AP CSA 4.4 Exercise 1: Both Directions

Unit 4: Data Collections · Lesson 4.4 · Exercise 1

Both Directions

Traversing Arrays. Write real Java, run it, and submit it to be graded against hidden test cases.

Back to the Lesson 4.4 page All Unit 4 lessons

Why this one is worth doing

Sometimes you need the index and sometimes you only need the value. This exercise asks for both in the same program so the choice between an indexed loop and an enhanced one has to be made deliberately.

What to write

Read an integer count, then that many integers into an array.
Print every element on its own line, front to back.
Print every element on its own line, back to front.
Print the index of the largest element. When it appears more than once, print the FIRST index it appears at.
Print the sum of the elements at even indexes.

Your program reads

An integer count of at least 1, then that many integers.

Your program prints

The elements forwards, then backwards, then the index of the maximum, then the sum at even indexes.

Worked examples

These are the cases you can see. There are more you cannot, and they use different values, so an answer that prints these numbers as constants will fail.

Example 1 input

5
3 9 4 9 1

Example 1 output

Example 2 input

1
42

Example 2 output

Your answer

Main.java

import java.util.Scanner;

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

    // Two traversals, then two answers that need the index.
  }
}

Input for the Run button

Run sends whatever is in the input box below. Submitting runs your program against every test case, including hidden ones with different input.

Stuck?

Hint 1

The backwards loop starts at length - 1 and tests i >= 0. Starting at length is the off by one that throws.

Hint 2

Use a strict > when hunting the maximum index, so a repeated maximum keeps the first position rather than the last.

Hint 3

Even INDEXES means 0, 2, 4 and so on, which is not the same as even values. Step the loop by 2.

Where to go next

Rework the Lesson 4.4 page Pick another Unit 4 lesson

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