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Keywords and Identifiers in C


C Keywords
Keywords are predefined, reserved words used in programming that have special meanings to the compiler. Keywords are part of the syntax and they cannot be used as an identifier. For example:
int money;
Here, int is a keyword that indicates 'money' is a variable of type integer. 
As C is a case sensitive language, all keywords must be written in lowercase. Here is a list of all keywords allowed in ANSI C.
Keywords in C Language
auto
double
int
struct
break
else
long
switch
case
enum
register 
typedef
char
extern
return
union
continue
for
signed
void
do
if
static 
while
default
goto
sizeof
volatile
const
float
short
unsigned
Along with these keywords, C supports other numerous keywords depending upon the compiler.
All these keywords, their syntax and application will be discussed in their respective topics. However, if you want a brief overview on these keywords without going further, visit list of all keywords in C programming.
C Identifiers
Identifier refers to name given to entities such as variables, functions, structures etc.
Identifier must be unique. They are created to give unique name to a entity to identify it during the execution of the program. For example:
int money;
double accountBalance;
Here, money and accountBalance are identifiers.
Also remember, identifier names must be different from keywords. You cannot use int as an identifier because int is a keyword.
Rules for writing an identifier
A valid identifier can have letters (both uppercase and lowercase letters), digits and underscores.
The first letter of an identifier should be either a letter or an underscore. However, it is discouraged to start an identifier name with an underscore.
There is no rule on length of an identifier. However, the first 31 characters of identifiers are discriminated by the compiler.
Good Programming Practice
You can choose any name for an identifier (excluding keywords). However, if you give meaningful name to an identifier, it will be easy to understand and work on for you and your fellow programmers.


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