition needed.

Selection

Use when the program must make a decision. Use IF for range/complex conditions. Use SWITCH when comparing one variable to several exact values.

Iteration

FOR — known number of iterations
WHILE — unknown count, might run 0 times
DO...UNTIL — must run at least once

DECISION GUIDE TABLE

Situation	Use
Repeat 10 times exactly	FOR
Keep going while valid	WHILE
Get input then validate	DO...UNTIL
Branch on a score range	IF/ELSEIF

Worked Example

All three constructs — number guessing game

THE SCENARIO

Player gets 3 attempts to guess a secret number. After each guess, tell them Too High, Too Low, or Correct. Use a constant for the secret number.

CONSTRUCTS USED:

FOR — 3 attempts IF/ELSEIF — feedback

OCR ERL PSEUDOCODE

const SECRET ← 7 FOR attempt = 1 TO 3 guess ← int(input("Guess: ")) IF guess == SECRET THEN print("Correct!") ELSEIF guess > SECRET THEN print("Too high") ELSE print("Too low") ENDIF NEXT attempt

Exam Practice

Sequence, selection & iteration — exam questions

Question 1 — 2 marks

State one difference between a WHILE loop and a DO...UNTIL loop in OCR pseudocode.

Answer — Q1 (1 mark for any one)

WHILE tests condition before each iteration (pre-test); DO...UNTIL tests after each iteration (post-test) [1] — A WHILE loop may never execute if the condition is initially False; DO...UNTIL always executes at least once [1]

Question 2 — 1 mark

A program needs to repeat a set of statements exactly 10 times. Which type of loop should be used? Give a reason.

Answer — Q2

FOR loop — because the number of iterations is known in advance (exactly 10) so a count-controlled loop is appropriate [1]

Question 3 — 4 marks

Write OCR ERL pseudocode for a program that asks the user for 5 numbers using a FOR loop, keeps a running total, then outputs the total and average. Use correct OCR ERL syntax throughout.

Exam Answers

Question 3 — answer and mark scheme

Q3 MODEL ANSWER

total ← 0 ← [1] total initialised FOR i = 1 TO 5 ← [1] FOR loop 1 TO 5 num ← int(input("Enter number: ")) total ← total + num NEXT i average ← total / 5 ← [1] average calculated print(total) ← [1] both outputs print(average)

COMMON MARK LOSSES

• Forgetting to initialise total ← 0 before the loop
• Missing NEXT i at the end of the FOR loop
• Putting the average calculation inside the loop
• Using = instead of ← for assignment

LOOP SELECTION GUIDE

Loop	Use when	Min runs
FOR	Known count	1
WHILE	Unknown, may skip	0
DO..UNTIL	Must run once first	1

WHILE INPUT VALIDATION — FULL PATTERN

num ← int(input("Enter 1–10: ")) ← prime WHILE num < 1 OR num > 10 DO print("Invalid, try again") num ← int(input("Enter 1–10: ")) ENDWHILE

This pattern is very common in exam questions — always prime the variable before the WHILE condition is tested for the first time.

Common Mistakes

Common mistakes — avoid these in the exam

MISTAKE 1 — Using FOR when the count is unknown

Using a FOR loop when repeating "until the user enters a valid value" — the count is unknown, so FOR is wrong

✓ Use WHILE (or DO...UNTIL if input must be collected at least once before validating)

MISTAKE 2 — Python range() off-by-one error

Writing range(1, 5) expecting 1,2,3,4,5 — but range() excludes the upper bound, giving 1,2,3,4

✓ Use range(1, 6) to get 1,2,3,4,5 · ERL FOR i = 1 TO 5 includes 5

MISTAKE 3 — Forgetting to update the loop variable (infinite loop)

In a WHILE loop, forgetting to update the variable that controls the condition — loop never terminates because the condition never becomes False

✓ Always include a statement inside the loop body that moves toward the exit condition

MISTAKE 4 — DO...UNTIL exit condition wrong direction

Writing UNTIL num < 1 OR num > 10 — this exits on invalid input, the opposite of what you want

✓ DO...UNTIL exits when condition is True — write the condition as what you want: UNTIL num >= 1 AND num <= 10

Summary

Key points — 2.2.1b

Sequence — statements run top to bottom, in order; getting the order wrong causes incorrect results or crashes

Selection — IF/ELSEIF/ELSE/ENDIF for ranges; SWITCH/CASE/DEFAULT/ENDSWITCH for exact values; Python uses elif not ELSEIF

FOR — known count, FOR i = 1 TO n ... NEXT i; Python range() excludes the end — use range(1, n+1)

WHILE — pre-test; condition checked before body; may run 0 times; always prime the variable first

DO...UNTIL — post-test; body runs first then condition checked; always runs at least once; exits when condition is True

⚡ Next topic: 2.2.1c — Arrays — storing and accessing multiple values under one variable name.

2.2.1b Complete

Sequence, Selection
& Iteration

Get the full resource pack at CSZone.co.uk

📄

Marked Worksheet

CSZone.co.uk

❓

Quiz

CSZone.co.uk

📊

Slides

CSZone.co.uk

Next Up

2.2.1c — Arrays

Programming FundamentalsSequence, Selection & Iteration

By the end of this video you will be able to...

Sequence — statements execute top to bottom, in order

Selection — IF / ELSEIF / ELSE

Selection — SWITCH / CASE

FOR loop — count-controlled iteration

WHILE loop — condition-controlled, pre-test

DO...UNTIL — post-test, always runs at least once

Choosing the right construct

All three constructs — number guessing game

Sequence, selection & iteration — exam questions

Question 3 — answer and mark scheme

Common mistakes — avoid these in the exam

Key points — 2.2.1b

Sequence, Selection& Iteration

Programming Fundamentals
Sequence, Selection & Iteration

Sequence, Selection
& Iteration