Classic recursion game of 8 Queens errors

BKizzle77 · #1 26-May-2008, 11:25

Hey I am a relatively amateur java programmer using Eclipse. I have been trying for a while now to create the classic recurrsion game of 8 Queens and am stumped. Here is what I have so far:

JAVA Code:

 
package queens;
 
public class Queens {
 
 
    public static void main(String[] args) {
 
        final int SIZE = 8;
 
         QueenClass q = new QueenClass();
int t[] = q.solve();
 
 
// print solution 
for(int i = 0; i < SIZE; i++) {
for(int j = 0; j < SIZE; j++) {
if (j == t[i])
System.out.print(" Q ");
else
System.out.print(" _ ");
}
}
    }
 
}

And a class:

JAVA Code:

 
package queens;
 
public class QueenClass {
 
    public static final int SIZE = 8;
 
 
public int [] solve() {
int t[] = new int[SIZE];
if (nqueens(t, 0)) {
return t;
} else {
return null;
}
}
 
 
private boolean nqueens(int[] t, int i) {
for(t[i] = 0; t[i] < SIZE; t[i] += 1) {
if (empty(t,i)) {
if (i == (SIZE-1)) 
    return true;
}
else {
if (empty(t,i+1)) 
     return true;
}
}
return false;
}
 
 
private boolean empty(int [] t, int i) {
 
for(int j = 0; j < i; j++) {
if (t[j] == t[i])
return false;
if (Math.abs(t[j] - t[i]) == (i - j))
return false;
}
return true;
}
 
}

I'm getting no luck with a bunch of different errors every time i try to change something. Can someone please help?

TurboPT · #2 26-May-2008, 17:50

Quote:

Originally Posted by BKizzle77

I'm getting no luck with a bunch of different errors every time i try to change something. Can someone please help?

What are these "bunch of different errors"?

Post the errors that you get so that we can see where the problem(s) may be. It might be helpful to check this post, or this post for troubleshooting Java errors.

Also, with future Jave code, paste the codes between [java] [/java] tags.

BKizzle77 · #3 27-May-2008, 10:18

The error that i initially get before I change anything is

Exception in thread "main" java.lang.NullPointerException
at queens.Queens.main(Queens.java:17)

which refers to the print statement

fakepoo · #4 27-May-2008, 10:44

This would be my guess:

JAVA Code:

//for(t[i] = 0; t[i] < SIZE; t[i] += 1)
for(i = 0; i < SIZE; i += 1)

TurboPT · #5 27-May-2008, 12:39

Quote:

Originally Posted by BKizzle77

The error that i initially get before I change anything is

Exception in thread "main" java.lang.NullPointerException
at queens.Queens.main(Queens.java:17)

Ok, this tells us you have a runtime error, not a compile error... (see why "bunch of different errors" is vague?)

fakepoo has already made reference to the offending line.

Also, see this post for a stack dump trace example. (different situation, but they also had a "null pointer" problem)

BKizzle77 · #6 27-May-2008, 12:44

It still gives me the same error.

Is there a different way to output it just to see if the program is working...then I will worry about properly printing it out

fakepoo · #7 27-May-2008, 12:59

Probably what is happening is with this:

JAVA Code:

int t[] = q.solve();

because the solve() method can return null. If it does return null, you will get that runtime error when you call t[i].

BKizzle77 · #8 30-May-2008, 15:58

hey i reformatted the program to make it more basic, using an 8 X 8 array and such and am getting there but am still gettting a common error. Here is the code:

JAVA Code:

package eightQueens;
public class EightQueens {
 public static void main(String[] args) {
  
  //Create Board
  boolean [][] game = new boolean [8][8];
  
  //Create Queens
  int row = 0;
 
  //Place First Queen
  game[row][(int)Math.random() * 8] = true;
 
  //Place Second Queen
  row++;
  placeQueen(game, row);
  
  //Place Third Queen
  row++;
  placeQueen(game, row);
  
  //Place Fourth Queen
  row++;
  placeQueen(game, row);
  
  //Place Fifth Queen
  row++;
  placeQueen(game, row);
 
  //Place Sixth Queen
  row++;
  placeQueen(game, row);
  
  //Place Seventh Queen
  row++;
  placeQueen(game, row);
  
  //Place Eigth Queen
  row++;
  placeQueen(game, row);
  
  //Print Board
  for (int r = 0; r < game.length; r++){
   for (int c = 0; c < game[0].length; c++)
    System.out.print(game[r][c] + " ");
   System.out.println();
  }
 }
 
 public static void placeQueen(boolean[][] board, int row){
  int column = (int)(Math.random()*8);
  while(canBePlaced(board, row, column) == false){
   column = (int)(Math.random()*8);
  }
  board[row][column] = true;
 }
 
 public static boolean canBePlaced(boolean [][] board, int row, int column){
  
  for (int i = row - 1; i >= 0 ; i--){
   
   //Check Diagonal Up Right to Left
   int lDiag = column - 1;
   if(board[row][column] == board[i][lDiag] && lDiag <= 0)
    return false;
   lDiag--;
   
   //Check Diagonal Up Left to Right
   int rDiag = column + 1;
   if(board[row][column] == board[i][rDiag] && rDiag <= 7)
    return false;
   rDiag++;
   
   //Check Columns
   if(board[row][column] == board[i][column])
    return false;
  }
    
  return true;
 }
 
 
}

the error i keep getting even when i change up the code, its an index out of bounds error, usually from a -1 or an 8, and im not sure why:

Code:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1
	at queens.Queens.canBePlaced(Queens.java:81)
	at queens.Queens.placeQueen(Queens.java:69)
	at queens.Queens.main(Queens.java:17)

and

Code:


Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 8
	at queens.Queens.canBePlaced(Queens.java:87)
	at queens.Queens.placeQueen(Queens.java:69)
	at queens.Queens.main(Queens.java:17)

Hope you guys can help because im stumped

TurboPT · #9 30-May-2008, 16:33

The number with the 'out-of-bounds' is the index value that is out of range.

The array is specified 8, so that is the range 0 through 7, of course.

In both cases, though, it seems that the handling between functions 'placeQueen' and 'canBePlaced' is causing a problem.

Add some print functions to show the calculations of the row and column values as it proceeds -- that should lead to the problem.
Here's an example for the -1:

Code:

My syntax notations:
pq implies placeQueen, 
cbp implies canBePlaced.
r/c are row/column values
======================
pq, before while -- r:1, c:1
cbp loop: r:1, c:1
pq loop: 4
cbp loop: r:1, c:4
pq loop: 5
cbp loop: r:1, c:5
pq loop: 1
cbp loop: r:1, c:1
pq loop: 6
cbp loop: r:1, c:6
pq loop: 3
cbp loop: r:1, c:3
pq loop: 6
cbp loop: r:1, c:6
pq loop: 1
cbp loop: r:1, c:1
pq loop: 6
cbp loop: r:1, c:6
pq loop: 1
cbp loop: r:1, c:1
pq loop: 5
cbp loop: r:1, c:5
pq loop: 1
cbp loop: r:1, c:1
pq loop: 5
cbp loop: r:1, c:5
pq loop: 4
cbp loop: r:1, c:4
pq loop: 1
cbp loop: r:1, c:1
pq loop: 0
cbp loop: r:1, c:0  // hint: what first happens to 'column' in the loop?
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1
        at EightQueens.canBePlaced(EightQueens.java:65)
        at EightQueens.placeQueen(EightQueens.java:52)
        at EightQueens.main(EightQueens.java:15)

That's how to debug your programs without relying on error message(s) or guessing what/where the problem may be.

If you get stumped again, post another reply.

TurboPT · #10 31-May-2008, 08:22

Quote:

Originally Posted by me

That's how to debug your programs without relying on error message(s) or guessing what/where the problem may be.

Actually, that's two alternate ways to debug but not always easier or reliable.
There's also the good old pencil and paper [aka: by hand] method to "trace-out" or plan the logic, and it's usually better to start that way.

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