If....else, if....else if....else.

Intro if-else

In C language there are three major decision making instruction as if statement, the if-else statement, and the switch statement. And one also minor decision making instruction is conditional operator.

The keyword if tells the compiler that what follows is a decision control instruction. The condition following the keyword if is always enclosed within a pair of parentheses. If the condition, whatever it is, is true, then the statement is executed, and If the condition is not true, then statement is not executed.

Let's demonstrates the use of if-else with c program:

/*demonstrates of if-else statement*/
#include<stdio.h>
#include<conio.h>
void main()
{
 int n=10;
 if(n>=15)
  printf("n is greater!!");
 else
  printf("n is less than!!");
 getch();
}
Output :
n is less than!!

On execution of this program, when compiler check condition 10>=15, the condition is false so its execute else part statement i.e. n is less than!!.

forms of if-else

There are many way to write if-else statement in program,it can be single if statement,both if-else statement,if-else if statement and etc. There are no limit on how deeply the if's and the else's can be nested. Now let's start to form of if-else  :

1. if(condition)

     do this;

2. if(condition)

   {

     do this;

     and this;

   }

3. if(condition)

     do this;

   else

     do this;

4. if(condition)

   {

     do this;

     and this;

   }

   else

     do this;

5. if(condition)

   {

     do this;

     and this;

   }

   else

   {

     do this;

     and this;

   }

6. if(condition)

     do this;

   else

   {

     if(condition)

       do this;
     else
     {
       do this;
       and this;
     }
   }
7.  if(condition)
    {
      if(condition)
       do this;
     else
     {
       do this;
       and this;
     }
   }
   else
     do this;
8.  if(condition)
      do this;
    else if(condition)
      do this;
    else if(condition)
      do this;
    else
      do this;


And now you thinking, what the hell condition? Don't worry, its quite simple and easy to use in programming. In condition part we put data,numeric values with with appropriate operator like as Logical operator, Arithmetic operator etc.

More if-else

Notice the following if statement :

if(expression)

  statement;

Here the expression can any valid expression including a relational expression. We can even use arithmetic expression in the if statement. For example all the following if statement are valid :

if(10 + 5 % 7)

  statement;

if(n=20)

  statement;

if(-15)

  statement;

if(-2.0)

  statement;

In C every expression is evaluated in terms of zero and non-zero values.
Note: that in C a non-zero value is considered to betrue, whereas a 0 is considered to be false.

The default scope of the if statement is only the next statement. So, to execute more than one statement they must be written in a pair of braces.

if else statement and flowchart

Decision Control Statements and Flowchart

The if Statement
It is used to execute an instruction or sequence/block of instruction only if a condition is fulfilled.
Difference forms of implements if-statement are:

• Simple if statement
• if-else statement
• Nested if-else statement
• else if statement

Figure: Simple if statement syntax and flowchart

Figure: if-else statement syntax and flowchart

Nested if-else statement

In nested if...else statement, an entire if...elseconstruct is written within either the body of the if statement or the body of  an else statement.

The syntax is as follows:

if(condition_1)

{

 if(condition_2)

 {

   block statement_1;

 }

 else

 {

   block statement_2;

 }

}

else

{

  block statement_3;

}

block statement_4;

Figure: nested if-else statement flowchart

Else if statement

It is used in multi-way decision on several conditions. This works by cascading comparisons. As soon as one of the conditions is true, the statement or block of statements following them is executed and no further comparison are performed.

The else...if syntax is as follows:

if(condition_1)

  block statement_1;

else if(condition_2)

  block statement_2;

else if(condition_n)

  block statement_n;

else

  default statement;

Figure: flowchart for else...if statement