AP CSA 4.5 Debugging: The Average That Rounds Itself

Unit 4: Data Collections · Lesson 4.5 · Debugging

The Average That Rounds Itself

Algorithms with Arrays. 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 4.5 page All Unit 4 lessons

Why debugging is its own skill

Dividing one int by another int throws the remainder away before anything else happens, so casting the result to double afterward preserves a number that is already wrong. The cast has to come first, and where you put it is the whole bug.

What is wrong with it

The average is computed by dividing two ints and then casting to double, which rounds down before the cast can help. Cast the total to double BEFORE the division so the fractional part survives.
The search reports the index of the target but starts scanning at index 1, so a target sitting at index 0 is reported as not found. Fix the starting index.
The total is computed correctly and should stay as int.

The program reads

A count, then that many integers, then a target value to find.

The program should print

Three lines: the total, the average as a double, and the index of the first occurrence of the target or -1.

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

2
7 0
7

Example 1 output

7
3.5
0

Example 2 input

4
1 2 3 4
3

Example 2 output

10
2.5
2

Your answer

Main.java

import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int n = input.nextInt();
    int[] data = new int[n];
    for (int i = 0; i < n; i++) {
      data[i] = input.nextInt();
    }
    int target = input.nextInt();

    int total = 0;
    for (int v : data) {
      total = total + v;
    }
    System.out.println(total);

    double average = (double) (total / data.length);
    System.out.println(average);

    int found = -1;
    for (int i = 1; i < data.length; i++) {
      if (data[i] == target) {
        found = i;
        break;
      }
    }
    System.out.println(found);
  }
}

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

Work out 7 / 2 in ints: the answer is 3, and the 0.5 is gone permanently. Casting 3 to double gives 3.0, not 3.5. The cast must apply to an operand, not to the finished result.

Hint 2

(double) total / data.length casts only total, and one double operand promotes the whole division to double arithmetic. That is why the parentheses placement matters so much.

Hint 3

Any search that starts at index 1 is claiming index 0 cannot hold the answer. Test with a target sitting at the front and the bug appears immediately.

Where to go next

Rework the Lesson 4.5 page Try the from-scratch version of this lesson

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