3.4. Multi-Selection: else-if Statements¶

Using if/else statements, you can even pick between 3 or more possibilites. Just add else if for each possibility after the first if, and else before the last possibility.

// 3 way choice with else if
if (boolean expression)
{
   statement1;
}
else if (boolean expression)
{
   statement2;
}
else
{
   statement3;
}

Coding Exercise

Run the code below and try changing the value of x to get each of the three possible lines in the conditional to print.

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

../_images/Condition-three.png — Figure 1: The order that statements execute in a conditional with 3 options: if, else if, and else¶

Note

Another way to handle 3 or more conditional cases is to use the switch and break keywords. For a tutorial on using switch see https://docs.oracle.com/javase/tutorial/java/nutsandbolts/switch.html.

Check your understanding

3-4-2: What does the following code print when x has been set to -5?

if (x < 0)
{
   System.out.println("x is negative");
}
else if (x == 0)
{
   System.out.println("x is zero");
}
else
{
   System.out.println("x is positive");
}

x is negative
When x is equal to -5 the condition of x < 0 is true.
x is zero
This will only print if x has been set to 0. Has it?
x is positive
This will only print if x is greater than zero. Is it?

3-4-3: What does the following code print when x has been set to 2000?

if (x < 0)
{
   System.out.println("x is negative");
}
else if (x == 0)
{
   System.out.println("x is zero");
}
else
{
   System.out.println("x is positive");
}

x is negative
This will only print if x has been set to a number less than zero. Has it?
x is zero
This will only print if x has been set to 0. Has it?
x is positive
The first condition is false and x is not equal to zero so the else will execute.

3-4-4: What does the following code print when x has been set to .8?

if (x < .25)
{
    System.out.println("first quartile");
}
else if (x < .5)
{
    System.out.println("second quartile");
}
else if (x < .75)
{
    System.out.println("third quartile");
}
else
{
    System.out.println("fourth quartile");
}

first quartile
This will only print if x is less than 0.25.
second quartile
This will only print if x is greater than or equal to 0.25 and less than 0.5.
third quartile
The first only print if x is greater than or equal to 0.5 and less than 0.75.
fourth quartile
This will print whenever x is greater than or equal to 0.75.

Coding Exercise

The else-if connection is necessary if you want to hook up conditionals together. In the following code, there are 4 separate if statements instead of the if-else-if pattern. Will this code print out the correct grade? First, trace through the code to see why it prints out the incorrect grade. Use the Code Lens button. Then, fix the code by adding in 3 else’s to connect the if statements and see if it works.

public class IfDebug
{
   public static void main(String[] args)
   {
     int score = 93;
     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);
   }
}
====
// Test Code for Lesson 3.4 - lccbIfDebug
 import static org.junit.Assert.*;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
 import java.io.*;

public class RunestoneTests extends CodeTestHelper
 {
     @Test
     public void testMainCorrectOutput() throws IOException
     {
         String output = getMethodOutput("main");
         String expected = "A\n";
         boolean passed = getResults(expected, output, "Expected output from main");
         assertTrue(passed);
     }

@Test
     public void testCodeContainsFourElses()
     {
         String code = getCode();
         String[] tokens = code.split("\\s+");

int expectedElseCount = 4;
         int actualElseCount  = 0;
         for (int i = 0; i < tokens.length; i++) {
             if (tokens[i].equals("else")) {
                 actualElseCount++;
             }
         }
         boolean passed = getResults(expectedElseCount, actualElseCount, "Expected number of else's");
         assertTrue(passed);
     }
 }

Finish the following code so that it prints “Plug in your phone!” if the battery is below 50, “Unplug your phone!” if it is above 100, and “All okay!” otherwise. Change the battery value to test all 3 conditions.

public class BatteryTest
{
    public static void main(String[] args)
    {
        int battery = 60;

System.out.println("All okay!");
    }
}
====
import static org.junit.Assert.*;
  import org.junit.*;;
  import java.io.*;

public class RunestoneTests extends CodeTestHelper
  {
      @Test
      public void testChangedCode() {
          String origCode = "public class BatteryTest {  public static void main(String[] args)  {  int battery = 60;  System.out.println(\"All okay!\");  }  }";

boolean changed = codeChanged(origCode);
          assertTrue(changed);
      }

@Test
      public void testCodeContains3(){
        boolean ifCheck1 = checkCodeContains("if statement for battery above 100", "if (battery > 100)");
          assertTrue(ifCheck1);
      }

@Test
      public void testCodeContains5(){
          boolean ifCheck1 = checkCodeContains("if statement for battery less than 50", "if (battery < 50)");
          assertTrue(ifCheck1);
      }

@Test
      public void testCodeContains4(){
        boolean ifCheck2 = checkCodeContains("else", "else");
        assertTrue(ifCheck2);
      }
  }

3.4.1. Programming Challenge : Adventure¶

This challenge is on repl.it.

One of the first games coded for early computers in the 1970s was called Colossal Cave Adventure. It was a text-based interactive fiction game where you had to make your way through an elaborate cave. The program only understood one word or phrase commands like north, south, enter, take, etc. You can try playing Adventure recreated online following some of the commands in this walkthrough. Part of the challenge is finding the commands that the code will understand.

In a game like Adventure, else if statements can be used to respond to commands from the user like n, s, e, w.

Try the program below or with this repl link. This is a very simple adventure game that lets the user move in 4 different directions. Right now, it only lets the user move north.
Add in else if statements to go in the directions of “s” for south and “e” for east, and an else statement that says “You can’t go in that direction”. Be creative and come up with different situations in each direction.

How many test-cases are needed to test all branches of your code?
If your class has time, your teacher may ask you to expand this game further or to come up with a different adventure location.

Copy and paste your all of your code from repl.it and run to see if it passes the autograder tests. Include the link to your repl.it code in comments. Note that this code will only run with the autograder’s input and will not ask the user for input.

// Copy in your link to your code on repl.it here:
// Copy in all of your code from repl.it below (include import and public class Main)

====
import static org.junit.Assert.*;
  import org.junit.*;
  import java.io.*;

public class RunestoneTests extends CodeTestHelper
  {
      public RunestoneTests() {
         super("Main", input1.replaceAll(" ", "\n")); // For Book
      }

private static int goal = 5;
      private static String input1 = "n s e w y y y y y y y y y y y y y y";
      private static String input2 = "s e w y n y y y y y y y y y y y y y";
      private static String input3 = "e w y n s y y y y y y y y y y y y y";
      private static String input4 = "w y n s e y y y y y y y y y y y y y";
      private static String input5 = "y n s e w y y y y y y y y y y y y y";
      private String output1, output2, output3, output4, output5;

@Test
      public void test1()
      {
          String input = input1.replaceAll(" ", "\n");
          String output = getMethodOutputWithInput("main", input);
          output1 = output;

String[] lines = output.split("\n");

boolean passed = lines.length >= goal;

passed = getResults(goal +"+ lines", "" + lines.length + " lines", "Outputs at least " + goal +" lines", passed);
          assertTrue(passed);
      }

@Test
      public void test2()
      {
          String input = input2.replaceAll(" ", "\n");
          String output = getMethodOutputWithInput("main", input);
          output2 = output;

input = input3.replaceAll(" ", "\n");
          output = getMethodOutputWithInput("main", input);
          output3 = output;

input = input4.replaceAll(" ", "\n");
          output = getMethodOutputWithInput("main", input);
          output4 = output;

input = input5.replaceAll(" ", "\n");
          output = getMethodOutputWithInput("main", input);
          output5 = output;

if (output1 == null) {
              input = input1.replaceAll(" ", "\n");
              output1 = getMethodOutputWithInput("main", input);
          }

boolean passed = !output1.equals(output2) && !output1.equals(output3) && !output1.equals(output4) && !output1.equals(output5);

passed = getResults("true", "" + passed, "Outputs different results for different inputs", passed);
          assertTrue(passed);
      }

@Test
      public void test3()
      {
          String code = getCode();
          int num = countOccurences(code, "if");
          boolean passed = num >= 4;

getResults("4", "" + num, "Number of if statements", passed);
          assertTrue(passed);
      }

@Test
      public void test4()
      {
          String code = getCode();
          int elseif = countOccurences(code, "else if");
          boolean passed = elseif == 3;

getResults(""+3, ""+elseif, "Number of else if statements", passed);
          assertTrue(passed);
      }

@Test
      public void test5()
      {
          String code = getCode();
          int num = countOccurences(code, "else {");
          boolean passed = num >= 1;

getResults("1", "" + num, "Number of else statements", passed);
          assertTrue(passed);
      }
  }

3.4.2. Summary¶

A multi-way selection is written when there are a series of conditions with different statements for each condition.
Multi-way selection is performed using if-else-if statements such that exactly one section of code is executed based on the first condition that evaluates to true.

// 3 way choice with else if
if (boolean expression)
{
   statement1;
}
else if (boolean expression)
{
   statement2;
}
else
{
    statement3;
}

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