C1 · Variables & Data Types

Spec reference: Section C - Programming Paradigms
Key idea: Understand how programs store, name, and type data.

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▶ Variables and Data Types in Python

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Data types

Every variable or constant holds a specific type of data. The type determines what values it can hold and what operations can be performed on it.

Data type	Description	Examples
Integer	Whole numbers	5, -3, 1000
Float / Real	Decimal numbers	3.14, -0.5, 99.9
String	Text (characters in quotes)	"Alice", "Hello, world!"
Boolean	True or False only	True, False
Char	A single character	'A', '5', '!'

Why data types matter

1. Storage - an integer uses less memory than a float.
2. Operations - you can add integers; you concatenate strings.
3. Validation - checking that age = 25 is an integer prevents errors.

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Declaring variables

In some languages (like Java, C#), you must declare the type before using a variable:

// Java  -  explicit type declaration
int score = 0;
String name = "Alice";
boolean isLoggedIn = false;

In Python, the type is inferred from the value:

# Python  -  type inferred automatically
score = 0           # integer
name = "Alice"      # string
is_logged_in = False  # boolean

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Type casting (conversion)

Sometimes you need to convert a value from one type to another:

age_string = "25"           # This is a string
age_number = int(age_string)  # Convert to integer

price = 9.99                # Float
price_text = str(price)     # Convert to string: "9.99"

Common error

If a user types their age as input, it arrives as a string - even if they type 25. You must convert it: age = int(input("Enter age: ")) before doing arithmetic on it.

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Scope: local vs global variables

Scope	Where it is accessible	Example
Local	Only inside the function it is declared in	A variable declared inside def calculate()
Global	Accessible everywhere in the program	A variable declared at the top of the file

total = 0  # global variable

def add_score(points):
    bonus = 5  # local variable  -  only exists here
    return points + bonus

print(total)   # works  -  global
print(bonus)   # ERROR  -  bonus is local to add_score

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Managing variables - best practices

• Use descriptive names: studentName not s.
• Use camelCase or snake_case consistently: totalScore or total_score.
• Initialise variables before use: count = 0 before count = count + 1.
• Don't use a variable name that is a reserved word (if, while, print, etc.).

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Summary

Term	Meaning
Variable	Named memory location whose value can change
Constant	Named value that is set once and never changes
Data type	The kind of data stored (integer, string, boolean, etc.)
Type casting	Converting a value from one data type to another
Local variable	Only accessible inside the function it was declared in
Global variable	Accessible throughout the entire program

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Test Yourself

Question 1 of 5

What is the key difference between a variable and a constant?