Programming Concepts

• Variables & Constants — named boxes that store data; variables can change, constants can't.
• Data Types — INTEGER, REAL, CHAR, STRING, BOOLEAN — the "shape" of data.
• Input & Output — how a program talks to the user (keyboard in, screen out).
• Sequence — code runs top to bottom, one line at a time, in order.
• Selection — IF and CASE statements that make decisions and branch the flow.
• Iteration — FOR, WHILE, REPEAT loops that repeat code.
• Totalling & Counting — running-total and counter patterns inside loops.
• String Handling — case conversion, length, substrings.
• Operators — arithmetic (+ - * /), relational (== > <), Boolean (AND OR NOT).
• Procedures & Functions — reusable named blocks of code; functions return a value, procedures don't.
• Local & Global Variables — where a variable "lives" and who can see it.
• Library Routines — pre-built, tested code like RANDOM() and ROUND().
• Maintaining Programs — writing readable, well-structured code that's easy to fix later.

What's actually going on here?

Think of your computer's memory as a giant wall of lockers. A variable is a labelled locker where you can put something in, take it out, and swap it for something else — as many times as you like while the program runs. The label (the identifier) never changes, but the contents can.

A constant is also a labelled locker, but it's sealed with a value at the start and never opened again for the rest of the program's life. This matters more than it sounds — if you use the literal number 3.142 for π in fifteen different places in your code and then discover you need more precision, you have to hunt down and change all fifteen. If you'd used CONSTANT PI ← 3.142 instead, you change it in exactly one place.

Naming rules for identifiers

Cambridge is picky about this, and it's an easy mark to lose. An identifier (the name of a variable, constant, procedure, or function) must:

• Be in mixed case — also called Pascal case, e.g. FirstName, not firstname or FIRSTNAME
• Only contain letters (A–Z, a–z) and digits (0–9)
• Start with a capital letter — and never start with a digit

Declaring a variable

DECLARE Age : INTEGER
DECLARE Price : REAL
DECLARE GameOver : BOOLEAN

score = int()   # whole number
cost = float()  # decimal number
light = bool()  # True/False only

Declaring a constant

CONSTANT PI ← 3.142
CONSTANT PASSWORD ← "letmein"
CONSTANT HIGHSCORE ← 9999

PI = 3.142
VAT = 0.20
PASSWORD = "letmein"