Identifier


Identifiers are the names given to program elements like as variables, arrays, etc. They are makes by using a sequence of letters (consisting of uppercase as well as lower case ), numerals(like 1,23,4 etc), and underscores.

Rules for identifiers in c


1. An Identifier can only have alphanumeric characters(a-z , A-Z , 0-9) and underscore(_).
2. The first character of an identifier can only contain alphabet(a-z , A-Z) or underscore (_).
3. Keep in mind name and Name are two different identifiers in C.
4. Keywords are not allowed to be used as Identifiers.
5. No special characters, such as semicolon, period, spaces, slash or comma are allowed to be used in or as Identifier.
6.keep in mind identifier  ‘VARIABLE’ is different from ‘variable’ and ‘Variable’.

Keywords

C has a set of reserved words often known as keywords that cannot be used as an identifier.All keywords are basically a sequence of characters that have a fixed meaning. All keywords must be written in lower case letters. C programming contains the list of 32 fixed keywords .

 In c programming language there are 32 reserved word that are predefined.These words are called keywords in c programming Keywords  can not use as identifier.All keyword 

Table 1 Keywords list C language


Basic data types


Data type are that which tell to compiler what type of data programmer going to use in program.There are four basic data type in c programming int, float,double, character.Here only simple definition given.But you will learn in further deeply with program example.

    Table 1.2
   no.
 Data type
 Size in byte
 use
   1.
   char
     1
 to store characters 
   2.
    int
     2
 to store integer numbers
   3.
   float
     4
 to store floating point
   4.
  double
     8
 to store  big floating point

Variables 


In programming languages variable can be defined as it is a name given  for help to give a memory in location in computer memory.Actually when programmer give a variable name then memory get located at a particular address.C language support two types of variable.
For example -
int a=12;
a=15;
here int is data type and keyword.And a is variable as well as identifiers.

              a
           12
      62343535
             
            a
           15
      62343535

 Here in example 62343535 is fixed address of a given variable and a is variable name.value at the address can be change during updating in program you will be understood further. In mathematics value of a variable can change and c programming value of a variable can be also change at a particular address.Here it is clear a variable give a memory location at a particular address.

Numeric variable 


Numeric variable use to store integer values or floating point values. Modifiers like short, long, signed, and unsigned can also be used with numeric variables. The difference between signed and unsigned numeric variables is that signed variables can be either negative or positive but unsigned variables can only be positive.

Character variable 


Character variables are just single characters enclosed within single quotes. Character set may be letters (‘a’, ‘A’), numerals (‘2’), or special characters (‘&’).

Declaration of variable


 Declaration of variable means specify the data type of a variable that programmer will use in program.Data types written before the variable indicate which type of data will be store in variable.A variable name can be written as user wants but heed on rule.
For example -
int a=12;
Here int is a data type written before the variable indicate integer type value will be store in variable a.

Characters Allowed  

Underscore(_)
Capital Letters ( A – Z )
Small Letters ( a – z )
Digits ( 0 – 9 )
Blanks & Commas are not allowed
No Special Symbols other than underscore(_) are allowed
First Character should be alphabet or Underscore
_var
var_num2
Tip 2 : blanks are not allowed

Invalid Names

Var 1
var 1
1 var
@var
addition of program

Initializing  of variable


While declaring the variables,we can also initialize them with some value.Initializing means declaration of variable with value.
For example, int emp_num = 100; 
float salary = 105.50;
char grade = ‘B’; 
double balance_amount = 800000000;

Constant


Constants are identifiers whose values do not change. While values of variables can be changed any time at particular address, values of constants can never be changed. 

Declaration  of constant


To declare a constant, precede the normal variable declaration with const keyword and assign it a value. const float pi = 3.1.

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