AP CSA 1.1 Debugging: Right Steps, Wrong Order

Unit 1: Using Objects and Methods · Lesson 1.1 · Debugging

Right Steps, Wrong Order

Introduction to Algorithms, Programming, and Compilers. 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 1.1 page All Unit 1 lessons

Why debugging is its own skill

Every instruction in this program is correct on its own. The order they run in is not, and the total comes out wrong by a few cents in a way no compiler will ever mention. Sequencing is part of the algorithm, not a detail of how it is typed.

What is wrong with it

The discount is applied AFTER the tax is calculated, so tax is charged on the full price instead of on the discounted price. Reorder the statements so the discount comes off first and the tax is computed on what remains.
The final total prints before the tax has been added to it, so it reports the pre-tax number. Move the printing to the end, after every calculation is finished.
The arithmetic in each individual line is correct. Only the order changes.

The program reads

A price in cents, then a discount in cents.

The program should print

Three lines: the discounted price, the tax at 10 percent of the discounted price, and the final total.

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

1000 200

Example 1 output

800
80
880

Example 2 input

500 0

Example 2 output

500
50
550

Your answer

Main.java

import java.util.Scanner;

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

    int tax = price * 10 / 100;
    int discounted = price - discount;
    int total = discounted;

    System.out.println(discounted);
    System.out.println(tax);
    System.out.println(total);

    total = discounted + tax;
  }
}

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

A statement can only use values that already exist when it runs. The tax line runs before discounted has been computed, so it has nothing to work from except the full price.

Hint 2

The last line of the starter computes the correct total, and then the program ends without printing it. Assigning a value after you have already printed the variable changes nothing anyone can see.

Hint 3

Write the three quantities in dependency order before you touch the code: discounted depends on price, tax depends on discounted, total depends on both. Statements have to follow that order.

Where to go next

Rework the Lesson 1.1 page Try the from-scratch version of this lesson

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