Instructions anyone can follow

Write down how to make a cup of tea. Give it to a friend. Can they follow it exactly?

"Boil the water." Which pot? "Add milk." How much? Good instructions are clear, in order and complete.

Before you write code, you plan it in this way.

Key words

Word	What it means
Flowchart	A diagram that shows the steps of a program as symbols joined by arrows.
BCS symbols	The standard flowchart shapes used in the UK. BCS is the Chartered Institute for IT. Its old name is the British Computer Society.
Pseudocode	Planning a program in plain words that look like code. No computer runs it.
Sequence	Steps that happen one after another.
Selection	A choice: the program takes one path or another. Also called branching.
Loop	Steps that repeat.
Iteration	Doing steps again and again. Also called a loop.
Control structure	Sequence, selection or iteration. They control the order of the steps.
Pre-defined code	Code that already exists, such as the built-in tools of a language, and that you reuse.

The cells students fill in are left blank in this preview.

Flowchart symbols

Symbol	Shape	Use it for	Example
Terminator	Rounded box	Start and end	Start
Process	Rectangle	A step that does something	Add 1 to total
Decision	Diamond	A yes or no question. Two exits.	Is age under 16?
Input or output	Parallelogram	Data in or out	Enter your age
Pre-defined process	Rectangle with double side lines	Code that already exists	Check the age is a number
Flow line	Arrow	Shows the order	From one step to the next

The cells students fill in are left blank in this preview.

Three control structures

Structure	In a flowchart	In pseudocode
Sequence	Steps in a line	SET price TO 5, then SET total TO price + 1
Selection	A diamond with two exits	IF age < 16 THEN OUTPUT "child" ELSE OUTPUT "adult" END IF
Iteration	An arrow that goes back up	FOR i FROM 1 TO 3, OUTPUT i, END FOR

The cells students fill in are left blank in this preview.

Try it: all three in Python

Code: python

# Sequence
price = 4
count = 3
total = price * count

# Selection
if total > 10:
    print("Big order:", total)
else:
    print("Small order:", total)

# Iteration
for i in range(1, 4):
    print("Ticket", i)

# Pre-defined code: len() and sum() already exist
print(sum([2, 4, 6]) / len([2, 4, 6]))

Writing pseudocode

Pseudocode looks like code but is for people. No computer runs it.

Use short capital words for the key ideas: INPUT, OUTPUT, SET, IF, THEN, ELSE, END IF, FOR, END FOR, WHILE. Indent (move to the right) the lines inside an IF or a loop. Write one step on each line. The sign < means "is less than".

The exact words can change from place to place. Pick one style and keep to it.