C Variables Tutorial | Declaration, Assignment, Data Types

What is a Variable — A Named Place to Store Data

A variable is a named area of storage that holds data.
Variable name: the name of the storage area. Variable value: the data stored there.
When you need the data, you simply refer to it by name.

variable a

variable b

variable c

Where Variables Live — Inside the Computer

The values of variables are stored in the computer's memory (RAM).

CPU (Processor)

Controls the program and performs calculations.

Memory (RAM)

Holds data used by the CPU. Variables are stored here.

Hard Disk (Storage)

Long-term data storage.

Inside Memory — The World of Bits (0/1)

Memory is a sequence of bits that can hold the values 0 or 1. A variable's value is recorded in several consecutive bits, and the variable name labels that region.

Binary representation of int num = 5; in memory (32 bits)

Change value:

Data Types — Every Variable Has a Type

Variables have types, and each type reserves a different number of bytes in memory.

Integer type: int

Example: -2, -1, 0, 1, 2, 3, ...
No fractional part.
Reserves 4 bytes.
int num = 5;

Floating-point: double

Example: 1.2, -2.68, 3.14, 79.69, ...
For calculations with decimals.
Reserves 8 bytes.
double pi = 3.14;

Size Comparison

char (1B)

int (4B)

float (4B)

double (8B)

Declaration, Assignment, and Overwriting

To use a variable, you first declare it (reserve storage in memory), then assign a value to it.

int num;         // Declaration: reserve memory for the variable
num = 5;         // Assignment: store a value ( = is "assign", NOT equality!)

Note: = is not the equals sign; it is assignment. It stores the right-hand value into the variable on the left.

Assigning to an undeclared variable is an error: x = 23; → "What is x??"

How Overwriting Works

Assigning a new value to a variable overwrites the previous value, which is lost. A variable can hold only one value at a time.

variable num

—

Assigning One Variable to Another

Writing x = y; copies the value of y into x (y itself does not change).

variable x

←

variable y

Step Execution — Watch Variables Change

variable_demo.c

Variable state

Name	Type	Value

Standard output

Try It Yourself — Variables

my_vars.c

Output

Click "Run" to execute...

Review Quiz

Check your understanding of this lesson!

Q1. What can an int variable hold?

Decimal numbers

Integers

Strings

The int type stores integers. For decimals, use float or double; for strings, use a char array.

Q2. Which of these is a valid variable name?

2value

my_var

int

Variable names must start with a letter or underscore. Names starting with a digit, or reserved words like int, are not allowed.

Q3. What is the difference between double and float?

double has higher precision

float has higher precision

No difference

double is double-precision floating point (~15 digits), while float is single-precision (~7 digits). double is typically preferred.

Lesson 3: Variables