📓2.4: Multiway Selection

Multiway Selection: Nested if Statements
- “Dangling” else Problem
Multiway Selection: else if Blocks
- Separate if Statements vs. else if Chains
Summary

📖 This page is a condensed version of CSAwesome Topic 2.4

Multiway Selection: Nested `if` Statements

If statements can be nested inside other if statements, allowing for more possible branches in a program.

This is like asking a “follow-up question” after passing an initial one.

image-small

if (boolean expression) {
    // This "nested if" is evaluated when the outer is true
    if (boolean expression) {
        statement;
    }
}

The boolean expression of the nested (inner) if statement is evaluated ONLY when the boolean expression of the outer if statement evaluates to true.

Example: 20 Questions

Have you ever played 20 Questions? 20 Questions is a game where one person thinks of an object, and the other players ask up to 20 questions to guess what it is.

There is great online version called Akinator that guesses whether you are thinking of a real or fictional character by asking you questions. Akinator is a simple artificial intelligence algorithm that uses a decision tree of yes or no questions to pinpoint the answer.

Although Akinator needs a very large decision tree, we can create a guessing game for just animals using a much smaller number of if-statements.

Here’s a simplified version of what the code might look like, using nested ifs:

“Dangling” `else` Problem

Sometimes with nested ifs we find a “dangling else” clause that could potentially belong to either if statement. This may cause unintended behavior.

The rule is that the else clause will always be a part of the closest unmatched if statement in the same block of code, regardless of indentation.

// Nested if with dangling else
if (boolean expression)
    if (boolean expression)
        doThisStatement;
else  // belongs to CLOSEST if
    doThatStatement;

You can (and should) use curly braces ({}) to enclose a nested if like below:

if (boolean expression)
{
    if (boolean expression)
        doThisStatement;
}
else  // belongs to FIRST if
    doThatStatement;

Now the else clause belongs to the top level if block instead!

In fact many experienced Java programmers always use curly braces, even when they are not technically required, to avoid this kind of confusion.

Multiway Selection: `else if` Blocks

With nested if/else structures, we can pick between 3 or more branches of code. Another type of multi-way selection is possible with else if blocks. This is convenient when there are a series of mutually-exclusive expressions to be tested, with different segments of code for each condition.

image-small

Multi-way selection with else if is performed such that no more than one segment of code is executed based on the first expression that evaluates to true in the block. If no expression evaluates to true, and there is a trailing else statement provided, then the body of the else is executed.

Syntax for multi-way selection with else if:

if (boolean expression) {
  statement1;
}
else if (boolean expression) {
  statement2;
}
else {
  statement3;
}

This structure can accomodate more than 3 choices easily – just add an else if clause for every possibility after the first if statement, and before the final else block.

Here is a flowchart for a conditional structure with 3 options like in the code above:

Here is a code example of a conditional structure that implements else if to test multiple possibilities for a single value:

int x = 2;
if (x < 0) {
  System.out.println("x is negative");
}
else if (x == 0) {
  System.out.println("x is 0");
}
else {
  System.out.println("x is positive");
}
System.out.println("after conditional block");

Try changing the value of x to get each of the three possible lines in the conditional to print.

Another way to handle 3 or more conditional cases is to use the switch and case keywords, but these will not be on the exam. For a tutorial on using switch-case, see the Java Documentation.

Initialize a variable battery of type int that represents your phone battery percentage.
Set up the structure of a conditional block with 4 possibilities, testing the value of battery.
Complete the code so the program prints:

"Unplug your phone!" if battery > 95
"Plug in your phone!" if battery < 50
"Low power mode!" if battery <= 20
"All okay!" otherwise

Separate `if` Statements vs. `else if` Chains

The core difference between multiple separate if statements and connected else if chains lies in their execution flow and the conditions under which their respective blocks of code are evaluated.

An else if statement is always used in conjunction with an initial if statement.
The conditions in an if/else-if/else chain are evaluated sequentially.
Once a condition is found to be true, its corresponding code block is executed, and the rest of the else if and else blocks in that chain are skipped.

This structure is ideal for handling mutually exclusive conditions, where only one outcome is expected or desired from a set of possibilities.

This program uses 4 separate if statements instead of the if/else-if pattern. First, trace through the code to see why it prints out the incorrect grade. Then, fix it by adding else if blocks to connect the conditions.

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int score = scan.nextInt();
        String grade = "";

        if (score >= 90) {
            grade = "A";
        }
        if (score >= 80) {
            grade = "B";
        }
        if (score >= 70) {
            grade = "C";
        }
        if (score >= 60) {
            grade = "D";
        }
        else {
            grade = "F";
        }

        System.out.println(grade);
    }
}

Summary

(AP 2.4.A.1) Nested if statements consist of if, if-else, or if-else-if statements within if, if-else, or if-else-if statements.
(AP 2.4.A.2) The boolean expression (condition) of the inner nested if statement is evaluated only if the Boolean expression of the outer if statement evaluates to true.
(AP 2.4.A.3) A multi-way selection (if/else-if) is used when there are a series of expressions with different segments of code for each condition. Multi-way selection is performed such that no more than one segment of code is executed based on the first expression that evaluates to true. If no expression evaluates to true and there is a trailing else statement, then the body of the else is executed.

// Multiway selection with else if
if (boolean expression) {
    statement1;
}
else if (boolean expression) {
    statement2;
}
else {
    statement3;
}

Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.

📓2.4: Multiway Selection

Table of Contents

Multiway Selection: Nested if Statements