Elements of the C Language  -Identifiers, Keywords, Data types and Data objects
This article deals with basic elements, which are used to create a C program. These elements are - the valid character set, identifiers, keywords, basic data types and their representation, constants and variables.

The C Character Set

C uses the uppercase English alphabets A to Z, the lowercase letters a to z, the digits 0 to 9, and certain special characters as building blocks to form basic program elements viz. constants, variables, operators, expressions and statements.
The special characters are listed below:

!	*	+	\	"	<
#	(	=	|	{	>
%	)	~	;	}	/
^	-	[	:	,	?
&	-	]	'	.	(blank)

Most versions of the language also allow some other characters, such as @ and $, to be included within strings and comments.
In addition certain combinations of these characters, such as '\b', '\n' and '\t', are used to represent special condition such as backspace, newline and horizontal tab, respectively. These character combinations are known as escape sequences.

Identifiers and Keywords

Identifiers are names given to various items in the program, such as variables, functions and arrays. An identifier consists of letters and digits, in any order, except that the first character must be a letter. Both upper and lowercase letters are permitted. Upper and lowercase letters are however not interchangeable (i.e., an uppercase letter is not equivalent to the corresponding lowercase letter). The underscore character (_) can also be included, and it is treated as a letter. Keywords like if, else, int, float, etc., have special meaning and they cannot be used as identifier names.
The following are examples of valid identifier names: A, ab123, velocity, stud_name, circumference, Average, TOTAL
The following names are not valid identifiers:

1st	- the first character must be a letter
"Jamshedpur"	- illegal characters (")
stud-name	- illegal character (-)
stud name	- illegal character (blank space)

Although an identifier can be arbitrarily long, most implementations recognize typically 31 characters. There are some implementations, which recognize only eight characters. The ANSI standard recognizes 31 characters. If a system recognizes only 8 characters and if you use a variable name with more than 8 characters, only the first 8 characters will be taken, the rest will be ignored. Thus, average_of_numbers and average_ will both be recognized as average_. It is therefore safer to try and use short meaningful names for your identifiers.

Exercise:

Identify which of the following are valid identifiers.

(a)	stud 1	(d)	switch	(g) Average of Numbers	(j) 123-45-678
(b)	1stud	(e)	%calc	(h) Average_of_Numbers
(c)	stud_1	(f)	_x	(i) Average-of-Numbers

Data Types and Sizes

There are only few basic data types in C. These are listed in the table below:

Data type	Description	Size	Range
char	single character	1 byte	0 - 255
int	integer number	4 bytes	-2147483648 to 2147483647
float	single precision floating point number (number containing fraction & or an exponent)	4 bytes	3.4E-38 to 3.4E+38
double	double precision floating point number	8 bytes	1.7E-308 to 1.7E+308

The list of data types can be increased by using the data type qualifiers such as - short, long, and unsigned. For example, an integer quantity can be defined as long, short, signed or unsigned integer. The memory requirement of an integer data varies depending on the compilers used. The qualified basic data types and their sizes are shown in table below.

Data type	Size	Range
short int	2 bytes	-32768 to 32767
long int	4 bytes	-2147483648 to 2147483647
unsigned short int	2 bytes	0 to 65535
unsigned int	4 bytes	0 to 4294967295
unsigned long int	4 bytes	0 to 4294967295
long double (extended precision)	8 bytes	1.7E-308 to1.7E+308

Note that the qualifier unsigned can be used along with the qualifiers short and long. The unsigned int occupies the same memory space as an ordinary int but differs on the possible content of the left-most bit. In case of an ordinary int it is reserved for sign (sign bit), but all the bits in an unsigned int are used for determining the magnitude of the integer quantity.
The char type is used to represent individual characters, and occupies one byte of memory. Each character has an equivalent integer representation (since each stores internally the ASCII value for the corresponding character). So char variables or constants can be used as integer data in arithmetic expressions.
The data objects to be manipulated in a C program are classified as variables and constants. The type of all the variables to be used in the program must be declared before they can be used. The operations that can be performed on the data objects are specified by a set of operators. Expressions used in a program combine the variables, constants and operators to produce new values.

Constants

The constants in C can be classified into four categories namely integer constants, floating point constants, character constants and string constants.
A character constant is written as for example - 'A' (always enclosed in single quotes).
Examples of string constants are - "Jamshedpur", "A", etc. Note that a string constant is always enclosed within double quotes.
A normal integer constant is written as 1234.
A long int is recognized by the presence of L (uppercase or lowercase) at the end of the constant, e.g. 2748723L.
The suffix u or U signifies the int to be an unsigned one.
The UL or ul at the end indicates the int quantity is of unsigned long type
Floating point constants contain a decimal point (167.903) or an exponent (1e-2) or both. Their type is double unless suffixed. The suffix of f or F indicates float; 1 or L indicates long double.
C also supports octal and hexadecimal data. The value of an integer data can be specified in either octal or hexadecimal form. A hexadecimal constant must begin with 0x or 0X, a leading 0 indicates the octal representation. Octal and hexadecimal constants may also be followed by U to indicate unsigned or L to determine long.
The number 0x2A5 is an example of a hexadecimal number. Internally the number is represented by the following bit patterns,

0x2A5 = 0010 1010 0101 = 2 * 16² + 10 * 16¹ + 5 * 16⁰ = 677
        ---- ---- ----
          2    A   5

The number 677 is the decimal equivalent of the number 0x2A5.

Example of an octal number can be 0347. To represent each digit of an octal number in the form of binary, we need maximum of three bits since the last digit in the octal number system is 7.

0347 = 011 100 111 = 3 * 8² + 4 * 8¹ + 7 * 8⁰ = 231 (in decimal)
       --- --- ---
        3   4   7

In numeric constants e.g. integer or floating point constants, blanks and any non-numeric characters cannot be included. The range of these constants will be limited by the maximum and minimum bounds (usually machine dependent).
A character constant is a single character enclosed in apostrophes such as 'A'. Such a constant is internally treated as an integer e.g. 'A' corresponds to the integer 65 (also known as its ASCII value). The ASCII value of 'a' (small) is 97. Hence character constants can participate in a numeric calculation just like any other integer, moreover, they also can be used for comparing with other character constants. Some character constants are of non-printing type which can be expressed by a combination of a back-slash (\) and a character. They are known as escape sequences. Each of them represents a single character even though they are written in terms of two or more characters.
Commonly used escape sequences are listed below:

Character	Escape Sequence	ASCII Value
Bell	\a	007
Backspace	\b	008
Null	\0	000
Newline	\n	010
Carriage return	\r	013
Vertical tab	\v	011
Horizontal tab	\t	009
Form feed	\f	012