Constants in C Plus Plus Programs

Constant may be defined as a quantity used in a program that cannot be changed during program execution. There are two types of constants in C++ programming language.

1. Literal Constants
2. Symbolic Constants

1. Literal Constants

A literal constant is a value that is directly typed in a program. For example 100 or 10.50 are examples of literal constants. Literal constants can be further divided into the following categories according to the type of value:

1. Integer Constant: positive or negative whole numbers like 100, 999, 2, 5056, 10, -233, -50 are called integer constants.
2. Real Constants: numeric values with fractional parts like 2.45, 999.99, -5.25, -6.023 etc are called Real constants or floating point constants.
3. Character Constants: a single character in single quotes like 'a', 'p' or '=' are called Character constants.
4. String Constants: a set of characters in double quotations like "Enter a number" or "Asad" is called a string constant.

2. Symbolic Constants

A symbolic constant may be defined as a name given to a constant quantity in a program. A symbolic constant can be useful to make a program more readable and easier to understand. A symbolic constant is defined for a constant literal value that is frequently used in a program. For example, in a mathematical program we can use PI as symbolic constant instead of the value 3.1415.

How To Declare a Symbolic Constant?

A symbolic constant can be defined in a C++ program, in two ways:

1. const Qualifier

const qualifier is used to define a constant in C++ programs. The syntax is as under:
 const datatype identifier = value
For example, to define PI, we will use the following C++ statement
const float PI=3.1415;

Example C Plus Plus Program Const Qualifier

/*
1. Symbolic Constants in C++
2. Use of const qualifier
Program to input radius and calculate
circumference of circle by formula=2xpixr
By: Mahmood Alam, Instructor(IT)
Govt. Post Graduate College of Commerce,
Rahim Yar Khan
*/
#include<iostream.h>
#include <stdio.h>
#include <conio.h>


int main()
{
float radius, circum;
const float PI=3.1415;
cout<<"Enter radius=";
cin>>radius;
circum = 2 * PI * radius;
cout<<"circumference="<<circum;
getch();
return 0;
}

2. define DIRECTIVE

define preprocessor directive is also to define a constant in a C++ program. The syntax to define a symbolic constant PI with define directive is as follows:
 #define identifier  value
For example to define a symbolic constant PI with define directive, we will type:
#define PI 3.1415

Example C Plus Plus Program #define Directive

/*
1. Symbolic Constants in C++
2. Use of #define directive
Program to input radius and calculate
circumference of circle by formula=2xpixr
By: Mahmood Alam, Instructor(IT)
Govt. Post Graduate College of Commerce,
Rahim Yar Khan
*/
#include<iostream.h>
#include <stdio.h>
#include <conio.h>
#define PI 3.1415

int main()
{
float radius, circum;
cout<<"Enter radius=";
cin>>radius;
circum = 2 * PI * radius;
cout<<"circumference="<<circum;
return 0;
}

The Difference between defining a symbolic constant by const qualifier or #define directive is:

Sr.No.	const qualifier	#define directive
1	Here we need to define data type of constant	here we do not need to define data type of constant
2	= symbols is used	= symbol is not needed
3	This statement is terminated by semi colon.	No semi colon at end