Variables and Constants in C

In C programming, variables and constants are used to store data values. Variables allow us to change the value during program execution, while constants hold values that remain fixed throughout the program.

1. Variables in C

A variable is a storage location in memory, identified by a name, where data can be stored, modified, and retrieved during program execution.

Declaring a Variable

To declare a variable, we specify the data type followed by the variable name. A variable can be declared without initialization or with an initial value.

int age;            // Declaration without initialization
int age = 25;       // Declaration with initialization

Rules for Naming Variables

A variable name must start with a letter (a-z, A-Z) or underscore (_).
The name can contain letters, digits, and underscores.
C is case-sensitive, so age and Age are different variables.
Keywords cannot be used as variable names.

Example:

int age = 20;      // Valid
float _height = 5.9; // Valid
int 1stRank = 1;   // Invalid, cannot start with a digit

Types of Variables

Variables in C are classified into three main types based on their scope, lifetime, and visibility:

a. Local Variables

Local variables are declared inside a function or block and can only be accessed within that function or block.

void function() {
    int localVar = 10;  // Local variable
}

b. Global Variables

Global variables are declared outside any function, typically at the top of the program. They can be accessed by any function within the program.

int globalVar = 50;   // Global variable
 
void function1() {
    printf("%d", globalVar); // Accessible here
}
 
void function2() {
    globalVar = 100; // Can be modified
}

c. Static Variables

Static variables maintain their value across function calls. If declared inside a function, they are not re-initialized when the function is called again.

void function() {
    static int counter = 0;
    counter++;
    printf("%d", counter); // Will retain its value across calls
}

2. Constants in C

A constant is a value that, once defined, cannot be changed during the execution of the program. Constants help ensure that certain values remain fixed.

Defining Constants

a. Using `#define` Preprocessor Directive

#define is a preprocessor directive used to define constant values before the program is compiled. It does not use any memory.

#define PI 3.14159
#define MAX_VALUE 100

Example:

int radius = 5;
float area = PI * radius * radius;

b. Using `const` Keyword

The const keyword is used to declare constant variables, which behave like regular variables but cannot be modified after initialization.

const int DAYS_IN_WEEK = 7;

Types of Constants

a. Integer Constants

Integer constants are whole numbers without any fractional part.

#define MAX_AGE 100
const int MIN_AGE = 18;

b. Floating-Point Constants

Floating-point constants are numbers with a fractional part.

#define GRAVITY 9.81
const float PI = 3.14159;

c. Character Constants

Character constants are enclosed in single quotes and represent a single character.

const char GRADE = 'A';

d. String Constants

String constants are enclosed in double quotes and represent sequences of characters.

const char NAME[] = "John Doe";

Why Use Constants?

Readability: Constants make the code easier to understand by giving meaningful names to values.
Maintainability: You can update the value in one place if needed, instead of updating it throughout the program.
Safety: Constants prevent accidental modification of values that should remain fixed.

Summary

In C programming, variables and constants are fundamental for storing and managing data. While variables allow dynamic changes in values during program execution, constants provide fixed values that cannot be altered.

Diagram of Variables and Constants in C

Data Types Operators