I am working a problem that seems to be somewhat famous among beginning programmers, the 8 queens puzzle. I have seen several solutions to this problems using 2D arrays, recursion etc, but this problem is an assignment given in CS course book chapter introducing 1D arrays, so the available techniques to solve this problem are limited.
The procedure I have used, is by first creating a 1D array with the size of 64, which makes possible positions to place queens from index 0 to 63. A random position index is then generated, and a test is preformed to check if there is any queens attacking this position. If this position is not attacked by any queens, a queen is placed by setting the board[position] = true. When a queen is placed, the queenCount is incremented, and this process repeats until 8 queens have been placed.
If queens are placed in such a way that it is impossible to place 8, the board resets after 1 millisecond by preforming a timecheck, and retries to place the 8 queens. At the best I am able to place 7 queens, but the last remaining one is never placed. Each attempt is printed, along with queenCount for this attempt. Is it possible to use this approach, or is it a dead end?
Code example below:
package ch7;
public class Chapter_07_E22_EightQueens64bool {
public static void main(String[] args) {
int queenCount = 0;
int attemptCount = 0;
boolean[] board = new boolean[8 * 8];
final long TIME_LIMIT = 1; //Milliseconds
long startTime = System.currentTimeMillis();
while (queenCount < 8) {
int position = placeQueen(board.length);
if(checkPosition(position, board) && !board[position]) {
board[position] = true;
queenCount++;
}
long timeCheck = System.currentTimeMillis();
if (timeCheck - startTime > TIME_LIMIT) {
clearBoard(board);
queenCount = 0;
startTime = System.currentTimeMillis();
}
System.out.println("Attempt #" + ++attemptCount);
System.out.println(queenCount + " queens placed.");
printBoard(board);
}
}
public static void printBoard(boolean[] board) {
for (int i = 0; i < board.length; i++) {
if (board[i])
System.out.print("|Q");
else
System.out.print("| ");
if ((i + 1) % 8 == 0)
System.out.println("|");
}
}
public static int placeQueen(int boardSize) {
return (int)(Math.random() * boardSize);
}
public static boolean[] clearBoard(boolean[] board) {
for (int i = 0; i < board.length; i++)
board[i] = false;
return board;
}
public static boolean checkPosition(int position, boolean[] board) {
return checkTop(position, board) && checkBottom(position, board) && checkLeft(position, board) &&
checkRight(position, board) && checkTopLeft(position, board) && checkTopRight(position, board) &&
checkBottomLeft(position, board) && checkBottomRight(position, board);
}
public static boolean checkTop(int position, boolean[] board) {
// Checks each field above the current position while i >= 8
for (int i = position; i >= 8; i -= 8) {
if (board[i - 8])
return false;
}
return true;
}
public static boolean checkBottom(int position, boolean[] board) {
// Checks each field below the current position while i <= 55;
for (int i = position; i <= 55; i += 8) {
if (board[i + 8])
return false;
}
return true;
}
public static boolean checkRight(int position, boolean[] board) {
// Checks each field to the right of the current position while i % 8 < 7
for (int i = position; i % 8 < 7; i += 1) {
if (board[i + 1])
return false;
}
return true;
}
public static boolean checkLeft(int position, boolean[] board) {
// Checks each field to the left of the current position while i % 8 != 0
for (int i = position; i % 8 != 0; i -= 1) {
if (board[i - 1])
return false;
}
return true;
}
public static boolean checkTopLeft(int position, boolean[] board) {
// Checks each field top left of the current position while i >= 9
for (int i = position; i >= 9; i -= 9) {
if (board[i - 9])
return false;
}
return true;
}
public static boolean checkTopRight(int position, boolean[] board) {
// Checks each field top right of the current position while i >= 7
for (int i = position; i >= 7; i -= 7) {
if (board[i - 7])
return false;
}
return true;
}
public static boolean checkBottomRight(int position, boolean[] board) {
// Checks each field below the current position while i <= 54
for (int i = position; i <= 54; i += 9) {
if (board[i + 9])
return false;
}
return true;
}
public static boolean checkBottomLeft(int position, boolean[] board) {
// Checks each field below the current position while i <= 56
for (int i = position; i <= 56; i += 7) {
if (board[i + 7])
return false;
}
return true;
}
}
