AP CSA 1.4 Debugging: Reading in the Wrong Order

Unit 1: Using Objects and Methods · Lesson 1.4 · Debugging

Reading in the Wrong Order

Assignment Statements and Input. 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 1.4 page All Unit 1 lessons

Why debugging is its own skill

Input arrives in one fixed order and the program consumes it in another, so every value lands in the wrong variable. Nothing crashes, the types all match, and the output is confidently mislabeled.

What is wrong with it

The spec says the quantity comes first and the price second, but the program reads price first. Swap the two reads so each variable receives the value it is named for.
The subtotal is assigned before the values have been read, so it uses the default 0. Move the assignment below the reads.
The printing at the bottom is correct and should not move.

The program reads

A quantity, then a unit price in cents.

The program should print

Three lines: the quantity, the unit price, and the subtotal.

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

3 250

Example 1 output

3
250
750

Example 2 input

1 99

Example 2 output

1
99
99

Your answer

Main.java

import java.util.Scanner;

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

    int quantity = 0;
    int price = 0;
    int subtotal = quantity * price;

    price = input.nextInt();
    quantity = input.nextInt();

    System.out.println(quantity);
    System.out.println(price);
    System.out.println(subtotal);
  }
}

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

nextInt does not know what the number means. It hands the next number to whichever variable is on the left of the assignment, so the ORDER of your reads has to match the order of the input exactly.

Hint 2

An assignment computes its right side once, at the moment it runs. Writing subtotal = quantity * price before the reads captures 0 times 0 and never updates itself later.

Hint 3

Test with a quantity and price that are different numbers. Using 5 and 5 makes a swapped read look perfectly correct.

Where to go next

Rework the Lesson 1.4 page Try the from-scratch version of this lesson

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