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Sequence, selection and iteration

GCSE Computer Science Updated Wed 7 Oct 2026

Every program is built from three structures: sequence (one step after another), selection (choosing a path) and iteration (repeating). Exam questions ask you to write them, spot which is which, and trace what a loop does.

Part 1 of 3: Learn it

In short

  1. Selection uses IF, ELSE IF and ELSE (or CASE) to choose which code runs.
  2. A count-controlled loop (FOR) repeats a fixed number of times.
  3. A condition-controlled loop (WHILE, REPEAT UNTIL) repeats until a condition changes.

Where this is in your specification

Spec points: AQA 3.2.2, OCR 2.2.1, Edexcel Topic 6

BoardTopic: Programming in exam style
AQA 85253.2.1 to 3.2.11, 3.7.1
Edexcel 1CP2Topic 6 (and Topic 1 for trace tables)
OCR J2772.2.1 to 2.2.3, 2.3.1, 2.3.2
Eduqas C5007 Principles of programming; 10 Program construction
Cambridge IGCSE 04788 Programming
National 5 C816 75Software design and development: implementation, testing

Selection

IF mark >= 70 THEN
OUTPUT "Grade A"
ELSE IF mark >= 50 THEN
OUTPUT "Grade B"
ELSE
OUTPUT "Try again"
ENDIF

Only one branch runs. The conditions are checked from the top, so put the most specific test first. A CASE (or switch) statement is a neater way to choose between many fixed values, such as menu options.

Iteration

LoopKindUse when
FOR i = 1 TO 10count-controlledyou know how many times to repeat
WHILE conditioncondition-controlled, checked at the startit may need to run zero times
REPEAT ... UNTIL condition (do while)condition-controlled, checked at the endit must run at least once, such as asking for input

Nesting and trace tables

Structures can sit inside each other: an IF inside a loop, or a loop inside a loop. To follow what code does, use a trace table: one column for each variable (and for output), and a new row each time a value changes.

Quick check

Which kind of loop would you use to keep asking for a password until it is correct?

Show the answer

A condition-controlled loop, such as WHILE or REPEAT UNTIL.

Part 2 of 3: See it worked

Worked examples

Example 1

Trace this code and give the final value of total: total = 0, then FOR i = 1 TO 4, total = total + i * 2, NEXT i.

  1. i = 1: total = 0 + 2 = 2
  2. i = 2: total = 2 + 4 = 6
  3. i = 3: total = 6 + 6 = 12
  4. i = 4: total = 12 + 8 = 20

Answer: 20.

Example 2

n = 50 and count = 0. WHILE n > 1: n = n DIV 2 and count = count + 1. What is count at the end?

  1. n goes 50, 25, 12, 6, 3, 1
  2. count goes up once for each halving: 1, 2, 3, 4, 5
  3. When n = 1 the condition n > 1 is false, so the loop stops

Answer: count = 5.

Common mistakes

  • Using a FOR loop when the number of repeats is not known in advance.
  • Writing a WHILE condition that never becomes false, so the loop never ends.
  • Ordering IF tests so a general condition catches values meant for a later branch.
  • Off-by-one errors: FOR i = 1 TO 10 runs 10 times, not 9.
Quick check

How many times does FOR i = 3 TO 7 run?

Show the answer

5 times (3, 4, 5, 6, 7).

Part 3 of 3: Test yourself

Check yourself

Answer each one in your head or on paper first, then open it to check.

Which kind of loop would you use to keep asking for a password until it is correct?

A condition-controlled loop, such as WHILE or REPEAT UNTIL.

How many times does FOR i = 3 TO 7 run?

5 times (3, 4, 5, 6, 7).

What are the three basic programming structures?

Sequence, selection and iteration.

Jobs that use this

Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).

Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses