Variables, Datatypes and Elements in C

July 25, 2016

11:03 am

2,047 Comments on Variables, Datatypes and Elements in C

Variables

A variable is nothing but a name given to a storage area that our programs can manipulate. It is defined by the following syntax.

Storage-class-specifier <space> type-specifier <space> variable-name,…

The Storage-class-specifier can be one of the following:

typedef	The symbol name “variable-name” becomes a type-specifier of type “type-specifier”. No variable is actually created, this is merely for convenience.
Extern	Inticates that the variable is defined outside of the current file. This brings the variables scope into the current scope. No variable is actually created by this.
Static	Causes a variable that is defined within a function to be preserved in subsequent calls to the function.
Auto	Causes a local variable to have a local lifetime (default).
register	Request that the variable be accessed as quickly as possible. This request is not guaranteed. Normally, the variable’s value is kept within a CPU register for maximum speed.

e.g. int I,j,k;

float x,y,z;

char ch;

Constants

ANSI C allows you to declare constants. When you declare a constants it is a bit like a variable declaration except the value cannot be changed.

The const keyword is to declare a constant, as shows below:

int const a = 1;

const int a = 2;

Note:

You can declare the const before or after the type.
It is usual to initialize a const with a value as it cannot get a value any other way.

The preprocessor #define is another more flexible method to define constants in a program.

Datatypes

Datatypes	Details	Format Specifier & Size	Range
char, signed char	Variable is large enough to store a basic character in character set. May be either signed or nonnegative	%c 8 bits	-128 to 127
unsigned char	Same as char, but unsigned only	%c 8 bits	0 to 255
short, signed short, short int, signed short int	Defines a short signed int.	%d 8 bits or 16 bits	-128 to 127 Or -32,768 to 32,767
unsigned short, unsigned short int	Defines an unsigned short integer.	%d 8 bits or 16 bits	0 to 255 or 0 to 65,535
int, signed int	Defines a signed integer	%d 16 bits	-32,768 to 32,767
unsigned int	Same as int, but unsigned values only.	%u 16 bits	0 to 65,535
long, signed long, long int, signed long int	Defines a long signed integer.	%ld 32 bits	-2.147,483,648 to 2,147,483,647
unsigned long, unsigned long int	Same as long, but unsigned values only.	%lu 32 bits	0 to 4,294,967,295
Float	A floating-point number.	%f 32 bits	1.17549435 * (10^-38) to 3.40282347*(10^+38)
Double	A more accurate floating-point number than float.	%lf 64 bits	2.2250738585072014 (10^-308 to 1.7976931348623157 (10^+308)
long double	Increase the size of double.	%Lf 80 bits	3.4 * (10^-4932) to 1.1 * (10^4932)

Note: Size of datatype depends on different implementation of c by different compilers.

C Elements

Tokens

In a C source program, the basic element recognized by the compiler is the “token”.

A token is source-program text that the compiler does not break down into component elements.

Tokens can be keyword, identifier, constant, string-literal, operator, punctuator.

Identifiers

“Identifiers” or “symbols” are the names you supply for variables, types, functions and labels in your program.
Identifier names must differ in spelling and case from any keywords.

Must start with alphabet (a..z A…Z) or underscore(_).
Next letter can be alphabet (a…z A…Z), underscore(_) or digit.
You cannot use keywords as identifiers.

Literals

Invariant program elements are called “literals” or “constants”. The terms “literal” and “constant” are used interchangeably here. Literals fall into four major categories:

integer
character
floating-point
string literals

A “string literal” is a sequence of characters from the source character set enclosed in double quotation marks (“ “). String literals are also used to represent a sequence of characters which, taken together, from a null-terminated string. The characters of a literal string are stored in order at continuous memory locations. A null character (represented by the \0 escape sequence) is automatically appended to and marks the end of each string literal.

e.g.

157 // integer constant
0xFE // integer constant
‘c’ // character constant
2 // floating constant
2E-01 // floating constant
“dog” string literal

Constants

A “constant” is a number, character or character string that can be used as a value in a program. Use constants to represent floating-point, integer, enumeration or character values that cannot be modified.

Keywords

“Keywords” are words that have special meaning to the C compiler.

An identifier cannot have the same spelling and case as a C keyword.

The C language uses following keywords:

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

Comments

Comments in the source code are ignored by the compiler. They are encapsulated starting with /* and ending with */. According to ANSI standard, nested comments are not allowed, although some implementations allow it.

Single line comments are becoming more common, although not defined in the ANSI standard. Single line comments begin with // and are automatically terminated at the end of the current line.

Punctuation and Special Characters

The punctuation and special characters in the C character set have various uses, from organizing program text to defining the tasks that the compiler or the compiled program carries out. They do not specify an operation to be performed. Some punctuation symbols are also operators. The compiler determines their use from context.

Punctuator: one of [ ] ( ) { } * , : = ; … #