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Variables, Constants, and Data Types

Variables, Constants, and Data Types

This lesson covers the fundamental building blocks of every program — variables, constants, and data types. Understanding these concepts is essential for GCSE Computer Science (AQA 3.2 / OCR J277 2.2) and forms the foundation of all programming topics.

What Is a Variable?

A variable is a named location in memory that stores a value which can change during the execution of a program. Think of it as a labelled box — the label is the variable name, and the contents of the box is the value.

Declaring and Assigning Variables

In pseudocode (AQA style):

name ← "Alice"
age ← 16
score ← 0

In Python:

name = "Alice"
age = 16
score = 0

The assignment operator (← in pseudocode, = in Python) stores the value on the right into the variable on the left. Variables can be reassigned at any time:

score = 10
score = score + 5  # score is now 15

Naming Rules

  • Variable names should be meaningful (e.g. totalScore not x).
  • They must start with a letter (not a number).
  • They cannot contain spaces — use camelCase (totalScore) or snake_case (total_score).
  • They cannot be reserved words such as if, while, print.

Exam Tip: The examiner wants you to use meaningful identifiers. A variable called t will often lose you marks compared with temperature.

What Is a Constant?

A constant is a named value that is set once and cannot be changed during the execution of the program. Constants make code easier to read and maintain.

In pseudocode:

CONST VAT_RATE ← 0.20
CONST PI ← 3.14159

In Python (by convention — Python does not enforce constants):

VAT_RATE = 0.20
PI = 3.14159

Why Use Constants?

Benefit Explanation
Readability VAT_RATE is clearer than 0.20 scattered through code
Maintainability Change the value in one place and it updates everywhere
Error prevention Prevents accidental modification of important values

Exam Tip: If a question asks the difference between a variable and a constant, state that a variable's value can change during execution whereas a constant's value is fixed once assigned.

Data Types

A data type defines the kind of value a variable can hold. The five data types you must know for GCSE are:

Data Type Description Example
Integer A whole number (positive or negative) 42, -7, 0
Real / Float A number with a decimal point 3.14, -0.5, 100.0
Boolean A value that is either True or False True, False
Character A single letter, digit, or symbol 'A', '7', '!'
String A sequence of characters "Hello", "GCSE2025"

Integer vs Real

An integer has no fractional part: 5, -12, 0. A real (also called a float) has a decimal point: 5.0, -12.75. This distinction matters because integers use less memory and arithmetic with them is faster.

Boolean

A Boolean can only hold one of two values: True or False. Booleans are central to selection and iteration because conditions evaluate to a Boolean result.

is_logged_in = True
has_passed = score >= 50  # evaluates to True or False

Character vs String

A character (char) is a single symbol, whereas a string is zero or more characters in sequence. In Python, there is no separate char type — a single character is simply a string of length 1.

letter = 'A'         # character (string of length 1 in Python)
greeting = "Hello"   # string of length 5

Casting (Type Conversion)

Sometimes you need to convert between data types. This is called casting.

age = int("16")        # string → integer
price = float("9.99")  # string → real
label = str(42)        # integer → string

In pseudocode:

age ← INT("16")
price ← REAL("9.99")
label ← STRING(42)

Casting is essential when reading user input, because input() in Python always returns a string:

age = int(input("Enter your age: "))

Exam Tip: A very common error in exam pseudocode answers is forgetting to cast the input to an integer before comparing it with a number. Always show the cast explicitly.

Summary

  • A variable stores a value that can change; a constant stores a value that stays the same.
  • The five GCSE data types are integer, real/float, Boolean, character, and string.
  • Casting converts values between data types.
  • Use meaningful names for variables and constants.
  • Constants improve readability, maintainability, and error prevention.
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