Variables, Data Types & Arithmetic

Variables, Data Types & Arithmetic

What is a variable?

Every useful program needs to store data — values typed by the user, results of calculations, or information the program needs to remember. A variable is a named storage location in the computer's memory that holds a single item of data.

Think of a variable like a labelled box: the label is the variable's name, and whatever is inside the box is its current value. When you put something new in the box, the old contents are permanently replaced — a variable can only hold one value at a time.

Variables must be given a meaningful name that describes what they store. A variable called score is immediately understandable; a variable called x is not. Meaningful names make code easier to read, maintain, and debug.

Declaring a variable

Some languages require a declaration to specify a variable’s name, data type and initial value. Python creates a variable when you first assign a value to it. Read the SQA reference-language declarations below; you do not write them.
SQA uses reference language in question papers. You need to be able to read it. You will never be asked to write it.

SQA reference language (read only)

DECLARE variableName AS DATATYPE INITIALLY startingValue

Examples:

SQA reference language (read only)

DECLARE score AS INTEGER INITIALLY 0
DECLARE playerName AS STRING INITIALLY ""
DECLARE average AS REAL INITIALLY 0.0
DECLARE firstLetter AS CHARACTER INITIALLY "R"
DECLARE gameOver AS BOOLEAN INITIALLY FALSE

Note that Python does not require variable declarations — it works out the type automatically when the variable is first assigned a value. This is called dynamic typing. Exam questions are written in SQA reference language, so you must be able to read it. When writing code answers, however, you may use any programming language; marks reward correct understanding and logic rather than exact reference-language syntax.

The five data types

A data type defines what kind of data a variable can store and what operations can be performed on it. There are five data types at N5:

1. Character
Stores a single letter, digit, or symbol. Internally the computer stores the ASCII code for that symbol.

SQA reference language (read only)

DECLARE firstInitial AS CHARACTER INITIALLY "R"
SET firstInitial TO "R"

Examples of character values: "A", "7", "@", "!"

2. String
Stores a sequence of characters — essentially any piece of text. A phone number is stored as a string (not an integer) because it may start with 0, and you never do arithmetic with it.

SQA reference language (read only)

DECLARE username AS STRING INITIALLY ""
SET username TO "coolcoder99"

Examples: "Edinburgh", "L3tme!n#@", "0131 252 5000"

3. Integer
Stores a positive or negative whole number. No decimal point is possible.

SQA reference language (read only)

DECLARE score AS INTEGER INITIALLY 0
SET score TO 42

Examples: 0, -6, 1000, -999. Key point: integers never appear in double quotes in code — quotes are for characters and strings only.

4. Real
Stores a positive or negative number that can include a decimal point. Python calls this type float; when identifying an N5 data type, use the course term real.

SQA reference language (read only)

DECLARE average AS REAL INITIALLY 0.0
SET average TO 7.35

Examples: 3.14, -0.5, 186.332

5. Boolean
Can only store two values: TRUE or FALSE. Booleans are commonly used as flags — to track whether something has happened yet.

SQA reference language (read only)

DECLARE score AS INTEGER INITIALLY 120
DECLARE gameOver AS BOOLEAN INITIALLY FALSE
IF score > 100 THEN
    SET gameOver TO TRUE
END IF

Booleans connect directly to how all digital computers work at hardware level — everything ultimately comes down to 1s and 0s, true or false.

Choosing the right data type

Use this approach to choose a suitable data type:

• Is it a whole number you'll do arithmetic with? → Integer
• Is it a number with a decimal point? → Real
• Is it text, or a number you won't calculate with (like a postcode or phone number)? → String
• Is it a single letter or symbol? → Character
• Can it only ever be true or false? → Boolean

Assignment in Python

In Python, store data using assignment. The variable is created on its first assignment:

variableName = value

The computer reads the right-hand side first, works out the value, then stores the result in the variable on the left.

score = 0
playerName = "Maya"
average = score / 10

Arithmetic operators in Python

When working with integers and real numbers, you can use arithmetic operators to perform calculations. The five operators at N5 are:

Arithmetic operators are processed by the ALU (Arithmetic Logic Unit) inside the processor. The computer always evaluates the right-hand side of an assignment before storing the result.

length = 5.0
breadth = 3.0
price = 10.0
area = length * breadth
total = price + (price * 0.2)

BODMAS — operator precedence

When an expression contains more than one operator, the order in which they are calculated matters. Python and SQA reference language follow the same mathematical rule: BODMAS.

Multiplication and division have equal priority and are evaluated left to right; addition and subtraction also have equal priority. Multiplication and division happen before addition and subtraction unless brackets override the order.

Common BODMAS trap:

num1 = 6
num2 = 9
num3 = 12
average = num1 + num2 + num3 / 3   # WRONG: divides num3 first
average = (num1 + num2 + num3) / 3  # CORRECT: adds all three first

String concatenation

Concatenation means joining two or more strings together to form a longer string. SQA reference language uses the & operator for concatenation:

SQA reference language (read only)

DECLARE firstName AS STRING INITIALLY ""
DECLARE surname AS STRING INITIALLY ""
SET firstName TO "Rohan"
SET surname TO "Singh"
SEND "Hello " & firstName & " " & surname & "!" TO DISPLAY

Output: Hello Rohan Singh!

Concatenation is also used to include variable values in output messages:

SQA reference language (read only)

SEND "Your score is " & score & " out of 100." TO DISPLAY

Use & to join each part of the output message in SQA reference-language SEND statements. Python uses + for string concatenation. Convert a number with str() before joining it to a string.