The decision control structure

In decision control structure the condition compared using relational operator like < <= > >= == != etc and prints required output.

if statement :
if (this condition is true)
execute this statement;

In the above syntax if is the key word followed by (relation condition check)
set of statements;

program:

                    main()

{

int num;

printf("Enter a number less than 10");

scanf("%d", & num);

if(num<10)

printf("What an obedient servant you are!");

}

conditional operators

conditional operators are:
= =            equal to                   x==y                x is equal to y
!=              Not equal                x!=y                 x is not equal to y
<               less than                   x<y                  x is less than y 
>               greater than              x>y                   x is greater than y
<=             less than or eual
>=             greater than or equal

logical operators

logical operators are
AND                  &&                             SELECTING TWO OR MORE CONDITIONS
OR                       ||                                SELECTING ANY ONE OF THE CONDITION
NOT                    !                                 NEGATION

Arithmetic instructions

arithmetic instructions are: +-*/ %
= is a assignment operator to which variable name is assigned at the left hand side and value of the variable or constant value is assigned at right hand side

c keywords

Key words are the words whose meaning has already been explained to the c compiler.The key words cannot be used as variable names.

There are only 32 key words available in C.They are:
auto
break
case
char
const
continue
default
do
double
else
enum
extern
float
far
for
goto
if
int
long
near
register
return
short
signed
static
struct
switch
typedef
union
unsigned
void
while

Constan, variable

A constant is a quantity that doesn't change in a program

C constants   
I PRIMARY CONSTANTS
int, float, char
II SECONDARY CONSTANTS
array, pointer, structure, union, enum etc.

Variables:
A quantity which may vary during program execution is called a variable.

C Charecter set

Alphabets                       A,B,C,D,..............Z
                                      a,b,c,d,..................z

Digits                             0,1,2,3,4................9

Special Symbols             `~?/>.<:;"'}]{[|\+=_-)(*&^%$#@!