C -Tokens - Keywords, Identifiers ,Constants

C TOKENS:

The Smallest individual units are known as ‘C’ Tokens.‘C’ as six types of tokens:

1. KEY WORDS

2. IDENTIFIERS

3. CONSTANTS

4.VARIABLES

5.READING VARIABLES or INPUT VARIABLE

6.PRINTING VARIABLE or WRITING VARIABLES or OUTPUT VARIABLE 

1. KEY WORDS:

Key words are the pre-defined names and the main meaning of keywords can’t be changed during the execution of programs and all the keywords must be written in Lower Case.

The following list shows the reserved words in ‘C’. These reserved words may not be used as constant value or as other identifier.

auto	double	int	struct
break	else	long	switch
case	enum	register	type def
char	extern	return	union
const	float	short	unsigned
continue	for	signed	void
default	goto	size of	volatile
do	if	static	while

2. IDENTIFIERS:

Identifiers refers to the names of the variables ,functions, arrays. These are user defined names and consists of a sequence letters and digits. Both upper and lower case letters are permitted.

NOTE: The Underscore(_) characters are also permitted in identifiers.

             Example:  Ram_charan123, Ram_123

3. CONSTANTS:

The term constant means that it does not change during execution of a program.

I)NUMERICAL CONSTANTS: They are of two types,

1 Integer constants

2.Floating point constants

1.INTEGER CONSTANTS:

These are three types of constants. Integer constants are allowed in ‘C’. They are

1.Decimalconstants

2.Octalconstants

3.HexaDecimalconstants

DECIMAL CONSTANTS:

The allowed digits in decimal constants are 0,1,2,3,4,5,6,7,8,9.

Example:(1459)₁₀ whose base or radix is 10 

OCTAL CONSTANTS:

The allowed digits in octal constants are 0,1,2,3,4,5,6,7. The octal constants are preceded  by‘0’

Example: (147)₈ whose base or radix is 8

HEXA DECIMAL CONSTANTS:

It is a sequence of digits from 0-9,10,11,12,13,14,15

                                                              A , B,  C,  D,  E,  F

Example: (9A4F)₁₆ =(0x9A4F)₁₆ whose base or radix is 16 

(14AF)₁₆=(0x14AF)₁₆

2.FLOATING POINT CONSTANTS:

These are represented in two ways. They are

1.Fractional Form

2.Exponential Form

FRACTIONAL FORM:

This must have atleast one decimal point to the right of decimal point.

Example:145.7,7.54

EXPONENTIAL FORM:

It consists of a Mantissa and an exponent. The Mantissa must have atleast one digit after the decimal .The Mantissa is followed by a letter ‘e’or‘E’ and the exponent. The exponent must be an integer with out a decimal point and must have atleast one digit.

Example:

1.4e3=(14/10)* ₁₀³=(14/10)*1000=1400.000000

15.67e4=(1567/10)*₁₀⁴=(1567/100)*10000=15670.000000

147.5e5=(1475/10)*₁₀⁵=(1475/10)*100000=14750000.000000

197.65e7=(19765/100)*₁₀⁷=19765*105=1976500000.000000

II) CHARACTER CONSTANTS: They are of 3 types. They are:

1.Single character constant

2.String constant

3. Back slash constant

1.SINGLE CHARACTER CONSTANT:

It contains a single character enclosed with in a pair of single quote marks.

Example:‘A’,’5’,‘H’

2.STRING CONSTANTS:

          It is a sequence of characters enclosed in double quotes. The character may be a number (or) a letter (or) a special character (or) blankspace.

Example: “A”, “Well Done”, “1943 Love Story”

3.BACK SLASH CHARACTER CONSTANTS :

'C' supports some special Back Slash Character Constants that are used in output functions. This character constants are known as ‘Escaped Sequences’.

Example:

\n–New line

\b–Back Space

\a–Bell Sound

\t–Tab

4.VARIABLES :

A variable is a data name that may be used to store a data value,unlike constants they do not remain unchanged during the execution of program.A variable may take different values at different times during execution

Example: sum,total etc

RULES FOR VARIABLES :

They must begin with a letter

ANSI standards recognizes a length of 31 characters However length should not be normally more then 8 characters since only the first 8 characters treated significant by many compilers

Upper case and lower case are significant 

Example: TOTAL & total are different

It should not be a keyword 

5.READING VARIABLES or INPUT VARIABLE :

A variable which reads the value from the keyboard with the function scanner is 

scanf("control string" ,argument list);

The control string parameter tells how many values and what type of values reading from the keyboard

The argument list parameter tells where to store the values of these variables ,here the address can be obtained by placing an am-percent symbol (&) before the variable name

6.PRINTING VARIABLE or WRITING VARIABLES or OUTPUT VARIABLE :

A variable which prints the values of the variables by using a function printf is 

printf("control string" ,argument list);

The control string parameter may include either of string of characters or format specifiers or both which tells the type of values to be printed  

The argument list parameter tells which variable to be printed

Example: printf("The answer to the above is %d",n);