C PROGRAMMING BASIC CONCEPTS

CHARACTER SET

Every programming language has its own set of characters to form the lexical elements. The characters used in C are grouped into the following three categories.

1 Alphabet    Upper Case A-Z and Lower Case a-z

2 Digits               0 to 9

3 Special Characters 

, comma + plus sign

. dot        - minu sign

: colon    * asterisk

; semicolon / slash

‘ opostrophe  % percentage

“ quotation mark   & ampersand

? question mark ^ caret

! exclamation mark    ~ tilde

_ underscore < less than

# hash    > greater than

= equal sign \ back slash

| pipeline character   ( left parenthesis

) right parenthesis         [ left bracket        ] right bracket

{ left brace          } right brace

C TOKENS

The basic and the smallest units of a C program are called C Tokens.

There are six types of tokens in C.

1 Keywords

2 Identifiers

3 Constants

4 String

5 Operators

6 Special Symbols

KEYWORDS

Every word in a C program is either a keyword or an identifier. All keywords (reserve words) are basically the sequence of characters that have one or more fixed meaning. All C keywords must be written in lowercase letters. Because, in C both uppercase and lowercase letters are significant.

Example for Keywords :

int	char	break	else	if	continue
do	float	goto	long	return	for
short	static	auto	union	struct	switch

IDENTIFIERS

Identifiers are names given to the program elements such as variables, arrays and functions.

Basically, identifiers are the sequences of alphabets and digits.

Rules for forming identifier name

· The first character must be an alphabet (upper case or lowercase) or an underscore ( _ )

· All succeeding characters must be either letters or digits

· Uppercase and lower case identifiers are different in C

· No special character or punctuation symbols are allowed except the underscore ( _ )

· No two successive underscores are allowed

· Keywords should not be used as identifiers.

CONSTANTS :

A Constant is an entity whose value does not change during program execution. The following are types of constants :

1 Integer Constant :

An integer constant is a whole number. It is a sequence of digits without a decimal point. It may prefixed with plus or minus sign.

Example : 246, 0, -3579, +25, -32028, 9999 etc.,

2 Floating Constant :

A floating constant is a number with a decimal point. It is defined as sequence of digits preceded and followed by the decimal point. They may have prefixed plus or minus sign.

Example : -246.01, +12.25, 0.0, 0.0005, 82.0, 9999.999, -0.00123 etc.,

3 Character Constant :

A character constant is a single character enclosed within pair of apostrophes.

Example : ‘a’, ‘?’, ‘#’, ‘ ‘(blank character) etc.,

4 String Constant :

A string constant is a sequence of characters enclosed within a pair of double quotes.

Example : “Hi”, “Welcome”, “2007”, “x+2”etc.,

VARIABLES

A variable is an entity used by a program to store values in the program. It is a symbolic name used for actual memory location. Therefore variable are also called as identifiers.

Example : sum, area, num, length, age, city etc.,

Rules for forming variable names

· The first character of a variable name must be an alphabet or an underscore

· All succeeding characters consists of letters and digits

· Both uppercase and lowercase variables are significant in C

· Keywords should be used as variables

· Special characters are not allowed

· There is not limit on the number of characters in a variable name

· Always choose an appropriate variable name that makes proper sense to the user

DECLARATION OF VARIABLES

All the variables must be declared before they are used in the program. A variable declaring consists of a data type name followed by a list of one or more variables of that type, separated by commas.

Syntax :

Datatype Variable_name,…..,….;

Example :

int num;

char name[10]; float rate;

ASSIGNING VALUES TO VARIABLES

We know that the variables represent some memory location, where the data is stored. Each variable is associated with one or more values. The process of giving values to variables is called assignment of values. The assignment operator ‘=’ is used to assign a value to a variable.

Syntax :

variable_name = value;

Example :

int num=5; float pi=3.142; x = 10;

sum = a + b;

name = “Saraswati”

DATA TYPES

Data type indicate the type of data that a variable can hold. The data may be numeric or non- numeric in nature. There are four fundamental C data types

Types	Keyword	Size(in bytes)
Integers	int	2 bytes
Real     (Floating point)	float	4 bytes
Double       Precision	double	8 bytes
Character	char	1 byte

int :

This is keyword used to indicate an integer number. Any integer number is a sequence ofdigits without a decimal point. A 8 bit computer the maximum integer that can be input is either - 128 or +127. Similarly, a 16 bit computer handles the integers from -32,768 to +32,767.

Example :

-248, 14042, 27246, +1996,0, 32760

float :

This is a keyword used to indicate a floating-point number. The floating-point numbers are

same as real numbers. They are called floating point. These numbers may be expressed in scientific notation.

Example : -263.238, 2.63238E+02, 0.0263238E+04, 26323.802

char :

This is a keyword used to indicate the character type data.

The data may be a character

Constant or a string constant. A character constant can be defined as any single character enclosed within a pair of apostrophes.

Example :

‘a’, ‘p’, ‘$’, ‘2’, ‘?’, ‘ ‘(blank) etc.,

double :

This is a keyword used to indicate a double precision floating point numbers. The precision is associated with the accuracy of data. The float usually stores a maximum or 6 digits after the decimal point. But double stored 16 significant digits after the decimal point.

Example : 234.0000000000000000, -0.0000001023999001

BACKSLASH CONSTANTS

A backslash constant a combination of two character in which the first character is always  the backslash( \ ). And the second character can be any one of the characters a, b, f, n, r, t, v , ’, “, \, and 0.The backslash characters also called as escape sequences. These statements are used in output statements.

Backslash Constant	Meaning
\a	System Alarm (bell or beep)
\b	Backspace
\f	Form feed
\n	New line
\r	Carriage return
\t	Horizontal tab
\v	Vertical tab
\”	Double quote
\’	Apostrophe
\0	Null character
\\	Back slash character (\)

SYMBOLIC CONSTANTS

A symbolic constant is a name that substitutes a numeric constant, a character constant or a string constant. For example, assume that we are computing the area of a circle using the formula.

area = 3.142 * radius * radius

In this formula, the numeric constant 3.142 can be replaced by symbolic constant name PI. Thus, the formula takes a new form as

area = PI * radius * radius

Here, we substituted PI for a numeric constant. During compile-time, each occurrence of PI is replaced by its value defined. Symbolic constants are defined using preprocessor statement #define.

Example :  

# define  PI 3.142

# define  NAME “PRABHU”

# define   FOUND 1

Symbolic constants must be defined at the beginning of a program. That is, before the main( ) function of a C Program.