C Programming Keywords, Identifiers, Variables and Constants

Character set

Character set are the set of alphabets, letters and some special characters that are valid in C language

Alphabets :

Uppercase : A, B, C, ...............................X, Y, Z.

Lowercase : a, b, c, ..................................x, y, z.

Digits :

0 1 2 3 4 5 6 7 8 9

Special Characters :

, < > . _ ( ) ; $ : % [ ] # ?

' & { } " ^ ! * / | - \ ~ +

Keywords :

Keywords are the reserved words used in programming. Each keywords has fixed meanings that cannot be changed by user. For example :

int money;

Here, int is a keyword that indicates, 'money' is of type integer.

As C programming is case sensitive all keywords must be written in lowercase. Here is the list of all keywords predefined by ASCII C.

auto          double       int             struct

break        else           long          switch

case          enum         register     typedef

char          extern        return       union

continue     for           signed       void

do             if              static         while

default       goto         sizeof        volatile

const         float         short         unsigned

Identifiers :

In C programming identifiers are the name given to C entities such as variables, functions, structures, etc. Identifiers are created to give unique name to C entities to identify it during the execution of program.

For example

int money;

int mango_tree;

Here, money is a identifier which denotes a variable of type integer. Similarly, mango_tree is another identifier which denotes another variable of type integer.

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: sum, car_no, count etc.

int num;

Here, num is a variable of integer type.

Rules for writing variable name in C

1. Variable name can be composed of letters (both uppercase and lowercase letters), digits and underscore '_' only.
2. The first letter of a variable should be either a letter or an underscore. But, it is discouraged to start variable name with an underscore though it is legal. It is because, variable name that starts with underscore can conflict with system names and compiler may complain.
3. 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.

In C programming, you have to declare variable before using it in the program.

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.

For example:

Decimal constants: 0, -9, 22 etc
Octal constants: 021, 077, 033 etc
Hexadecimal constants: 0x7f, 0x2a, 0x521 etc

Notes:

1. You can use small caps a, b, c, d, e, f instead of uppercase letters while writing a hexadecimal constant.
2. 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. For example: \n is used for newline. The backslash( \ ) causes "escape" from the normal way the characters are interpreted by the compiler.

                  Escape Sequences
Escape Sequences      Character
\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};

Here, the variable name is color and yellow, green, black and white are the enumeration constants having value 0, 1, 2 and 3 respectively by default.