Variables & Constants: How Programs Store Data

Programs need a place to store information while they run.

That place is a variable—a named container in memory that holds a value you can read and change. Sometimes you also need a constant—a container you cannot change after setting it.

In this lesson you’ll see how data lives in memory cells, what a memory address is, what an identifier means, and how to choose the right type without wasting memory.

1️⃣ What is a Variable?

🔹 A variable is a container for storing data values you’ll reuse during program execution.

🔹 Think “kitchen containers”: you pick the right size and label it for what it holds.

2️⃣ Anatomy of a Variable (RAM view)

For a single variable you always have:

• Address (e.g., 0x7fff...) → Location in memory (hexadecimal)
• Type (e.g., Int, Float, String, Boolean)
• Identifier (Name) (e.g., Age)
• Value (e.g., 45)
• Memory Cell (the physical slot that holds the bits)

Program can change the variable content at runtime :

Example :

• Type: Int
• Identifier (Name): Age
• Value: 45
• Memory cell: a slot in RAM that actually stores the bits
• Memory address: where this cell lives (shown in hex, e.g., 0x7fff5694dc58)

Updating a variable:

Age = Age + 2 → from 45 to 47. The same cell gets a new value.

3️⃣ Primary Variable Types

• Integer (Int): whole numbers (e.g., 0, 45, −3)
• Float/Double: numbers with fractions (e.g., 3.14)
• String: text (e.g., "Mohammed")
• Boolean: True/False (1/0)
  

4️⃣ What is a Constant?

🔹 A constant is also a container for storing data, but read-only after initialization.
🔹 If you try to assign a new value → Error.

Example: Const Float PI = 3.14 → PI = PI + 2 ❌ (not allowed).

5️⃣ Performance Tips — Don’t Waste Memory!

• Choose the smallest sensible type for the real-world range.
• Example : human Age will never be 4,294,967,295; typical max ~150 → don’t reserve a huge type unnecessarily.
• Fewer/lighter variables → fewer memory space → better performance.

🧬 Characteristics

• 1️⃣ Variable → mutable (can be changed) value in a memory cell.
• 2️⃣ Constant → immutable (cannot be changed) value; compile/runtime prevents changes.
• 3️⃣ Identifier → human-friendly name bound to a memory location.
• 4️⃣ Address → precise location in RAM (shown in hexadecimal).
• 5️⃣ Type → defines what kind of data and how much memory to use.

🔗 Interconnection

• Type decides size/format → impacts performance and limits.
• Identifier ↔ Address: name you use vs where it lives in RAM.
• Variable vs Constant: mutable vs read-only; both are containers but with different rules.

By carefully choosing names, types, and mutability, you write code that’s clear, safe, and fast. 🚀