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Ternary Conditional Operators - Beyond if Statements

What is the Ternary Conditional Operator?

The Ternary Conditional Operator results in a value based on whether a Boolean expression is evaluated to true or false.

The ternary conditional operator consists of a question mark (?) and a colon (:), as follows:

<Boolean expression> ? <expression if true> : <expression if false>

Ternary Conditional Operator consists of three expressions:

<Boolean expression> ? <expression 1> : <expression 2>

The first expression is a Boolean expression expression 1 and expression 2 can be any expression:

Example 1: Even or Odd

To determine whether x is even or odd, we can use the mod operator to determine whether x is divisible by 2. If the result of x % 2 is 0, then x is even otherwise, it is odd.

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To re-write this if statement using ternary conditional operators, we will put the boolean expression in first.

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The ? operator is used after the boolean expression. Think of it as asking if the boolean expression is true. If it is true, then the next part of the expression is evaluated and Even is assigned to result.

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A colon (:) is used to separate what happens when the boolean expression is true and when it evaluates to false. If the boolean expression evaluates to false in this example, Odd is assigned to result.

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To Recap

These two statements are equivalent just written differently.

 String result;
 if (x % 2 == 0) {
    result = "Even";
 } else {
    result = "Odd";
 }

Produces the same result as:

String result = (x % 2 == 0) ? "Even" : "Odd";

We could use the ternary conditional operator to print whether the value assigned to x is even or odd simply by putting the expression inside a System.out.println statement:

System.out.println((x % 2 == 0) ? "Even" : "Odd");

Example 2: Calling Methods

Consider a class called Calculator that has methods to calculate the sum and positive difference of two values as follows:

public double sum(double value1, double value2);
public double posDifference(double value1, double value2);

Based on a String input of + for sum and +- for positive difference, we can call the appropriate method:

Calculator calc = new Calculator();
double x = //input from user;
double y = //input from user;
String input = //input from user;
double answer = input.equals("+") ? calc.sum(x, y) : calc.posDifference(x, y);

Example 3: Nested Ternary

Expanding on the last example.

Assume Calculator has product and quotient methods as well.

Calculator calc = new Calculator();
double x = //input from user;
double y = //input from user;
String input = //input from user;
double answer
   = (input.equals("+") ? calc.sum(x, y)  
   : ((input.equals("+-") ? calc.posDifference(x, y)
   : ((input.equals("*") ? calc.product(x, y)
   : calc.quotient(x, y))));

Re-written as an if..else statement:

double answer;
if (input.equals("+")) {
   answer = calc.sum(x, y);
} else {
   if (input.equals("+-")) {
      answer = calc.posDifference(x, y);
   } else {
      if (input.equals("*")) {
         answer = calc.product(x, y);
      } else {
         answer = calc.quotient(x, y);
      }
   }
}

Ternary and void Method Calls

Remember, when using the Ternary Conditional Operator, the result is a value. You can not use it to call void methods. For example, consider a Robot class that has methods for turning the robot to the left and right as follows:

public void turnRight()
public void turnLeft()

Assuming randy is a properly declared Robot and direction is an int variable, we can NOT write any of the following:

(direction == 39) ? randy.turnRight() : randy.turnLeft();

boolean b = (direction == 39) ? randy.turnRight() : randy.turnLeft();

System.out.println ((direction == 39) 
                     ? randy.turnRight() : randy.turnLeft());

Instructional Ideas to Engaging Students

Ternary Conditional Operators and the AP CSA Free Response Questions

Things to Note on Using Ternary Conditional Operator and the AP CSA FRQs

Example 1: 2024 Free Response Question 1

In 2024, question 1 can be re-written using the ternary conditional operator. The full question and solution can be found here.

To summarize, this question has a class called Feeder that has an instance variable currentFood and two methods simulateOneDay and simulateManyDays. Students are asked to implement these two methods. We are looking at writing the simulateOneDay method.

To simulate a day at the bird feeder, students will need to do the following:

An example solution is as follows: 

public void simulateOneDay (int numBirds) {
   boolean bearPresent = false;
   double bear = Math.random() * 100;
   if (bear <= 5) {
      bearPresent = !bearPresent;
   }
   int birdConsumed = ((int) (Math.random() * 41) + 10) * numBirds;
   if (bearPresent)
   {
      currentFood = 0;
   }
   else{
      if (birdConsumed >= currentFood) {
          currentFood = 0; 
      else {
          currentFood = currentFood - birdConsumed;
   }
}

This following part of the if statement can be re-written using the ternary conditional operator.

The original if statement:

if (birdConsumed >= currentFood) {
    currentFood = 0;
else {
    currentFood = currentFood - birdConsumed;
}

Re-written as: 

currentFood = (birdConsumed >= currentFood) ? 0 : (currentFood – birdConsumed);

This ternary conditional operator expression is nested inside another if statement as follows: 

if (bearPresent)
{
    currentFood = 0;
}
else {
    currentFood = (birdConsumed >= currentFood) ? 0 : (currentFood – birdConsumed);
}

This can be re-written using nested ternary conditional operators as following: 

currentFood = (bearPresent) ? 0 
                 : ((birdConsumed >= currentFood) ? 0 : (currentFood – birdConsumed);  
}

Scoring and Teaching

Resources

Calculator - Using Conditionals and Lambda Mini-Lab