Variables, sequence, selection and iteration

These four ideas are the heart of programming. Every algorithm — no matter how complex — is built from a combination of variables, sequences, selections (decisions) and iterations (loops).

Variables and constants

A variable is a named storage location whose value can change as the program runs.

SET score TO 0
SET score TO score + 1

A constant is a name whose value is fixed throughout execution. Use constants for values that should never change (e.g. VAT rate, pi). They protect you against accidental edits and document intent.

CONSTANT VAT TO 0.20
SET price TO net * (1 + VAT)

In AQA pseudocode, the syntax may also appear as vatRate ← 0.20. The arrow and SET ... TO are interchangeable.

Naming conventions — use clear, lower-case-camelCase names: totalScore, isLoggedIn, numberOfAttempts. Avoid single letters except for loop counters i, j.

Sequence

Statements run in the order written, top to bottom. This is the simplest construct.

USERINPUT name
USERINPUT age
OUTPUT "Hello " + name
OUTPUT "You are " + age

If you swap the first two lines, the meaning changes — sequence matters.

Selection (IF / ELSE)

A decision picks between alternatives based on a condition.

USERINPUT score
IF score >= 70 THEN
  OUTPUT "Distinction"
ELSEIF score >= 50 THEN
  OUTPUT "Pass"
ELSE
  OUTPUT "Fail"
ENDIF

Boolean conditions use comparison operators (=, !=, <, <=, >, >=) and logical operators (AND, OR, NOT).

IF age >= 18 AND hasLicense = true THEN
  OUTPUT "Can drive"
ENDIF

Iteration — counted (FOR) and conditional (WHILE)

A counted loop (FOR) repeats a fixed number of times.

FOR i FROM 1 TO 10
  OUTPUT i * i
ENDFOR

This prints 1, 4, 9, …, 100. The loop counter i runs from 1 to 10 inclusive.

A conditional loop (WHILE) repeats as long as a condition is true. The condition is checked before each iteration, so the body might not run at all if the condition is false initially.

USERINPUT pin
WHILE pin != 1234
  OUTPUT "Wrong PIN. Try again."
  USERINPUT pin
ENDWHILE
OUTPUT "Access granted"

If the user enters the right PIN first time, the loop body never executes.

A REPEAT … UNTIL loop (post-test) checks the condition after the body — so the body always runs at least once. AQA pseudocode uses REPEAT … UNTIL condition.

Choosing FOR or WHILE

• You know how many times in advance → FOR.
• The number depends on input or a condition during execution → WHILE.

E.g. iterating through every element of a 10-item array → FOR. Reading lines until a special "end" marker → WHILE.

Combining the constructs

SET total TO 0
SET count TO 0
USERINPUT n
WHILE n != 0
  IF n > 0 THEN
    SET total TO total + n
    SET count TO count + 1
  ENDIF
  USERINPUT n
ENDWHILE
IF count > 0 THEN
  OUTPUT "Average: " + (total / count)
ELSE
  OUTPUT "No positive numbers entered"
ENDIF

This nests selection inside a loop, then uses another selection to handle the edge case of no input.

Common pitfalls

1. Off-by-one errors. FOR i FROM 1 TO 10 runs 10 times, not 9 or 11.
2. Forgetting to update the loop variable in a WHILE — gives an infinite loop.
3. Using = instead of == in some languages (Python: == for comparison; = for assignment). AQA pseudocode uses = for both, by context.
4. Mixing AND/OR. age >= 18 AND age <= 65 is correct; age >= 18 OR age <= 65 is true for everyone.
5. Reusing a variable name for two different purposes — confusing.

USERINPUT n
SET total TO 0
FOR i FROM 1 TO n
  SET total TO total + i
ENDFOR
OUTPUT total