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Constants In C++

The keyword const can be added to the declaration of an object to make that object a constant rather than variable.
 
The general form of constant declaration is as follows:
 
Syntax: const int a=50;
 
Constants can be classified based on their data type.
 
The various types are-
 
Numeric type constants
character constants
String
 
Numeric type constants
 
Numeric constants consist of a series of digits. Integer constants can be written in different
 
number systems:
hexadecimal (base 16), octal (base 8) or in decimal (base 10).
 
A decimal integer constant is any normal whole number (can consist of digits from 0 to 9):
 
2,34, 100, 900, 1456 etc.
 
An octal integer constant can contain digits from 0 to 7 only and it should start with a zero (so that the computer will know it is an octal number).
 
For example:
 
023, 0567, 0214 etc.
 
A hexadecimal integer constant should start with 0x or 0X and can consist of digits from 0 to 9 and A to F (uppercase and lowercase letters are allowed).
 
For example:
 
0x10, 0x1FF etc.
Floating point constants will have the decimal point in the number.
 
Character and String Constants
 
Character constants are single characters enclosed within two single quotes (or between two apostrophes).
 
For example:
 
‘a’ , ‘b’ , ‘x’ , ‘1’ , ‘A’ , ‘*’
 
A single character enclosed within single quotes is a character constant. All character constants will have an integer value (determined from the ASCII table). A C++ statement:
 
char ch = ‘B’;
 
will assign the character constant ‘B’ to the character variable ch.
 
String constants consist of a series of characters enclosed within double quotes.
 
For example:
 
“hello”
“This is a string”
“x”
 
Even “x” is a string because it is enclosed in double quotes. “x” is different from ‘x’ (this is a character constant). The reason is because “x” actually consists of ‘x’ and ‘\0’ (the null character which will be explained later).
 
Structure
 
A structure is a collection of data of different data types under one name. e.g.
 
Struct employees
 
{
char Name[10];
int Age;
int Salary;
};
 
Union
 
It is a collection of data of different types sharing common memory space. e.g.
 
Union item
{
int m;
float x;
char c;
};
 
Enumerated Data types
 
This data types gives us an opportunity to invent your own data type and define what values the variable of this data type can take.
 
Example:
enum colors
 
{
red, green, blue, cyan
};
 
colors foreground, background;
 
Here the declaration has two parts:
 
The first part declare the data type and specifies its possible values.
The second part declare variable of this data type.
 
Now we can give the values to these variables:
foreground=red;
background=blue;
 
But remember we can’t use values that aren’t in the original declaration. Thus, the following declaration cause error.
 
foreground=yellow;

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