Question

Mailboxes Java

Peter the postman was bored one evening and he decided to experiment with the mailboxes in the mailroom. The mailboxes were numbered from zero to 200 (there were 201 total mailboxes). Initially all boxes were closed. Starting with mailbox #2, he opened the door of every even numbered mailbox. Next, beginning with mailbox #3 and going to every third mailbox, he opened its door if it was closed and closed it if it was open. Then he repeated this procedure with every fourth mailbox starting with mailbox #4; and then every fifth mailbox starting with mailbox #5; continuing in this manner all the way up to the changing every 200th mailbox starting with mailbox #200 (which of course just changes mailbox #200).

Your job is to write a program to determine which mailboxes were closed at the end of the experiment.The program should ask the user for how many mailboxes are in the mailroom, and then perform the desired experiment for that many mailboxes. This will allow us to run the experiment for a small mailroom with only 201 mailboxes, or a large mailroom with thousands of mailboxes.

Notes:

1. Simply create an array of integers of the desired size. Each mailbox will be represented by the array element whose index matches the mailbox number; i.e., mailbox #0 will be represented by the array element at index 0. After creating an array of the needed size, be sure to initialize the array so that all mailboxes start (or are filled) in the "closed" position.
2. Let the integer value 0 represent a closed mailbox and the integer value 1 represent an open mailbox. Alternatively, you could use an array of booleans and let the values true and false represent open and closed.
3. You will need a double-nested loop (a loop inside a loop) to conduct the experiment.
4. Print the numbers of the boxes that are in the closed position, starting at mailbox #0. Print out the distance between the closed mailboxes. See the example execution below for the format of the output.
5. You should write separate methods to tackle the different sections of this problem a) create and initialize the array, b) the actual experiment, and c) printing of the results.
   

Example execution (user input is 5)

This is an example execution to show you how Mailboxes.java should behave, it is run for only 5 mailboxes

How many mailbox are in the mailroom? 5

When done, here are all the mailboxes that are closed:

Box #0; distance from previous box: 0

Box #1; distance from previous box: 1

Box #4; distance from previous box: 3

Answer 1

Mailboxes.java

import java.util.Arrays;
import java.util.Scanner;

public class Mailboxes {
  
private static final int OPEN = 1;
private static final int CLOSED = 0;
  
public static void main(String[]args)
{
Scanner sc = new Scanner(System.in);
System.out.print("How many mailbox are in the mailroom? ");
int nMailbox = Integer.parseInt(sc.nextLine().trim());
int mailboxes[] = new int[nMailbox];
Arrays.fill(mailboxes, CLOSED);
  
// open every even numbered mailbox starting from mailbox 2
for(int i = 2; i <= nMailbox; i++)
{
if(i % 2 == 0)
mailboxes[i] = OPEN;
}
  
// start from 3rd mailbox and change every 3rd box
// repeat the above process for all the boxes
for(int i = 3; i <= nMailbox; i++)
{
int j = i;
while(j < nMailbox)
{
if(mailboxes[j] == OPEN)
{
mailboxes[j] = CLOSED;
}
else if(mailboxes[j] == CLOSED)
{
mailboxes[j] = OPEN;
}
j += i;
}
}
  
int previous = 0;
for(int i = 0; i < nMailbox; i++)
{
if(mailboxes[i] == CLOSED)
{
System.out.println("Box #" + i + "; distance from previous box: " + (i - previous));
previous = i;
}
}
}
}

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