C Variables and Constants

Variables

Variables are memory location in computer's memory to store data. To indicate the memory location, each variable should be given a unique name called identifier. Variable names are just the symbolic representation of a memory location. Examples of variable name: product, rem, ctr etc.

int input; 

Here, input is a variable of integer type.

Rules for writing variable name in C

• Variable name can be composed of letters (both uppercase and lowercase letters), digits and underscore '_' only.
• The first letter of a variable should be either a letter or an underscore. 
• There is no rule for the length of length of a variable. However, the first 31 characters of a variable are discriminated by the compiler. So, the first 31 letters of two variables in a program should be different.

Constants

Constants are the terms that can't be changed during the execution of a program. For example: 1, 2.5, "Programming is easy." etc. In C, constants can be classified as:
Integer constants

Integer constants are the numeric constants(constant associated with number) without any fractional part or exponential part. There are three types of integer constants in C language: decimal constant(base 10), octal constant(base 8) and hexadecimal constant(base 16) .

Decimal digits: 0 ,1 ,2 ,3 ,4 ,5 ,6 ,7 ,8 ,9

Octal digits: 0 ,1 ,2 ,3 ,4 ,5 ,6 ,7

Hexadecimal digits: 0 ,1 ,2 ,3 ,4 ,5 ,6 ,7 ,8 ,9 ,A ,B ,C ,D ,E ,F.

Note : - Every octal constant starts with 0 and hexadecimal constant starts with 0x in C programming.

Floating-point constants

Floating point constants are the numeric constants that has either fractional form or exponent form. For example:-2.0 0.0000234 -0.22E-5


Note:Here, E-5 represents 10-5. Thus, -0.22E-5 = -0.0000022.

Character constants

Character constants are the constant which use single quotation around characters. For example: 'a', 'l', 'm', 'F' etc.

Escape Sequences
Sometimes, it is necessary to use newline(enter), tab, quotation mark etc. in the program which either cannot be typed or has special meaning in C programming. In such cases, escape sequence are used. 

Escape Sequences

• \b Backspace
• \f Form feed
• \n Newline
• \r Return
• \t Horizontal tab
• \v Vertical tab
• \\ Backslash
• \' Single quotation mark
• \" Double quotation mark
• \? Question mark
• \0 Null character

String constants

String constants are the constants which are enclosed in a pair of double-quote marks. For example:"good" //string constant "" //null string constant " " //string constant of six white space "x" //string constant having single character. "Earth is round\n" //prints string with newline

Enumeration constants

Keyword enum is used to declare enumeration types. For example:enum color {yellow, green, black, white};