AP CSA 1.3 Debugging: Precedence Without Parentheses

Unit 1: Using Objects and Methods · Lesson 1.3 · Debugging

Precedence Without Parentheses

Expressions and Assignment 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 1.3 page All Unit 1 lessons

Why debugging is its own skill

Multiplication happens before addition whether or not that is what you meant. The expression reads left to right on the page and does not execute that way, and the only thing standing between the two readings is a pair of parentheses nobody typed.

What is wrong with it

The average of three values is written as a + b + c / 3, which divides only c by 3 because division binds tighter than addition. Add the parentheses that make the whole sum divide.
The area of the border is written as width * 2 + height * 2 but the spec calls for twice the sum of width and height plus four corners. Fix the expression so it matches the stated formula.
The remainder line is correct.

The program reads

Three integers a, b and c, then a width and a height.

The program should print

Three lines: the integer average of the three values, a modulo b, and the border count.

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 3 3 2 5

Example 1 output

3
0
18

Example 2 input

10 4 7 3 3

Example 2 output

7
2
16

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 width = input.nextInt();
    int height = input.nextInt();

    int average = a + b + c / 3;
    System.out.println(average);

    System.out.println(a % b);

    int border = width * 2 + height * 2;
    System.out.println(border);
  }
}

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

Evaluate a + b + c / 3 with a = 3, b = 3, c = 3 by hand. Division first gives 3 + 3 + 1, which is 7, not 3. Parentheses are the only way to make the addition happen first.

Hint 2

The border formula in the spec is twice the sum, plus four. The starter computes twice each side and never adds the corners, so it is short by exactly 4 on every input.

Hint 3

When an expression mixes operators, write the intended order in words first, then add parentheses until the code matches the words. Extra parentheses cost nothing.

Where to go next

Rework the Lesson 1.3 page Try the from-scratch version of this lesson

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