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What is Constant in C++?

  • Constants refer to fixed values that the program may not alter and they are called literals.
  • Constants can be of any of the basic data types and can be divided into Integer Numerals, Floating-Point Numerals, Characters, Strings and Boolean Values.
  • Again, constants are treated just like regular variables except that their values cannot be modified after their definition.
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Integer literals

  • An integer literal can be a decimal, octal, or hexadecimal constant.
  • A prefix specifies the base or radix: 0x or 0X for hexadecimal, 0 for octal, and nothing for decimal.
  • An integer literal can also have a suffix that is a combination of U and L, for unsigned and long, respectively.
  • The suffix can be uppercase or lowercase and can be in any order.
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  • Here are some examples of integer literals:
212            // Legal
215u          // Legal
0xFeeL      // Legal
078            // Illegal: 8 is not an octal digit
032UU      // Illegal: cannot repeat a suffix
  • Following are other examples of various types of Integer literals:
#include <iostream>using namespace std;
int main(){    
    int number;

    cout << "Enter an integer: ";
    cin >> number;

    cout << "You entered " << number;    
    return 0;}

Floating-point literals

  • A floating-point literal has an integer part, a decimal point, a fractional part, and an exponent part.
  • You can represent floating point literals either in decimal form or exponential form.
  • While representing using decimal form, you must include the decimal point, the exponent, or both and
  • while representing using exponential form, you must include the integer part, the fractional part, or both.
  • The signed exponent is introduced by e or E.
  • Here are some examples of floating-point literals:
3.14159             // Legal
314159E-5L     // Legal
510E                // Illegal: incomplete exponent
210f                 // Illegal: no decimal or exponent
.e55                 // Illegal: missing integer or fraction
#include <iostream>using namespace std;
int main(){
    double firstNumber, secondNumber, productOfTwoNumbers;
    cout << "Enter two numbers: ";

    // Stores two floating point numbers in variable firstNumber and secondNumber respectively
    cin >> firstNumber >> secondNumber;
    // Performs multiplication and stores the result in variable productOfTwoNumbers
    productOfTwoNumbers = firstNumber * secondNumber;  

    cout << "Product = " << productOfTwoNumbers;    
    return 0;}

Boolean literals

  • There are two Boolean literals and they are part of standard C++ keywords:
    • A value of true representing true.
    • A value of false representing false.
  • You should not consider the value of true equal to 1 and value of false equal to 0.
 if (x >= 0)
  POSX = true;
  }//end of if for bool
  POSX = false;

Character literals

  • Character literals are enclosed in single quotes. If the literal begins with L (uppercase only), it is a wide character literal (e.g., L'x') and should be stored in wchar_t type of variable . Otherwise, it is a narrow character literal (e.g., 'x') and can be stored in a simple variable of char type.
  • A character literal can be a plain character (e.g., 'x'), an escape sequence (e.g., '\t'), or a universal character (e.g., '\u02C0').
  • There are certain characters in C++ when they are preceded by a backslash they will have special meaning and they are used to represent like newline (\n) or tab (\t).
  • Here, you have a list of some of such escape sequence codes:
Escape sequence Meaning
\\ \ character
\' ' character
\" " character
\? ? character
\a Alert or bell
\b Backspace
\f Form feed
\n Newline
\r Carriage return
\t Horizontal tab
\v Vertical tab
\ooo Octal number of one to three digits
\xhh . . . Hexadecimal number of one or more digits
  • Following is the example to show few escape sequence characters:
#include <iostream>
using namespace std;

int main() {
   cout << "Hello\tWorld\n\n";
   return 0;
  • When the above code is compiled and executed, it produces the following result:
Hello   World

String literals

  • String literals are enclosed in double quotes. A string contains characters that are similar to character literals:
    • plain characters
    • escape sequences, and
    • universal characters
  • You can break a long line into multiple lines using string literals and separate them using whitespaces.
  • Here are some examples of string literals. All the three forms are identical strings.
#include <iostream>using namespace std;
int main(){
    char str[100];

    cout << "Enter a string: ";
    cin >> str;
    cout << "You entered: " << str << endl;

    cout << "\nEnter another string: ";
    cin >> str;
    cout << "You entered: "<<str<<endl;

    return 0;}
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Defining Constants

  • There are two simple ways in C++ to define constants:
    • Using #define preprocessor.
    • Using const keyword.

The #define Preprocessor

  • Following is the form to use #define preprocessor to define a constant:
#define identifier value
  • Following example explains it in detail:
#include <iostream>
using namespace std;

#define LENGTH 10   
#define WIDTH  5
#define NEWLINE '\n'

int main() {

   int area;  
   area = LENGTH * WIDTH;
   cout << area;
   cout << NEWLINE;
   return 0;
  • When the above code is compiled and executed, it produces the following result:

The const Keyword

  • You can use const prefix to declare constants with a specific type as follows:
const type variable = value;
#include <iostream>
using namespace std;

int main() {
   const int  LENGTH = 10;
   const int  WIDTH  = 5;
   const char NEWLINE = '\n';
   int area;  
   area = LENGTH * WIDTH;
   cout << area;
   cout << NEWLINE;
   return 0;
  • When the above code is compiled and executed, it produces the following result:
  • Note that it is a good programming practice to define constants in CAPITALS.

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