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:

Nested if Structure 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.

Nested if Statement 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
using namespace std;
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   return 0;
 }

Output of 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
using namespace std;
main()
 {
   int a, b, c, d;
   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    return 0;
 } // 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
using namespace std;
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   return 0;
 }

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