Constants in C Language A symbolic constant is a constant represented by a name (symbol) in your program. Like a literal constant, a symbolic constant can’t change. Whenever you need the constant’s value in your program, you use its name as you would use a variable name. The actual value of the symbolic constant needs to be entered only once, when it is first defined.

• C has two methods for defining a symbolic constants In  C language the #define directive and the const keyword

#define CONSTNAME
Ex: #define PI 3.14159
Defining Constants with the const Keyword
The second way to define a symbolic constant is with the const keyword. const is a modifier that can be applied to any variable declaration. A variable declared to be const can’t be modified during program execution–only initialized at the time of declaration. Here are some examples:
1.const int COUNT = 100;
2.const float PI = 3.14159;
3.const long DEPT= 12000000,
4.float TAX_RATE = 0.21;
Difference between #define and const
1. When you use #define, all the constants get replaced with their actual values before compilation by the preprocessor) and const is used by the compiler.
2. #define is always global, const can be local (global also)
3. #define doesn’t have type checking where type checking is part of const
Literal Constants In C language:
Numeric constants In C Language
1. Integer constants:

• Decimal: 524, +45, -369, 8000, ,9876L, 100L
• Octal: 071, 032, (078 is illegal because 8 is not octal)
• Hexadecimal: 0x39FEE, 0x53aDbe (0 to 9 and A to E)

2. Real constants: +456.25, -500.2523,30.0, 3.42e-4, 85.,-1.2E-1
Character Constants
1. Single Character constants: ‘A’, ‘f’, ‘4’, ‘=’
2. String constants:”Hello”,”1987″
Backslash Character Constants or escape sequences: ‘\a’,’\b’,’\n’,’\r’,’\t’,’\”,’,’\\’,’\?’

NOTE:
All ‘C’ statements must end with semicolon.

• ‘C’ is case-sensitive. This is upper case and lower case characters are different. Generally the statements are type in lower case.
• A ‘C’ statement can be write in one line or it can split into multiple lines.
• Braces must always match upon pairs, i.e., every opening brace { must have a matching closing brace }.
• No restriction in using blank lines, which separate different words or different parts of program. Blank spaces improve the readability of the statements. But blank spaces are not allow within a variable, constant or keyword.
• Comments cannot be nest. For example, /* A simple ‘C’ program/* it prints a message */*/ is not valid.
• A comment can be split into more than one line.
• Finally, the global variables can be declare outside of all functions. Generally, a variable, which is use in more than one function, can be identified as global variable