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C Language – The C language is one of the powerful programming languages used to write computer programs for variety of applications. It was developed by Dennis Ritchie at Bell Lab during 1970’s. C has flexible features to write programs for numerical, commercial and graphical applications. May operating system programs for the latest computers and compilers are written using C language.
                        This language was developed from B language which was modified from a language called Basic Combined Programming Language (BCPL).

Salient features of C:
C is a general purpose, structured programming language. Among the two types of programming languages discussed earlier, C lies in between these two categories. That’s why it is often called a middle level language. It means that it combines the elements of high level languages with the functionality of assembly language. It provides relatively good programming efficiency (as compared to machine oriented language) and relatively good machine efficiency as compared to high level languages). As a middle level language, C allows the manipulation of bits, bytes and addresses – the basic elements with which the computer executes the inbuilt and memory management functions. C code is very portable, that it allows the same C program to be run on machines with different hardware configurations. The flexibility of C allows it to be used for systems programming as well as for application programming.

C is commonly called a structured language because of structural similarities to ALGOL and Pascal. The distinguishing feature of a structured language is compartmentalization of code and data. Structured language is one that divides the entire program into modules using top-down approach where each module executes one job or task. It is easy for debugging, testing, and maintenance if a language is a structured one. C supports several control structures such as while, do-while and for and various data structures such as strucs, files, arrays etc. as would be seen in the later units. The basic unit of a C program is a function - C’s standalone subroutine. The structural component of C makes the programming and maintenance easier.


Characters used in C –
·         Alphabets
Upper case letters A to Z
Lower case letter a to z

·         Numbers
0 to 9

·         Special Characters

+
Plus
Double Quote
-
Minus
&
Ampersand
*
Asterisk
#
Hash
/
Slash
$
Dollar
\
Back slash
^
Caret
%
Percent
< 
Lesser than
|
Vertical bar
> 
Greater than
~
Tilde
=
Equal to
?
Question mark
(
Open Parenthesis
!
Exclamation Mark
)
Close Parenthesis
,
Comma
[
Open bracket
.
Full stop
]
Close Bracket
;
Semicolon
{
Open Set Bracket
:
Colon
}
Close Set bracket
Apostrophe
_
Underscore

Identifier – An identifier is a name having a few letters, numbers and special character _ (Underscore). It is used to identify a variable, function, symbolic constant and so on. An identifier can be written with a maximum 31 characters. It is a good practice to have identifiers with few letters; less than 8 letters is commonly followed with the first letter being an alphabet.
Ex:-
            c3
            sum
            PI
            sigma
            matadd

Rules for Forming Identifiers -
Identifiers are defined according to the following rules:
1.      It consists of letters and digits.
2.      First character must be an alphabet or underscore.
3.      Both upper and lower cases are allowed. Same text of different case is not equivalent, for example: TEXT is not same as text.
4.      Except the special character underscore ( _ ), no other special symbols can be used.

Keywords or Reserved words – C language uses the following keywords which are not available to users to use them as variables/function names. Generally all keywords are in lower case although uppercase of same names can be used as identifiers.
auto
break
case
enum
volatile
while
char
extern
switch
typedef
const
struct
short
union
continue
return
signed
unsigned
register
goto
sizeof
void
for
if
static
float
double
int
default
do
else
long

Tokens – There are six types of tokens are used in C language.

C Tokens
Examples
Keywords
Auto, float, int etc
Constants
-25, 3.24 etc
Identifiers
Sum, PI etc
String literals
“Total Amount is” “Sum =“ etc
Operators
+, -, * etc
Separators
,   :    ;

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