Solving Eight Queens problem with 2-d array: IndexOutOfBounds error
One of my homework assignments is to solve the Eight Queens problem using a two dimensional array to represent the board. I keep getting an index out of bounds error:8 in my "isUnderAttack" method at:
if (board[row][j] == QUEEN)
and in my "placeQueen" method at:
if (isUnderAttack(row, column)) {
and
queenPlaced = placeQueens(column+1);
Where might I have gone wrong? I would have added more tags to this post but I'm a new user and I can't create "new tags". Sorry!
Here is what I've created:
public class Queens {
// squares per row or column
public static final int BOARD_SIZE = 8;
// used to indicate an empty square
public static final int EMPTY = 0;
// used to indicate square contains a queen
public static final int QUEEN = 1;
private int board[][]; // chess board
public Queens() {
// -------------------------------------------------
// Constructor: Creates an empty square board.
// -------------------------------------------------
board = new int[BOARD_SIZE][BOARD_SIZE];
} // end constructor
public void clearBoard() {
// -------------------------------------------------
// Clears the board.
// Precondition: None.
// Postcondition: Sets all squares to EMPTY.
// -------------------------------------------------
//loops through rows
for (int i = 0; i < BOARD_SIZE; i++){
//loops through columns
for (int j = 0; j <BOARD_SIZE; j++){
board[i][j] = EMPTY;
}
}
} // end clearBoard
public void displayBoard() {
// -------------------------------------------------
// Displays the board.
// Precondition: None.
// Postcondition: Board is written to standard
// output; zero is an EMPTY square, one is a square
// containing a queen (QUEEN).
// -------------------------------------------------
for (int i = 0; i < BOARD_SIZE; i++){
System.out.println("");
for (int j = 0; j <BOARD_SIZE; j++){
System.out.print(board [i][j] + " ");
}
}
} // end displayBoard
public boolean placeQueens(int column) {
// -------------------------------------------------
// Places queens in columns of the board beginning
// at the column specified.
// Precondition: Queens are placed correctly in
// columns 1 through column-1.
// Postcondition: If a solution is found, each
// column of the board contains one queen and method
// returns true; otherwise, returns false (no
// solution exists for a queen anywhere in column
// specified).
// -------------------------------------------------
if (column > BOARD_SIZE) {
return true; // base case
}
else {
boolean queenPlaced = false;
int row = 1; // number of square in column
while ( !queenPlaced && (row <= BOARD_SIZE) ) {
// if square can be attacked
if (isUnderAttack(row, column)) {
++row; // consider next square in column
} // end if
else { // place queen and consider next column
setQueen(row, column);
queenPlaced = placeQueens(column+1);
// if no queen is possible in next column,
if (!queenPlaced) {
// backtrack: remove queen placed earlier
// and try next square in column
removeQueen(row, column);
++row;
} // end if
} // end if
} // end while
return queenPlaced;
} // end if
} // end placeQueens
private void setQueen(int row, int column) {
// --------------------------------------------------
// Sets a queen at square indicated by row and
// column.
// Precondition: None.
// Postcondition: Sets the square on the board in a
// given row and column to QUEEN.
// --------------------------------------------------
board [row][column] = QUEEN;
} // end setQueen
private void removeQueen(int row, int column) {
// --------------------------------------------------
// Removes a queen at square indicated by row and
// column.
// Precondition: None.
// Postcondition: Sets the square on the board in a
// given row and column to EMPTY.
// --------------------------------------------------
board [row][column] = EMPTY;
} // end removeQueen
private boolean isUnderAttack(int row, int column) {
// --------------------------------------------------
// Determines whether the square on the board at a
// given row and column is under attack by any queens
// in the columns 1 through column-1.
// Precondition: Each column between 1 and column-1
// has a queen placed in a square at a specific row.
// None of these queens can be attacked by any other
// queen.
// Postcondition: If the designated square is under
// attack, returns true; otherwise, returns false.
// --------------------------------------------------
//i = rows and j = columns
//----------------------------------------
//
// Checks before and after square
//
//----------------------------------------
//looks behind for queens
for (int j = 0; j < column; j++){
if (board[row][j] == QUEEN)
return true;
}
//looks in front for queens
for (int j =column+1; j < BOARD_SIZE; j++){
if(board[row][j] == QUEEN)
return true;
}
//-----------------------------------------------------------------
//
// Checks diagonally from square
//
//-----------------------------------------------------------------
for (int i = 1+row, j = 1+column; i < BOARD_SIZE && j < BOARD_SIZE; i++, j++){
if (board[i][j] == QUEEN)
return true;
}
for (int i= 1-row, j = 1-column; i >= 0 && j >=0; i--, j--){
if (board[i][j] == QUEEN)
return true;
}
for (int i= 1+row, j = 1-column; i < BOARD_SIZE && j >= 0; i++, j--){
if (board[i][j] == QUEEN)
return true;
}
for (int i= 1-row, j = 1+column; i >= 0 && j < BOARD_SIZE; i--, j++){
if (board[i][j] == QUEEN)
return true;
}
return false;
} // end isUnderAttack
private int index(int number) {
// --------------------------------------------------
// Returns the array index that corresponds to
// a row or column number.
// Precondition: 1 <= number <= BOARD_SIZE.
// Postcondition: Returns adjusted index value.
// --------------------------------------------------
return number -1;
} // end index
} // end Queens
Answer
Change this line:
while ( !queenPlaced && (row <= BOARD_SIZE) ) {
to:
while ( !queenPlaced && (row < BOARD_SIZE) ) {
because BOARD_SIZE is defined to 8, which is the array size (8x8), the isUnderAttack() function gets out of bounds when it tries to access board[8][j]. It can access rows from 0-7, not 8.
Also, you may need to change the line that is just above that one (int row = 1;) to set row to 0 instead of one (assumong you want to start from the 1st row).