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Variable, Constant Declaration/Definition

In computer programming, a variable is a named container that is associated with the storage location in the computer’s physical memory. Before designing and using the suitable variable name, we must declare and define it. A declaration is the creation of a variable name, whereas the definition is the assignment of some value to it.

The general syntax for declaring a variable is:

type_qualifier data_type variable_name1, variable_name2,...., variable_namen;

The general syntax for variable declaration with a definition is

type_qualifier data_type variable_name1 = INITIAL_VALUE;

Example:

// declaring single variable of type unsigned long integer
unsigned long int my_var;

// declaring three variables of type int x,y,z
int x, y, z; 

// declare x of type integer and initialize with 30
int x = 30;

// * to make run these statements, you must write it inside main function
// * Check what happens if you assign a value to a variable that is
// out of range to that container

Constant declaration, definition:

The constants do not allow update or change once it is initialized. In C, we use keyword const to make a variable of type constant. Constant should be initialized during the declaration. The general syntax for creating a constant in C is.

type_qualifier data_type constant_name = VALUE;

Example

// declaring value of pie as constant
unsigned float PI = 3.1416;

The following is the sample example that can be run with a C compiler or IDE:

/*
    simple program to demonstrate variable and constants
    in C Programming
*/

#include <stdio.h>

int main()
{
    char my_character = 'A';
    int my_integer = 1;
    const float  PI = 100;

    // print the current values of the variables and constants
    printf("Value of Character: %c\n", my_character);
    printf("Value of Integer: %d\n", my_integer);
    printf("Value of PI: %f\n", PI);

    // you can change the value of variable
    my_integer = 123;

    // print changed value
    printf("New Value of Integer: %d\n", my_integer);

    // if you try to change the value of constant compiler will flag

    return 0;
}

The Expected output is:

Value of Character: A
Value of Integer: 1
Value of PI: 100.000000
New Value of Integer: 123
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