AP CSA 1.6 Debugging: The Operator That Is Two Operators

Unit 1: Using Objects and Methods · Lesson 1.6 · Debugging

The Operator That Is Two Operators

Compound Assignment Operators. 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 1.6 page All Unit 1 lessons

Why debugging is its own skill

x =+ 1 compiles. It is not a typo the compiler will catch, because it is a perfectly ordinary assignment of positive one, and it silently replaces the value instead of adding to it. Two characters in the wrong order change accumulation into overwriting.

What is wrong with it

The running total uses total =+ value, which assigns the value rather than adding it, so the total ends up holding only the last number. Change it to the compound operator that adds.
The countdown uses count =- 1, which assigns negative one every time instead of subtracting. Change it to the compound operator that subtracts.
The multiply-by-two line already uses a compound operator correctly. Use it as your model.

The program reads

Three integers.

The program should print

Three lines: the sum of the three values, the value 10 reduced by 1 three times, and the third value doubled.

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

1 2 3

Example 1 output

6
7
6

Example 2 input

10 20 30

Example 2 output

60
7
60

Your answer

Main.java

import java.util.Scanner;

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

    int total = 0;
    total =+ a;
    total =+ b;
    total =+ c;
    System.out.println(total);

    int count = 10;
    count =- 1;
    count =- 1;
    count =- 1;
    System.out.println(count);

    int doubled = c;
    doubled *= 2;
    System.out.println(doubled);
  }
}

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

Read total =+ a as total = (+a). The plus is a unary sign on a, not part of the assignment. So the statement means "replace total with a", and after three of them total holds only c.

Hint 2

Similarly count =- 1 means count = (-1), which is why the countdown from 10 lands on -1 instead of 7 no matter how many times it runs.

Hint 3

The compound operator always has the symbol FIRST and the equals second: += and -=. If the equals comes first, you have written a plain assignment with a signed value.

Where to go next

Rework the Lesson 1.6 page Try the from-scratch version of this lesson

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