john john - 7 months ago 35
Java Question

java:implement 8 queen using depth first search

i am try to implement 8 queen using depth search for any initial state it work fine for empty board(no queen on the board) ,but i need it to work for initial state if there is a solution,if there is no solution for this initial state it will print there is no solution

Here is my code:

public class depth {
public static void main(String[] args) {
//we create a board
int[][] board = new int[8][8];
board [0][0]=1;
board [1][1]=1;
board [2][2]=1;
board [3][3]=1;
board [4][4]=1;
board [5][5]=1;
board [6][6]=1;
board [7][7]=1;

eightQueen(8, board, 0, 0, false);
System.out.println("the solution as pair");
for(int i=0;i<board.length;i++){
for(int j=0;j<board.length;j++)
if(board[i][j]!=0)

System.out.println(" ("+i+" ,"+j +")");
}
System.out.println("the number of node stored in memory "+count1);
}
public static int count1=0;
public static void eightQueen(int N, int[][] board, int i, int j, boolean found) {

long startTime = System.nanoTime();//time start

if (!found) {
if (IsValid(board, i, j)) {//check if the position is valid
board[i][j] = 1;

System.out.println("[Queen added at (" + i + "," + j + ")");
count1++;
PrintBoard(board);


if (i == N - 1) {//check if its the last queen
found = true;
PrintBoard(board);
double endTime = System.nanoTime();//end the method time

double duration = (endTime - startTime)*Math.pow(10.0, -9.0);
System.out.print("total Time"+"= "+duration+"\n");
}
//call the next step
eightQueen(N, board, i + 1, 0, found);
} else {

//if the position is not valid & if reach the last row we backtracking
while (j >= N - 1) {
int[] a = Backmethod(board, i, j);
i = a[0];
j = a[1];

System.out.println("back at (" + i + "," + j + ")");
PrintBoard(board);
}
//we do the next call
eightQueen(N, board, i, j + 1, false);
}
}

}

public static int[] Backmethod(int[][] board, int i, int j) {
int[] a = new int[2];
for (int x = i; x >= 0; x--) {
for (int y = j; y >= 0; y--) {
//search for the last queen
if (board[x][y] != 0) {
//deletes the last queen and returns the position
board[x][y] = 0;
a[0] = x;
a[1] = y;
return a;
}
}
}
return a;
}

public static boolean IsValid(int[][] board, int i, int j) {

int x;
//check the queens in column
for (x = 0; x < board.length; x++) {
if (board[i][x] != 0) {
return false;
}
}
//check the queens in row
for (x = 0; x < board.length; x++) {
if (board[x][j] != 0) {
return false;
}
}
//check the queens in the diagonals
if (!SafeDiag(board, i, j)) {
return false;
}
return true;
}

public static boolean SafeDiag(int[][] board, int i, int j) {

int xx = i;
int yy = j;
while (yy >= 0 && xx >= 0 && xx < board.length && yy < board.length) {
if (board[xx][yy] != 0) {
return false;
}
yy++;
xx++;
}
xx = i;
yy = j;
while (yy >= 0 && xx >= 0 && xx < board.length && yy < board.length) {
if (board[xx][yy] != 0) {
return false;
}
yy--;
xx--;
}
xx = i;
yy = j;
while (yy >= 0 && xx >= 0 && xx < board.length && yy < board.length) {
if (board[xx][yy] != 0) {
return false;
}
yy--;
xx++;
}
xx = i;
yy = j;
while (yy >= 0 && xx >= 0 && xx < board.length && yy < board.length) {
if (board[xx][yy] != 0) {
return false;
}
yy++;
xx--;
}
return true;
}
public static void PrintBoard(int[][] board) {
System.out.print(" ");
for (int j = 0; j < board.length; j++) {
System.out.print(j);
}
System.out.print("\n");
for (int i = 0; i < board.length; i++) {
System.out.print(i);
for (int j = 0; j < board.length; j++) {
if (board[i][j] == 0) {
System.out.print(" ");
} else {
System.out.print("Q");
}
}
System.out.print("\n");
}
}


}


for example for this initial state it give me the following error:

Exception in thread "main" java.lang.StackOverflowError


i am stuck, i think the error is infinite call for the method how to solve this problem.

any idea will be helpful,thanks in advance.

note:the broad is two dimensional array,when i put (1) it means there queen at this point.

note2:
we i put the initial state as the following it work:

board [0][0]=1;
board [1][1]=1;
board [2][2]=1;
board [3][3]=1;
board [4][4]=1;
board [5][5]=1;
board [6][6]=1;
board [7][1]=1;

Answer

[EDIT: Added conditional output tip.]

To add to @StephenC's answer:

This is a heck of a complicated piece of code, especially if you're not experienced in programming Java.

I executed your code, and it outputs this over and over and over and over (and over)

back at (0,0)
 01234567
0
1 Q
2  Q
3   Q
4    Q
5     Q
6      Q
7       Q
back at (0,0)

And then crashes with this

Exception in thread "main" java.lang.StackOverflowError
    at java.nio.Buffer.<init>(Unknown Source)
    ...
    at java.io.PrintStream.print(Unknown Source)
    at java.io.PrintStream.println(Unknown Source)
    at Depth.eightQueen(Depth.java:56)
    at Depth.eightQueen(Depth.java:60)
    at Depth.eightQueen(Depth.java:60)
    at Depth.eightQueen(Depth.java:60)
    at Depth.eightQueen(Depth.java:60)
    ...

My first instinct is always to add some System.out.println(...)s to figure out where stuff is going wrong, but that won't work here.

The only two options I see are to

  • Get familiar with a debugger and use it to step through and analyze why it's never stopping the loop
  • Break it down man! How can you hope to deal with a massive problem like this without breaking it into digestible chunks???

Not to mention that the concept of 8-queens is complicated to begin with.


One further thought:

System.out.println()s are not useful as currently implemented, because there's infinite output. A debugger is the better solution here, but another option is to somehow limit your output. For example, create a counter at the top

private static final int iITERATIONS = 0;

and instead of

System.out.println("[ANUMBERFORTRACING]: ... USEFUL INFORMATION ...")

use

conditionalSDO((iITERATIONS < 5), "[ANUMBERFORTRACING]: ... USEFUL INFORMATION");

Here is the function:

private static final void conditionalSDO(boolean b_condition, String s_message)  {
   if(b_condition)  {
       System.out.println(s_message);
   }
}

Another alternative is to not limit the output, but to write it to a file.

I hope this information helps you.

(Note: I edited the OP's code to be compilable.)

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