Variables and Constants in C++ (Day Seven)

Variables and Constants in C++

What is a Variable ?

A variable refers to a storage area whose contents vary during processing. For example, to store salary of a employee, we need a variable.

Declaration of a Variable –

A variable name is declared by

data_type variable_name

where data_type is the data types provided by C++ and variable_name is the identifier. Thus, the naming convention rules for naming an identifier is also applicable to name of the variable. To store salary of an employee,

int emp_salary;

if more than one variable is to be defined, then variable name is separated by commas.

int day, month, year;

Symbolic Variables –

The variables are referred as symbolic variables as they are named locations or it can be said that we give a name to a location in the computer memory. There are 2 values associated with a symbolic variable –

rvalue (pronounced as r-value) – data value of variable stored at some location
lvalue (pronounced as el-value) – location value of a variable, the memory address at which its data value is stored

lvalue of emp_salary = 1001                lvalue of character = 1006
rvalue of emp_salary = 20000               rvalue of character = d

Initialization of Variables –

If we do not give the data value of variable, then the variable is called uninitialized variable. and the variable data valuse is said to be undefined. Static Initialization – A variable is initialized by declaring/assigning its rvalue. C++ supports two forms of variable initialization at the time of variable definition :

int val = 100;  (or)
int val(100) ;

Dynamic (or) Run Time Initialization – If the initialization of variable is done at run time. A variable can be initialized at run time using expressions at the place of declaration. For instance,

float avg = sum/count ;

Constants –

As said earlier, a constant never change their value during a program run. Access Type : Read only. How to create a constant? const keyword is required to create a constant. For example

const int c=100 ;   (or)
const c=100 ;

If the data type is not declared in initialization of constant, then automatically it is said to be an integer constant.

Difference Between Variables and Constants in c++

Basis	Variables	Constants
Definition	A variable is a named location, whose value changes during a program run	A constant is a named location, whose value never changes during a program run.
Syntax	data_type variable_name	const data_type variable_name= value
Example	int a;	const int a=100;
Static Initialization	Variables can be initialize after its declaration also.	constants must be initialized at the time of decelaration only.
Dynamic Initialization	Variables can be initialize dynamically	Dynamic Initialization of constant is not possible.

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