Pseudocode Structures

Pseudocode structures — sequence, selection, and iteration — are the three building blocks used to express any algorithm. Mastering them lets you plan solutions to complex problems clearly.

Every algorithm, no matter how complex, is built from just three fundamental structures: sequence, selection, and iteration. These are sometimes called the three constructs of structured programming. Understanding them in pseudocode is the key to being able to plan and communicate any algorithm clearly.

1. Sequence

Sequence simply means that instructions execute one after another in order, from top to bottom. It is the default structure.

SET price ← 12.99
SET quantity ← 3
SET total ← price * quantity
OUTPUT "Total: £" + total

Each line runs once, in order. Nothing is repeated, nothing is skipped.

2. Selection

Selection (also called branching) allows the algorithm to choose between different paths based on a condition. In pseudocode this is written with IF, ELSE, and ENDIF.

IF score >= 70 THEN
    OUTPUT "Pass"
ELSE
    OUTPUT "Fail"
ENDIF

Multiple branches use ELSE IF:

IF score >= 90 THEN
    OUTPUT "Distinction"
ELSE IF score >= 70 THEN
    OUTPUT "Pass"
ELSE
    OUTPUT "Fail"
ENDIF

Note

ELSE IF in pseudocode corresponds to elif in Python. It adds as many extra branches as you need without deeply nesting multiple IF blocks.

3. Iteration (Loops)

Iteration means repeating a block of steps. There are two key forms in pseudocode:

Count-controlled loop (FOR)

Use when you know how many times to repeat:

FOR counter ← 1 TO 5 DO
    OUTPUT counter
ENDFOR

Condition-controlled loop (WHILE)

Use when repetition depends on a condition:

SET guess ← 0
WHILE guess ≠ 42 DO
    INPUT guess
ENDWHILE
OUTPUT "Correct!"

Tip

Choose the right loop type when writing pseudocode. If you know the number of repetitions upfront, use FOR. If you are waiting for a condition to change, use WHILE.

Combining All Three Structures

Real algorithms combine all three constructs. Here is pseudocode for a number-guessing game:

SET secret ← 42
SET attempts ← 0
SET found ← FALSE

WHILE found = FALSE DO
    INPUT guess
    SET attempts ← attempts + 1

    IF guess < secret THEN
        OUTPUT "Too low"
    ELSE IF guess > secret THEN
        OUTPUT "Too high"
    ELSE
        OUTPUT "Correct! Attempts: " + attempts
        SET found ← TRUE
    ENDIF
ENDWHILE

This uses:

Sequence — each step in order
Iteration — the WHILE loop keeps going until the player guesses correctly
Selection — the IF-ELSE IF-ELSE checks three different outcomes

Warning

When tracing pseudocode in an exam, work through each line carefully, updating variable values as you go. Keep a table of variable values — this helps you avoid making arithmetic mistakes mid-trace.

Nesting Structures

Structures can be nested inside each other. In the example above, the IF statement is nested inside the WHILE loop. This is perfectly normal and is how complex behaviour is built from simple building blocks.

Practice Problems

Identify the three structures (sequence, selection, iteration) in this pseudocode: SET total ← 0; FOR i ← 1 TO 10 DO; INPUT num; IF num > 0 THEN SET total ← total + num ENDIF; ENDFOR; OUTPUT total.

Show answer

Sequence: SET total ← 0 and OUTPUT total run in order before and after the loop. Iteration: the FOR loop repeats 10 times. Selection: the IF statement inside the loop only adds num to total if it is positive.

Rewrite this Python code as pseudocode: count = 0; for n in range(1, 6): if n % 2 == 0: count += 1; print(count)

Show answer

SET count ← 0; FOR n ← 1 TO 5 DO; IF n MOD 2 = 0 THEN; SET count ← count + 1; ENDIF; ENDFOR; OUTPUT count. (Note: MOD is the pseudocode equivalent of Python's % operator.)