AP CSA 4.2 Debugging: The Loop That Cannot Write Back

Unit 4: Data Collections · Lesson 4.2 · Debugging

The Loop That Cannot Write Back

Introduction to Using Data Sets. 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 4.2 page All Unit 4 lessons

Why debugging is its own skill

The doubling loop is written correctly in every visible way and the array comes out completely unchanged. The enhanced for loop hands you a copy of each element, so assigning to the loop variable updates the copy and throws it away.

What is wrong with it

The doubling loop uses an enhanced for loop and assigns to the loop variable, which only changes a copy. Rewrite it as an indexed loop so the values in the array are actually replaced.
The count of values above the threshold compares against the wrong variable: it uses the loop index instead of the element. Fix the comparison.
The printing loop at the end is correct, and an enhanced for loop is the right choice there because it only reads.

The program reads

A count, then that many integers, then a threshold.

The program should print

Every element after doubling, one per line, then the count of the doubled values that are strictly greater than the threshold.

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

4
1 5 9 2
8

Example 1 output

Example 2 input

1
3
5

Example 2 output

6
1

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 threshold = input.nextInt();

    for (int value : data) {
      value = value * 2;
    }

    for (int value : data) {
      System.out.println(value);
    }

    int above = 0;
    for (int i = 0; i < data.length; i++) {
      if (i > threshold) {
        above++;
      }
    }
    System.out.println(above);
  }
}

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

The enhanced for loop variable is a fresh copy of the element, not the slot it came from. Writing to it is like changing a photocopy: legal, silent, and useless. Only data[i] = ... reaches the array.

Hint 2

The rule worth memorizing: enhanced for to READ, indexed for to WRITE. It applies to the exam and to every array you touch afterward.

Hint 3

In the counting loop, i is a position (0, 1, 2) and data[i] is a value. Comparing a position against a threshold meant for values is a type-correct, meaning-free comparison, which is why the compiler stays quiet.

Where to go next

Rework the Lesson 4.2 page Try the from-scratch version of this lesson

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