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Design: Pseudocode

SDD · Lesson 4 of 19 Software Design & Development Approx 60 min
Learning intentions
  • Describe what pseudocode is and explain its purpose in program design
  • Read and interpret pseudocode that includes sequence, selection, and repetition
  • Write structured-English pseudocode, adding refinements where detail is needed
  • Explain the advantages and disadvantages of pseudocode as a design notation
Success criteria
  • I can explain what pseudocode is in my own words
  • I can read a piece of pseudocode and explain what it does step by step
  • I can write a clear algorithm in structured English and refine steps that need detail
  • I can use numbering and indentation as optional aids to clarity
  • I can describe decisions and fixed or conditional loops in words, with clear block boundaries
  • I can give one advantage and one disadvantage of pseudocode
Warm up — what do you already know?

Answer before the lesson begins. These check prior knowledge — it's fine if you're unsure.

1. Which of the following best describes pseudocode?

2. In pseudocode, what does a refinement do?

3. Which design technique from the previous lesson shows the structure of a program as a tree diagram?

Key vocabulary

Pseudocode
A design technique that uses structured English to describe an algorithm — it has no strict syntax rules and cannot be run by a computer.
Algorithm
A precise sequence of steps that solves a problem — pseudocode is one way to represent an algorithm.
Main steps
The top-level steps giving an overview of the algorithm. Numbering them is an optional clarity aid.
Refinement
Detail added to a main step so its logic can be understood and checked. Optional numbers such as 2.1 and 2.2 link the detail to its main step.
Selection
A decision that chooses steps according to a condition. In structured English, describe the condition and the actions for each outcome.
Repetition
Repeating steps a known number of times in a fixed loop, or until a condition is met in a conditional loop. Describe the boundaries in words.

Design: Pseudocode

What is pseudocode?

Pseudocode is a text-based design technique used to plan a program before writing real code. It describes the logic of an algorithm using structured English — there is no fixed syntax to follow, which means programmers can focus on the logic rather than worrying about the exact rules of a programming language.

The SQA specification describes pseudocode as "a natural language-based design methodology used to define an algorithm and refinement." It is intended to be read by humans, not run by computers. Structured English combines ordinary words with explicit conditions, repetition and actions so the algorithm can be checked before implementation.

Because pseudocode has no strict rules, it can be written in any text editor or on paper. Two programmers might write pseudocode slightly differently and both be correct. What matters is that the logic is clear and that anyone reading it could translate it into real code.

SQA uses reference language in question papers. You need to be able to read it. You will never be asked to write it.

Three things, three jobs

  • Pseudocode: write structured English for design questions.
  • SQA reference language: read only — read, trace and explain supplied code.
  • Python: write for code questions.

Main steps and refinements

A useful method is to write an algorithm of main steps, then add refinements where more detail is needed. This lesson models that method; a separate two-part layout is not a marking requirement.

Main steps give the top-level overview of the algorithm. Optional numbers such as 1, 2, 3… help identify steps. They describe what the program does at a high level, using broad descriptions like "Get scores from user" or "Calculate average". Main steps are typically only a few words long.

Refinements supply the detail needed to understand and check a main step. Optional numbers such as 1.1 and 1.2 show which step is being refined. A refinement may become several lines of code; a step that is already clear needs no extra detail.

Clear logic matters. Numbering, indentation and a separate refinements section can help a reader follow your design, but none earns a mark simply by being present. A clear detailed list can also communicate a valid design.

1. Get and total 20 scores 1.1 Set total to 0 1.2 Start fixed loop 20 times 1.3    Get score and add it to total 1.4 End fixed loop 2. Calculate average as total divided by 20 3. If average is at least 50       Display Pass Otherwise display Fail Main step — top-level description Refinement — supplies necessary detail Loop count — repeat exactly 20 times Natural-language boundary — loop ends here Selection — condition chooses the outcome

Including selection and repetition

A selection chooses actions according to a condition. Repetition repeats actions. The course specification, pseudocode appendix (printed pp49–50), says selection and repetition may be highlighted with indentation. Here we use indentation and natural-language boundaries to make the blocks easy to follow.

Describe a decision and both outcomes in structured English:

If score is at least 50
    Display Pass
Otherwise
    Display Fail
End selection

A fixed loop repeats a known number of times:

Set total to 0
Start fixed loop for each of the 30 pupils
    Get score from user
    Calculate total by adding score to total
End fixed loop

A conditional loop repeats until its condition is met:

Start conditional loop
    Ask user for an answer
    If answer is incorrect
        Display Please try again
    End selection
Repeat until answer is correct

Why use pseudocode?

Pseudocode lets programmers plan an algorithm without getting distracted by syntax errors. It can be written quickly in any text editor or even on paper. It is also easy to show refinements progressively — a complex main step can be broken down until the necessary inputs, calculations, conditions and outputs are clear. This helps when implementing and testing the design.

However, pseudocode has a drawback: there is no universal standard notation. Different schools, companies, and textbooks use different conventions, which can cause confusion when sharing designs. It also does not show the flow of a program as visually as a flowchart does — loops and branches are less immediately obvious in text than in a diagram.

Worked examples

Example 1 — Swimming pool volume (sequence only)

Problem: Write pseudocode for a program that calculates the volume of a swimming pool.

1
Write the main steps first — identify the three top-level tasks: get the dimensions, calculate the volume, display the result. 
Algorithm
1  Ask user to enter dimensions of a swimming pool in metres
2  Calculate volume of pool
3  Display message stating the volume of the pool
2
Write refinements for step 1 — make clear that all three dimensions must be obtained and stored. 
1.1  Get length of pool in metres from user
1.2  Get width of pool in metres from user
1.3  Get depth of pool in metres from user
3
Write refinements for steps 2 and 3. State the calculation and the output clearly. 
2.1  Calculate volume as length multiplied by width multiplied by depth
3.1  Display the volume of the pool in cubic metres

For a pool 10 metres long, 5 metres wide and 2 metres deep, the volume is 10 × 5 × 2 = 100 cubic metres. Add detail where it helps; do not force extra refinements onto an already clear step.

Example 2 — Class average (fixed loop + selection)

Problem: Write pseudocode for a program that takes 20 scores from the user, calculates the average, and displays whether the class passed (average ≥ 50).

1
Identify the main steps: initialise, get scores, calculate, display. 
Algorithm
1  Initialise variables
2  Get scores
3  Calculate average
4  Display result
2
Refine step 1 (initialise total to 0) and step 2 (a fixed loop for 20 scores). 
1.1  Set total to 0
2.1  Start fixed loop for each of the 20 pupils
2.2      Get score from user
2.3      Calculate total by adding score to total
2.4  End fixed loop
3
Refine steps 3 and 4. Step 4 chooses the pass or fail message. 
3.1  Calculate average as total divided by 20
4.1  If average is at least 50
4.2      Display The class passed and the average
4.3  Otherwise
4.4      Display The class did not pass and the average
4.5  End selection

Main step 2 (Get scores) contains a fixed loop in its refinements. Main step 4 (Display result) contains selection. Both use indentation to show structure clearly.

Example 3 — Input validation (conditional loop)

Problem: Write pseudocode for a program that asks the user to enter an age and keeps asking until a valid age (between 0 and 120 inclusive) is entered.

1
The main steps are straightforward: get a valid age, then display it. 
Algorithm
1  Get valid age from user
2  Display confirmed age
2
Refine step 1. The program must ask once, then loop while the input is invalid. This is a conditional loop because we do not know in advance how many times it will repeat. 
1.1  Get age from user
1.2  If age is below 0 or above 120
1.3      Start conditional loop
1.4          Display Invalid age. Please enter an age between 0 and 120 inclusive
1.5          Get age from user
1.6      Repeat until age is between 0 and 120 inclusive
1.7  End selection
3
Refine step 2 — a single display statement. 
2.1  Display the confirmed age

The first Get age from user is a priming read: it supplies the age before the first validity check. If that age is valid, the loop is skipped. Otherwise, the conditional loop displays an error and gets a new age until it is valid. Both 0 and 120 are valid; −1 and 121 are invalid.

Now you try

A programmer is writing a program that asks the user to enter 5 temperatures, calculates the average, and displays a message saying whether the average is above 20°C or is 20°C or below.

Write pseudocode for this program. Your design needs these logical steps; main steps with refinements are one useful way to organise them:

  1. Set the total to 0 before getting the readings
  2. A fixed loop to get and total exactly 5 temperatures, then calculate their average
  3. A selection to display the correct message and average for both outcomes

Pseudocode — structured-English design:

Algorithm
1  Initialise variables
2  Get temperatures
3  Calculate average
4  Display result

Refinements
1.1  Set total to 0
2.1  Start fixed loop for each of the 5 readings
2.2      Get temperature from user
2.3      Calculate total by adding temperature to total
2.4  End fixed loop
3.1  Calculate average as total divided by 5
4.1  If average is above 20
4.2      Display Above 20°C and the average
4.3  Otherwise
4.4      Display 20°C or below and the average
4.5  End selection

Mark yourself: ✓ total starts at 0 · ✓ exactly 5 readings · ✓ each reading is added inside the loop · ✓ average calculated after the loop · ✓ average compared to 20 · ✓ both outcomes displayed with the average. Accept any clear alternative design.

SQA reference language (read only)

DECLARE total AS REAL INITIALLY 0
DECLARE temperature AS REAL INITIALLY 0
DECLARE average AS REAL INITIALLY 0
FOR reading FROM 1 TO 5 DO
    RECEIVE temperature FROM KEYBOARD
    SET total TO total + temperature
END FOR
SET average TO total / 5
IF average > 20 THEN
    SEND "Above 20°C " & average TO DISPLAY
ELSE
    SEND "20°C or below " & average TO DISPLAY
END IF

Python — implementation

total = 0
for reading in range(5):
    temperature = float(input("Enter temperature: "))
    total = total + temperature
average = total / 5
if average > 20:
    print("Above 20°C", average)
else:
    print("20°C or below", average)

All three versions get exactly 5 readings. The reference-language loop includes both endpoints, 1 and 5. Python range(5) gives 0, 1, 2, 3, 4 — also 5 iterations. For the class example, range(20) gives 20 iterations, matching a reference-language loop from 1 to 20. Read the reference panel to compare the logic; do not copy its notation for a design answer.

Check the calculation with 18, 20, 22, 24, 26: total 110, average 22, so display Above 20°C. Five readings of 20 give total 100 and average 20, so display 20°C or below.

Common mistakes
✕
Unclear block boundaries — Make it clear in words where repeated steps and decisions begin and end. This helps a reader follow the logic. Missing formal END keywords is not a pseudocode mark rule: in SQA 2024 Q11(a)(i), a missing loop start or end was tolerated.
✕
Too little detail — “Calculate total” may leave the method unclear. Explain that total starts at 0 and each score is added inside the loop. Sufficient detail makes a design clear and checkable; refinement numbers are optional.
✕
Overview without the necessary actions — Main steps help organise a design, but a reader also needs the conditions, inputs, calculations and outputs. Supply these in refinements or in a clear detailed algorithm.
✕
Unclear grouping — Indentation may highlight selection and repetition. Use it to show which actions belong together; the specification does not make indentation a separate marking requirement.
✕
Real code syntax in a design answer — Write structured English such as “Display hello” and “Start fixed loop 5 times”. Python function calls, colons and range expressions belong in the Python panel. Formal reference-language keywords belong in a labelled reading example.
Exam tip

For a design question, write clear structured-English pseudocode. An algorithm with refinements can help you supply the necessary detail. For a code question, write Python. When reference language is supplied, read it to work out its behaviour.

SQA 2024 Q11(a)(i), marking instructions p6, gives four logic points for that validation design: a conditional loop, the correct condition for the valid range 5–75, input inside the loop and an error message inside the loop. A missing loop start or end was tolerated when interpreting the design. These are criteria for that question, not a universal keyword checklist.

In the 2024 commentaries, Candidate 14 Q11(a)(i), a minimalist design earned 4/4. Focus on making the intended logic clear, rather than adding numbering or keywords to try to earn marks.

Task Set

Questions 1–4 are auto-checked. Questions 5–8 are self-marked — write your answer, then reveal the model answer to check your work.

1. What does the refinement number 3.2 mean? TYPE 1

2. Which of the following is a correct reason to use pseudocode rather than a flowchart? TYPE 1

3. A design says “Start conditional loop; get a score; repeat until the score is at least 0”. What type of construct is this? TYPE 1

4. Which of the following is a disadvantage of pseudocode? TYPE 1

5. This design should get 10 numbers, total them, then display Positive if the total is above 0, or Zero or negative otherwise. Identify four logic errors and explain each correction. TYPE 2

Get number from user
Start fixed loop 9 times
    Calculate total by adding number to total
End fixed loop
If total is below 0
    Display Positive
Otherwise
    Display Zero or negative
End selection
  • Missing initialisation: set total to 0 before adding any numbers.
  • Wrong count: repeat 10 times, not 9.
  • Input outside the loop: move Get number from user inside so each iteration gets a new number.
  • Wrong condition: test whether total is above 0 to choose Positive.
Set total to 0
Start fixed loop 10 times
    Get number from user
    Calculate total by adding number to total
End fixed loop
If total is above 0
    Display Positive
Otherwise
    Display Zero or negative
End selection

Mark yourself: ✓ initial total is 0 · ✓ 10 inputs are totalled · ✓ each new input is inside the loop · ✓ the two messages match positive and non-positive totals. Accept any clear alternative design with this logic.

6. A program asks a user to enter a PIN number. The PIN must be exactly 4 digits long; treat it as text so leading zeros are kept. The program keeps asking until the user enters a valid PIN, then confirms it was accepted. Write structured-English pseudocode for this program. Use refinements where more detail is needed. TYPE 2

Algorithm
1  Get valid PIN from user
2  Confirm PIN accepted

Refinements
1.1  Get PIN as text from user
1.2  If PIN does not have 4 characters or contains a non-digit character
1.3      Start conditional loop
1.4          Display Invalid PIN. Must be 4 digits.
1.5          Get PIN as text from user
1.6      Repeat until PIN has 4 characters and every character is a digit
1.7  End selection
2.1  Display PIN accepted

Mark yourself: ✓ PIN is kept as text so leading zeros are preserved · ✓ validity checks both length and digit-only content · ✓ invalid input gets an error message and a new input inside the loop · ✓ valid first input skips the loop · ✓ confirmation follows validation. Accept any clear alternative design. Test 0123 as valid; 123, 12345 and 12a3 as invalid.

7. Give one advantage and one disadvantage of pseudocode as a design technique. TYPE 2

Advantage (any one of):

  • Can be written quickly without worrying about programming language syntax
  • No special tools required — can be written on paper or in any text editor
  • Easy to show refinements progressively, making translation to code straightforward

Disadvantage (any one of):

  • There is no standard notation, so different programmers may write it differently, causing confusion
  • Does not show the flow of the program as clearly as a flowchart (loops and branches are less visually obvious)

8. A program takes 10 exam scores between 0 and 100 inclusive from a user, finds the highest score, and displays the message "New high score!" each time a score beats the current highest. After all scores have been entered, the program displays the final highest score. Write pseudocode for this program. Use a fixed loop and a decision in structured English. Add refinements where detail is needed. TYPE 3

Algorithm
1  Initialise variables
2  Get scores and track highest
3  Display final result

Refinements
1.1  Set highest to 0
2.1  Start fixed loop for each of the 10 exams
2.2      Get score from user
2.3      If score is greater than highest
2.4          Set highest to score
2.5          Display New high score!
2.6      End selection
2.7  End fixed loop
3.1  Display the highest score

Mark yourself: ✓ highest starts at 0 for non-negative scores · ✓ exactly 10 scores are entered inside the loop · ✓ each score is compared with highest · ✓ a greater score updates highest and displays New high score! · ✓ equal scores do not announce a new high · ✓ final highest displayed after the loop. Accept any clear alternative design.

Teacher notes — Shift+T to hide

Lesson context: This is SDD lesson 4, following structure diagrams and flowcharts. Teach three distinct jobs: pupils write structured-English pseudocode for designs, read SQA reference language, and write Python for code questions. The bridge shows the same temperature algorithm in all three forms.

Common misconceptions: Do not make Python or reference-language syntax the definition of pseudocode. Main steps with refinements are useful for planning; a clear detailed list is also acceptable. Numbering and indentation are optional clarity aids. Do not claim penalties for absent refinements or END keywords.

Assessment evidence: SQA 2024 Q11(a)(i) awards four logic points for its validation design: conditional loop, correct 5–75 condition, input inside the loop and error message inside the loop. The marking instructions tolerate a missing loop start/end. Candidate 14’s minimalist answer earned 4/4 in the 2024 commentaries. Use the sources in the exam tip; do not generalise this into annual exam-weight or formatting claims.

Differentiation: Q1–Q4 are auto-checked. Q5 tests four logic faults. Q6 validates a text PIN, including leading zeros; Q8 places a decision inside a fixed loop. Check age boundaries 0 and 120, temperature equality at 20, and tied highest scores. Accept any clear alternative design meeting the logic criteria.

SQA command words covered: "write pseudocode", "identify an error", "give an advantage/disadvantage", "describe".