AP CSA 2.7 Debugging: One Short, Every Time

Unit 2: Selection and Iteration · Lesson 2.7 · Debugging

One Short, Every Time

while Loops. 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 2.7 page All Unit 2 lessons

Why debugging is its own skill

This loop terminates, which is the good news. It just does the wrong amount of work, and the accumulator is updated in the wrong order, so both the count and the total are off. Neither mistake announces itself.

What is wrong with it

The program should print every integer from 1 through n. It stops at n - 1 because the condition excludes n itself. Fix the condition.
The running total is supposed to be the sum of every number printed. The counter is increased BEFORE the value is added, so the first number is never counted and an extra one is. Move the update so the total matches what was printed.
Do not convert this to a for loop. The exercise is about getting a while loop right.

The program reads

A single positive integer n.

The program should print

The integers from 1 through n, one per line, then the sum of those integers on the last line.

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

Example 1 output

Example 2 input

Example 2 output

1
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 i = 1;
    int total = 0;
    while (i < n) {
      i++;
      System.out.println(i);
      total = total + i;
    }
    System.out.println(total);
  }
}

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

Trace n = 3 on paper with two columns, i and total, writing a new row every time the body runs. The starter never prints 1 at all, because i is incremented before the first println. The first thing the body does should be the work, not the update.

Hint 2

A counting while loop has a fixed shape: initialize before, test at the top, do the work, update last. Moving the update to the top is what skips the first value.

Hint 3

i <= n and i < n differ by exactly one iteration. When the spec says through n inclusive, the condition has to let n into the body.

Where to go next

Rework the Lesson 2.7 page Try the from-scratch version of this lesson

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