Variables, Constants, and Data Types

This lesson covers variables, constants, and the fundamental data types as required by OCR J277 Section 2.3. These concepts form the foundation of all programming and are essential for both Paper 1 (theory) and Paper 2 (practical programming) of the OCR GCSE Computer Science exam.

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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 a variable as a labelled box — the label is the variable name, and the contents of the box is the value.

OCR Pseudocode:

name = "Alice"
age = 15
name = "Bob"

Python:

name = "Alice"
age = 15
name = "Bob"  # The value of name has changed from "Alice" to "Bob"

Key rules for variable names:

Rule	Example	Valid?
Must start with a letter	score	Yes
Can contain letters, numbers, underscores	player_1	Yes
Cannot start with a number	1st_place	No
Cannot contain spaces	my score	No
Should be meaningful/descriptive	x vs totalScore	Both valid, but totalScore is better

OCR Exam Tip: Always use meaningful variable names in your exam answers. Names like x or a will not lose you marks but may make your code harder for the examiner to follow. Names like totalScore or userName demonstrate good programming practice.

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What Is a Constant?

A constant is a named value that is set once and cannot change during the execution of a program. Constants are used for values that remain fixed, such as the value of pi or the VAT rate.

OCR Pseudocode:

const VAT_RATE = 0.20
const PI = 3.14159
price = 100
total = price + (price * VAT_RATE)
print(total)

Python:

VAT_RATE = 0.20  # Python has no true const keyword; convention uses UPPER_CASE
PI = 3.14159
price = 100
total = price + (price * VAT_RATE)
print(total)  # Output: 120.0

Why Use Constants?

Reason	Explanation
Readability	VAT_RATE is clearer than 0.20 scattered throughout code
Maintainability	Change the value in one place rather than every occurrence
Prevents accidental changes	The value cannot be modified during execution

OCR Exam Tip: In the exam, if a question asks "why might a programmer use a constant instead of a variable?", always mention that it prevents accidental modification and makes the program easier to maintain.

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Data Types

A data type defines the kind of value a variable can hold and what operations can be performed on it. The OCR specification requires knowledge of five data types:

Data Type	Description	Example Values
Integer	A whole number (positive, negative, or zero)	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 (text)	"Hello", "GCSE", ""

OCR Pseudocode Examples:

age = 15                  // integer
temperature = 36.6        // real
isLoggedIn = true         // Boolean
initial = 'J'             // character
fullName = "John Smith"   // string

Python Examples:

age = 15                  # int
temperature = 36.6        # float
is_logged_in = True       # bool (note: capital T)
initial = 'J'             # str (Python doesn't distinguish char from string)
full_name = "John Smith"  # str

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Casting (Type Conversion)

Sometimes you need to convert a value from one data type to another. This is called casting.

Conversion	OCR Pseudocode	Python
String to integer	int("42")	int("42")
String to real	real("3.14")	float("3.14")
Integer to string	str(42)	str(42)
Real to integer	int(3.7) → 3	int(3.7) → 3
ASCII to character	ASC(65) → 'A'	chr(65) → 'A'
Character to ASCII	CHR('A') → 65	ord('A') → 65

OCR Exam Tip: A very common error is forgetting to cast the result of input() to an integer. In Python, input() always returns a string. To do arithmetic, you must write int(input()) or float(input()). This is also true in OCR pseudocode.

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Summary

• A variable stores a value that can change during program execution.
• A constant stores a value that cannot change — use const in OCR pseudocode.
• The five data types are: integer, real, Boolean, character, and string.
• Casting converts values between data types.
• Use meaningful names for variables and constants.

OCR Exam Tip: If asked to identify the data type of a value, look carefully. "42" is a string (because of the quotes), but 42 is an integer. True is a Boolean, not a string. This distinction is frequently tested.