Multiple Selections: Nested if

In the previous sections, you learned how to implement one-way and two-way selections in a program. Some problems require the implementation of more than two alternatives. For example, suppose that if the checking account balance is more than $50000, the interest rate is 7%; if the balance is between $25000 and $49999.99, the interest rate is 5%; if the balance is between $1000 and $24999.99, the interest rate is 3%; otherwise, the interest rate is 0%. This particular problem has four alternatives—that is, multiple selection paths. You can include multiple selection paths in a program by using an if...else structure, if the action statement itself is an if or if...else statement. When one control statement is located within another, it is said to be nested.

Assume that all variables are properly declared, and consider the following statements:

if(score >= 90) //Line 1

cout<<"The grade is A"<<endl; //Line 2

else //Line 3

if(score >= 80) //Line 4

cout<<"The grade is B"<<endl; //Line 5

else //Line 6

if(score >= 70) //Line 7

cout<<"The grade is C"<<endl; //Line 8

else //Line 9

if(score >= 60) //Line 10

cout<<"The grade is D"<<endl; //Line 11

else //Line 12

cout<<"The grade is F"<<endl; //Line 13

These statements illustrate how to incorporate multiple selections using a nested if...else structure.

A nested if...else structure demands the answer to an important question: How do you know which else is paired with which if? Recall that in C++ there is no stand-alone else statement. Every else must be paired with an if. The rule to pair an else with an if is as follows:

Pairing an else with an if: In a nested if statement, C++ associates an else with the most recent incomplete if—that is, the most recent if that has not been paired with an else.

Using this rule, in the preceding example, the else at Line 3 is paired with the if at Line 1. The else at Line 6 is paired with the if at Line 4. The else at Line 9 is paired with the if at Line 7, and the else at Line 12 is paired with the if at Line 10.

To avoid excessive indentation, some programmers prefer to write the preceding code as follows:

if(score >= 90)

cout<<"The grade is A"<<endl;

else if(score >= 80)

cout<<"The grade is B"<<endl;

else if(score >= 70)

cout<<"The grade is C"<<endl;

else if(score >= 60)

cout<<"The grade is D"<<endl;

else

cout<<"The grade is F"<<endl;

The following examples will help you to see the various ways in which you can use nested if structures to implement multiple selection.