Variables in C Programming

Overview of Variables in C

• Variables represent named memory locations to store various data types.
• A variable is essential for a C program and can store different values throughout its lifecycle.
• The size of a variable is determined by its data type.

Variable Syntax

• Declaration format: data_type variable_name = value;
• Multiple declaration format: data_type variable_name1, variable_name2;
• Examples of data types:
  • int for integers
  • char for characters
  • float for floating-point numbers

Components of Variable Definition

• Variable Declaration: Informs the compiler about a variable's name and data type; does not allocate memory.
• Variable Definition: Allocates memory and may assign a random garbage value until initialized.
• Variable Initialization: Assigns a meaningful value during the variable's creation, e.g., int var = 10;.

Initialization vs. Assignment

• Initialization occurs during declaration, assigning an initial value.
• Assignment updates the value of an already declared variable and can happen multiple times, e.g., a = 10;.

Variable Usage Example

• Example program demonstrates declaration, definition, and initialization of variables, outputting defined values after assignment.

Variable Naming Rules

• Can include letters, digits, and underscores.
• Must start with a letter or underscore, not a digit.
• No whitespace is allowed in variable names.
• Cannot be a reserved keyword in C.

Types of Variables in C

• Local Variables: Declared within functions or blocks, accessible only within that scope.
• Global Variables: Declared outside functions, accessible throughout the program.
• Static Variables: Retain their value for the program's lifetime; scope determined by declaration region.
• Automatic Variables: Default for local variables, existing only within the block where declared.
• Register Variables: Suggest the compiler store variable in CPU registers for faster access; address cannot be accessed.
• Constant Variables: Read-only variables defined using the const keyword; values cannot be modified after initialization.

Practical Examples of Variable Types

• Local variable example shows its limited scope within a function.
• Global variable example demonstrates accessibility across multiple functions.
• Static variable example illustrates value retention across function calls.
• Register variable example confirms no address access.

Constant Variables

• Constant variables must be initialized at the time of definition and cannot have their values changed later.