Constants refer to fixed values that the program may not alter during its execution. These fixed values are also called literals.

Constants can be of any of the basic data types like an integer constant, a floating constant, a character constant, or a string literal. There are enumeration constants as well.

Constants are treated just like regular variables except that their values cannot be modified after their definition.

Types of C Constant

1. Integer constants
2. Real or Floating point constants
3. Octal & Hexadecimal constants
4. Character constants
5. String constants
6. Backslash character constants

Rules for constructing C Constatn

1. Integer Constant
   • An integer constant must have at least one digit.
   • It must not have a decimal point.
   • It can either be positive or negative.
   • No commas or blanks are allowed within an integer constant.
   • If no sign precedes an integer constant, it is assumed to be positive.
   • The allowable range for integer constants is -32768 to 32767.
2. Real Constant
   • A real constant must have at least one digit
   • It must have a decimal point
   • It could be either positive or negative
   • If no sign precedes an integer constant, it is assumed to be positive.
   • No commas or blanks are allowed within a real constant.
3. Character and String Constant
   • A character constant is a single alphabet, a single digit or a single special symbol enclosed within single quotes.
   • The maximum length of a character constant is 1 character.
   • String constants are  enclosed within double quotes.
4. Backslash Constant
   • There are some characters which have special meaning in C language.
   • They should be preceded by backslash symbol to make use of special function of them.
   • Given below is the list of special characters and their purpose.

We can define constants in a C program in the following ways.

1. By “const” keyword
2. By “#define” preprocessor directive