Constants / Literals
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.
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.
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:
85 // decimal 0213 // octal 0x4b // hexadecimal 30 // int 30u // unsigned int 30l // long 30ul // unsigned long
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
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.
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 <constream.h> void main() { cout << "Hello\tWorld\n\n"; getch(); }
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.
"hello, dear" "hello, \ dear" "hello, " "d" "ear"
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 <constream.h> #define LENGTH 10 #define WIDTH 5 #define NEWLINE '\n' void main() { int area; clrscr(); area = LENGTH * WIDTH; cout << area; cout << NEWLINE; getch(); }
When the above code is compiled and executed, it produces the following result:
50
The const Keyword:
You can use const prefix to declare constants with a specific type as follows:
const type variable = value;
Following example explains it in detail:
#include <constream.h> void main() { const int LENGTH = 10; const int WIDTH = 5; const char NEWLINE = '\n'; int area; clrscr(); area = LENGTH * WIDTH; cout << area; cout << NEWLINE; getch(); }
When the above code is compiled and executed, it produces the following result:
50
Note that it is a good programming practice to define constants in CAPITALS.
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