AP CSA 2.3 Debugging: Four Ifs Where One Chain Belongs

Unit 2: Selection and Iteration · Lesson 2.3 · Debugging

Four Ifs Where One Chain Belongs

if Statements. This is not a blank editor: it is someone else's attempt. Find what is wrong, fix it, and submit to be graded against hidden test cases.

Back to the Lesson 2.3 page All Unit 2 lessons

Why debugging is its own skill

A chain of else if stops at the first branch that matches. Four separate ifs do not stop at all, and every one of them gets tested. The output tells you immediately which one you wrote, if you read it.

What is wrong with it

Run the program on a score of 95. It prints more than one letter grade. The bands were written as four independent if statements instead of one if / else if chain, so a high score satisfies several of them. Rewrite it as a single chain that prints exactly one letter.
The PASS and FAIL line prints PASS for every score, including a 12. There is a stray semicolon immediately after the if condition, which ends the statement before the body. Find it and remove it.
The distance-from-100 line at the bottom is correct.

The program reads

A single integer score from 0 to 100.

The program should print

Three lines: exactly one letter grade, then PASS or FAIL, then how far the score is from 100.

Worked examples

These show what the FIXED program should print. There are more cases you cannot see, and they use different values, so patching around just these numbers will fail.

Example 1 input

Example 1 output

A
PASS
5

Example 2 input

Example 2 output

F
FAIL
88

Your answer

Main.java

import java.util.Scanner;

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

    if (score >= 90) {
      System.out.println("A");
    }
    if (score >= 80) {
      System.out.println("B");
    }
    if (score >= 70) {
      System.out.println("C");
    }
    if (score >= 60) {
      System.out.println("D");
    }
    if (score < 60) {
      System.out.println("F");
    }

    if (score >= 60);
    {
      System.out.println("PASS");
    }

    System.out.println(100 - score);
  }
}

Input for the Run button

Run sends whatever is in the code box below, bugs and all, so you can see the crash or the wrong answer for yourself before you fix anything.

Stuck?

Hint 1

Trace a score of 95 by hand through all five ifs. It is at least 90, and also at least 80, and also at least 70, and also at least 60. Four of the five bodies run. else if is what makes the second test unreachable once the first one matched.

Hint 2

A semicolon right after if (condition) is a complete, empty statement: the if runs it and does nothing. The block underneath is then just a plain block that always executes. This compiles without a single warning, which is what makes it worth recognizing on sight.

Hint 3

The FAIL case has to print something. Once you remove the stray semicolon you still need an else, or a failing score prints no second line at all.

Where to go next

Rework the Lesson 2.3 page Try the from-scratch version of this lesson

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