Pseudocode & Flowcharts

1. What pseudocode actually is

Pseudocode means “fake code”. It is a plan for a program, written in short English sentences and laid out like code, with the important words in CAPITALS.

The thing to get straight first: no computer ever runs pseudocode. A person reads it. That is the whole point of it existing.

• It has no fussy rules. No colons, no brackets, no semicolons to forget. You cannot get a syntax error, because nothing is checking.
• It works for any language. The same plan can be turned into Python, Java, JavaScript or C#. The plan is the idea; the language is just how you spell it.
• It is how you think first and type second. Sort out what the program does while it is cheap to change your mind, then translate.
• It is on the exam. Writing pseudocode shows you can think like a programmer without the marker needing to check whether you remembered Python’s punctuation.

Think of a recipe. “Boil the water, add the pasta, wait 10 minutes, drain it.” Nobody can eat the recipe. It is instructions for a human, in order, and that is exactly what pseudocode is for a program.

OUTPUT "What is your name?"
INPUT name
OUTPUT "Hello " + name

name = input("What is your name?")
print("Hello " + name)

Same idea, twice. The pseudocode says what happens. The Python says it in a way a computer will accept.

2. The four shapes

A flowchart is the same plan as a picture. You follow the arrows from the top, and the shape of each box tells you what kind of step it is. There are only four you need.

Arrows join the shapes and show the order. Only the diamond ever has two arrows leaving it, and they must be labelled Yes and No.

The same program, both ways

Here is a program that asks your age and tells you whether you can vote. On the left is the flowchart, on the right is the pseudocode. Read them side by side: every shape becomes a line.

BEGIN
    OUTPUT "How old are you?"
    INPUT age
    IF age >= 18 THEN
        OUTPUT "You can vote"
    ELSE
        OUTPUT "Too young"
    ENDIF
END

Which shape holds this line?

Six lines of pseudocode. Tap the shape each one would be drawn in.

3. The words you are allowed to use

Pseudocode keywords go in CAPITALS. Everything else, your own variable names and messages, stays normal. That contrast is what makes it readable at a glance.

4. Writing an IF, line by line

This is the bit worth practising, because it is the bit the exam asks for. We will build one up a line at a time. The job: ask how tall someone is, and tell them whether they can go on the ride (140 cm or more).

Step 1. Every program is wrapped in BEGIN and END. Write those first and fill in the middle.

BEGIN

END

Step 2. Ask the question. Asking is something the user sees, so it is an OUTPUT.

BEGIN
    OUTPUT "How tall are you in cm?"

END

Step 3. Read the answer, and keep it in a variable. This is the line the first mark is for. INPUT height, not just INPUT, because you are about to need that number.

BEGIN
    OUTPUT "How tall are you in cm?"
    INPUT height

END

Step 4. Now the decision. Test the variable, indent what happens if the answer is yes, and close it with ENDIF.

BEGIN
    OUTPUT "How tall are you in cm?"
    INPUT height
    IF height >= 140 THEN
        OUTPUT "You can ride"
    ENDIF
END

Step 5. As it stands, someone too short gets nothing at all. ELSE covers everything the IF did not.

BEGIN
    OUTPUT "How tall are you in cm?"
    INPUT height
    IF height >= 140 THEN
        OUTPUT "You can ride"
    ELSE
        OUTPUT "Sorry, too short"
    ENDIF
END

That is the finished answer, and it is the same skeleton every question of this type wants:

• Ask, then read. An OUTPUT question, then an INPUT that stores the answer in a variable.
• Test the variable. The IF line has to mention the thing you just read in. Testing anything else scores nothing.
• Both branches. THEN covers yes, ELSE covers no. Exactly one of them happens.
• Close it. ENDIF, then END. Indent so the shape is obvious.

5. ENDIF, curly brackets and indentation

Every language needs a way of saying “the IF stops here”. Without it, nothing could tell which lines belong inside the decision and which come afterwards. Languages just disagree about how to say it.

IF age >= 18 THEN
    OUTPUT "You can vote"
ELSE
    OUTPUT "Too young"
ENDIF

if (age >= 18) {
    System.out.println("You can vote");
} else {
    System.out.println("Too young");
}

if age >= 18:
    print("You can vote")
else:
    print("Too young")

Three ways of drawing the same box round the same two lines. Java writes }, pseudocode writes ENDIF, and Python has nothing to write at all: in Python the block ends when the indenting stops. That is why a stray space matters so much in Python and matters not at all in Java.

6. Put the lines in order

Here are the nine lines of a weather program, shuffled. It should ask how hot it is, then say Hot day if it is 30 or more and Not too bad if it is not.

Tap the lines in the right order. The indenting is drawn in for you, so watch the shape appear. Tap a line you have already placed to take it back out.

7. How the marks are given

Pseudocode questions are marked on structure, not spelling. Here is a real one, worth 3 marks, with the marks pointed out.

BEGIN
    OUTPUT "How old are you?"
    INPUT age                        <- mark 1
    IF age >= 18 THEN              <- mark 2
        OUTPUT "You can vote"       <- mark 3
    ELSE
        OUTPUT "Too young"          <- mark 3
    ENDIF
END

Two answers that dropped marks

BEGIN
    OUTPUT "How old are you?"
    INPUT
    IF age >= 18 THEN
        OUTPUT "You can vote"
    ELSE
        OUTPUT "Too young"
    ENDIF
END

BEGIN
    OUTPUT "How old are you?"
    INPUT age
    IF age >= 18 THEN
        OUTPUT "You can vote"
    ENDIF
END

Both students knew what they were doing. Both lost a mark on one missing line, which is why the four rules in part 4 are worth memorising as a shape rather than as a paragraph.

8. Quick check Part 1 of 2

Eight quick questions. This is half of completing the module. The exam task below is the other half.