3. Variables & Data Types

Teaching your program to remember things

Your computer has two types of memory: RAM (Random Access Memory), which is fast and temporary — it forgets everything when you turn off the computer — and storage (SSD/HDD), which is slower but permanent.

When your program runs, it lives in RAM. Every value your program needs — a number, a name, a true/false flag — is stored somewhere in RAM. Variables are how you name those memory locations so you can use them.

At the deepest level, everything in a computer is binary — 1s and 0s. The number 5 is stored as 00000101. The letter 'A' is stored as 01000001. An image is millions of these bits organized in a specific pattern. Programming languages handle this translation for you.

Variables and Constants

A variable is a named container for a value. You give it a name, and you can store, change, and retrieve its value throughout your program.

A constant is a variable whose value should never change after it's set. In Python, we signal this by writing the name in ALL_CAPS — the language doesn't enforce it, but it's a strong convention:

Primitive Data Types

Primitive types are the fundamental building blocks of all data in programming. Every complex data structure is ultimately made of these.

• Integers (int): Whole numbers, positive, negative, or zero. No decimal point.
  score = 100 temperature = -5 year = 2025
• Floats (float): Numbers with decimal points. Used for anything requiring precision.
  price = 29.99 pi = 3.14159 percentage = 85.5
  ⚠️ Watch Out

  Floats are stored as approximations in binary. This means 0.1 + 0.2 might give you 0.30000000000000004, not exactly 0.3. For financial calculations, use the decimal module in Python.

• Strings (str): Text. Any sequence of characters enclosed in quotes. Strings are one of the most-used types in real programs.
  first_name = 'Ama' last_name = "Mensah" full_name = first_name + ' ' + last_name print(full_name) # Output: Ama Mensah
• Booleans (bool): The simplest type: either True or False. Used in every decision your program makes.
  is_logged_in = True has_premium = False x = 10 print(x > 5) # True print(x == 3) # False

Type Conversion & Casting

Sometimes you need to convert one type to another. Python provides built-in functions for this:

Dynamic vs Static Typing

In Python and JavaScript, variables can hold any type — and you can even change a variable's type. This is called dynamic typing.

In languages like Java, C++, or TypeScript, you must declare what type a variable will hold — and it can only ever be that type. This is static typing. It catches bugs earlier but requires more upfront code.