Decision Control Structure:

There are some circumstances where we need different sets of action to be performed at a particular instant of time.A C language has three major decision making instructions------
1->The if statement.
2->The if else statement.

3->switch statement.

Apart from these three we have fourth type also i.e. conditional operators.
In this post I will show you about the if ,if else method, rest I will describe in another post.

The if Statement:

C uses a keyword if to implement the decision control instructions,it's syntax is
                               if(condition true)                                                                                                                                                  {    execute these set of statement;.....
                                                __________________}    

Flow Chart of" if "Structure

The if statement  tells the compiler that to check the condition is true or not enclosed within a pair of parentheses. If true then it will undergo the execution block of "if".otherwise proceed to other statements.
NOTE: In C a non zero value is considered to be true and 0 is considered to be false. i.e.

if(5)
//true statements are executed
if(-5)

//here too statements gets executed true
if(0)

//false it will terminate the if block

there are operators which can be used for particular conditions like:
RELATIONAL OPERATORS
1)

"= =" :Equal to
example:
 a==b means a is equal to b

2)

"!=" : Not equal to
example:
a!=b means a is not equal to b.

3)
"<" :Less then
example:
a<b means a is less then b.

4)

">":Greater then
example

a>b means a is greater then b.

5)

"<=" :Less then or equal to

example

a<=b means a is less then or equal to b

6)

">=" :Greater then or equal to

example

a>=b means a is greater then or equal to b.

LOGICAL OPERATORS
7) The logical And is represented as " && "

8) The logical OR is represented as "  || "

9) The logical NOT is represented as " ! "

The if -else Statement.

The main drawback of  if statement is that the following set of statements within "if block " executes only when the condition is true .It does nothing when expression evaluates to false.This is what exactly the purpose of  else statement  is being used.

syntax:


          if(condition)
         {........//statements; }
        else
    {,,,,,,,,,,,,,,,,,,,,,,,,,,//statements;}

Flow chart for " if-else"structure.

NOTE:The else block should be written exactly below the "if "block.

Nested if-else :

We can also write an entire if-else construct within either the body of if statement  or the body of else statement. This is called nesting of ifs

The example of nested if-else is shown below:

Nested if-else.