C Variables Constants #define, const, scope: Tutorial, Examples, FAQs & Interview Tips

Variables

A variable is a named memory location that stores a value. In C, you must declare a variable before using it by specifying its data type and name.

// Syntax
type variableName;
type variableName = value;  // declaration with initialization

// Examples
int age;
int age = 25;
float salary = 50000.50;
char initial = 'J';

Variable Naming Rules

Must start with a letter (a"z, A"Z) or underscore _
Can contain letters, digits (0"9), and underscores
Cannot contain spaces or special characters (@, #, $, etc.)
Cannot be a C keyword (e.g., int, return, if)
Case-sensitive: age and Age are different variables
No length limit (but only first 31 characters are significant in older standards)

Valid Names	Invalid Names
`age`, `_count`, `total_price`, `num1`	`1num` (starts with digit)
`firstName`, `MAX_SIZE`	`my-var` (contains hyphen)
`x`, `y`, `z`	`int` (reserved keyword)

Variable Scope

Local variables are declared inside a function and can only be used within that function. Global variables are declared outside all functions and can be accessed from anywhere in the program.

Local vs Global Variables

#include <stdio.h>

int globalVar = 100;  // global variable - accessible everywhere

void display() {
    int localVar = 50;  // local variable - only inside display()
    printf("Inside display() - localVar: %d\n", localVar);
    printf("Inside display() - globalVar: %d\n", globalVar);
}

int main() {
    int localVar = 10;  // different localVar - local to main()
    printf("Inside main() - localVar: %d\n", localVar);
    printf("Inside main() - globalVar: %d\n", globalVar);

    globalVar = 200;    // modify global variable
    display();

    // printf("%d", localVar from display()); // ERROR - not accessible here

    return 0;
}

/*
Output:
Inside main() - localVar: 10
Inside main() - globalVar: 100
Inside display() - localVar: 50
Inside display() - globalVar: 200
*/

Constants

A constant is a value that cannot be changed after it is defined. C provides two ways to define constants:

#define Macro (Preprocessor Constant)

The #define directive creates a macro - the preprocessor replaces every occurrence of the name with the value before compilation. No memory is allocated.

#define PI 3.14159
#define MAX_SIZE 100
#define GREETING "Hello, World!"

const Keyword

The const keyword declares a variable whose value cannot be modified. Unlike #define, it has a type and occupies memory.

const float PI = 3.14159f;
const int MAX = 100;

#define and const Constants

#include <stdio.h>

#define PI        3.14159    // preprocessor macro - no type, no semicolon
#define MAX_SCORE 100
#define AUTHOR    "Dennis Ritchie"

int main() {
    // const variable - has type, stored in memory
    const int MIN_AGE = 18;
    const float GRAVITY = 9.81f;

    float radius = 5.0;
    float area = PI * radius * radius;

    printf("Area of circle: %.2f\n", area);
    printf("Max score: %d\n", MAX_SCORE);
    printf("Author: %s\n", AUTHOR);
    printf("Min age: %d\n", MIN_AGE);
    printf("Gravity: %.2f m/s^2\n", GRAVITY);

    // MIN_AGE = 21;  // ERROR: assignment of read-only variable

    return 0;
}

/*
Output:
Area of circle: 78.54
Max score: 100
Author: Dennis Ritchie
Min age: 18
Gravity: 9.81 m/s^2
*/

Tip: Prefer const over #define when possible - it provides type safety and is visible to the debugger. Use #define for simple text substitution or when you need a value usable in array size declarations (in older C standards).

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