When multiple conditions have to check in the program then "nested if statements" are used to solve the problem. The nested if structure may contain multiple if statements that may be nested up to any level.
If statement within another if statement is called nested if structure. The 'if statement' that contains another if statement is called outer if statement. Similarly, the if statement that is inside the outer if statement is called inner if statement.
The control enters into the "inner if structure" only when condition of the "outer if statement" is true.

Declaration

In C++, the general syntax of "nested if" structure is as follows:

if (condition-1)
  {
      if (condition-2)
         {
             Block-1;
         }
         else
         {
             Block-2;
          }
 else
   {
            Block-3;
   }
}

In the above syntax, the "if statement" that has "condition-1" contains another "if statement" that has "condition-2".
Where:

Flowchart 

The general flowchart of  this type of selection structure is as follows:

Execution

The following is the logic of the execution of nested if statement:-

• First of all, the condition of the "outer if statement" is evaluated.
• If the condition of the "outer if statement" is true, then "inner if statement" is executed.
  • If condition of the inner if statement is true, then Block-1 is executed.
  • If condition of the inner if statement is false, then Block-2 is executed.
• If condition of the "outer if statement" is false, then Block-3 is executed.

Example Programs

Example 1: The following source code of the program inputs three numbers and finds out the smallest value using "nested if statement".

#include

#include

main()

 {

   float num1, num2, num3, mini;

   clrscr();

   cout
   cin>>num1;

   cout
   cin>>num2;

   cout
   cin>>num3;

   if ( num1 
      if (num1 
            mini = num1;

           else

            mini = num3;

   else

       if (num2 
            mini = num2;

           else

            mini = num3;

   cout
   getch();

 }

Output of the Program:
(suppose entered values are 12.5, 48, 8.3 respectively)

Enter First Value: 12.5
Enter Second Value: 48
Enter Third Value: 8.3
The Minimum Value is 8.3

Example 2: The following source code of the program inputs four integer values from the user and finds out if these values or equal or different using "nested if statement".

#include

#include

main()

 {

   int a, b, c, d;

   clrscr();

   cout
   cin>>a;

   cout
   cin>>b;

   cout
   cin>>c;

   cout
   cin>>d;

   if (a == d)

      if ( b == c && c == d)

         cout
      else

         cout
   else

       cout
   getch();

 } // end main function

Example 3: The following source code of C++ program finds out the grade of a student based on the average marks obtained in three subjects. (suppose subjects are Mathematics, Physics and Chemistry).
The grade is calculated as:

• If average is greater than 80, grade is A.
• If average is less than 80 and greater than 60, grade is B. 
• If average is less than 60 and greater than 33, grade is C.
• If average is less than 33, grade is F.

#include

#include

main()

 {

   int math, physics, chemistry, average;

   char grade;

   clrscr();

   cout
   cin>>math;

   cout
   cin>>physics;

   cout
   cin>>chemistry;

   average = (math + physics + chemistry) / 3; //to find average marks

   if (average > 33)

         if (average > 60)

               if ( average > 80)

                    grade = 'A';

              else

                    grade = 'B';

              else

                    grade = 'C';

   else

         grade = 'F';

   cout   cout
   getch();

 }

Output of the Program:
(Suppose entered marks are 78, 65, 57 respectively)

Enter Marks of Mathematics: 78
Enter Marks of Physics: 65
Enter Marks of Chemistry: 57
Average Marks = 66
Grade = A