AP CSA 1.9 Debugging: Arguments in the Wrong Slots

Unit 1: Using Objects and Methods · Lesson 1.9 · Debugging

Arguments in the Wrong Slots

Method Signatures. 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 1.9 page All Unit 1 lessons

Why debugging is its own skill

A method signature is a contract about order as much as about types. Hand it two ints in the wrong order and every type checks out, the call compiles, and the answer is quietly the answer to a different question.

What is wrong with it

The call to describe passes the height first and the width second, but the signature declares width first. Swap the arguments at the CALL, not in the method.
The percent method divides part by whole in the wrong order, so it reports the reciprocal, which is 0 for any real input under integer division. Fix the order inside the method.
The signatures themselves are correct and must not change. Only the call and the body of percent need work.

The program reads

A width, a height, then a part and a whole.

The program should print

Two lines: the area described as width times height, then the part as an integer percentage of the whole.

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 4 25 50

Example 1 output

12
50

Example 2 input

5 5 1 4

Example 2 output

25
25

Your answer

Main.java

import java.util.Scanner;

public class Main {
  public static int describe(int width, int height) {
    return width * height;
  }

  public static int percent(int part, int whole) {
    return whole * 100 / part;
  }

  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int width = input.nextInt();
    int height = input.nextInt();
    int part = input.nextInt();
    int whole = input.nextInt();

    System.out.println(describe(height, width));
    System.out.println(percent(part, whole));
  }
}

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

Multiplication is commutative, so the area happens to come out right anyway. That is exactly what makes this dangerous: the same swapped call in a method that subtracts or divides would be visibly wrong, and here it is invisible. Fix it because the contract says so.

Hint 2

In percent, ask what fraction you want: part out of whole. Multiplying by 100 first keeps the integer division from collapsing to 0, but the part still has to be on top.

Hint 3

Parameter names are documentation for the caller. When a call site reads describe(height, width) against a signature that says (width, height), the call is wrong even if the result looks fine.

Where to go next

Rework the Lesson 1.9 page Try the from-scratch version of this lesson

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