N queens on a NxN chessboard - c

Question

#include <stdio.h>
#include <math.h>
int n=4;
int GetQueenSettings(int board[4][4],int currentRow,int n)
{
//decide when the recursion stops
if(currentRow==n)
return 1; //successful setting
//otherwise we set column by column in this row and continue
int TotalSettingCount=0;
for(int i=0;i<n;i++)
{
//make sure it can be set (it is unset at that moment)
if(board[currentRow][i]==0)
{
board[currentRow][i]==1+currentRow;
//use row related info for settings
//now set invalid positions for remaining rows
setInvalid(board,currentRow,n,i);
//recover after this before trying the next
TotalSettingCount += GetQueenSettings(board,currentRow+1,n);
board[currentRow][i]=0;
RecoverBoard(board,currentRow,n);
}
}
return TotalSettingCount;
}
void setInvalid(int board[4][4],int currentRow,int n,int i)
{
//vertical and diagonal elements
for(int row=currentRow+1;row<n;row++) //start from the next line
{
//firstly make sure board can be set
if(board[row][i]==0)//vertical position
board[row][i]=-(1+currentRow);
//now check diagonal
int rowGap=row-currentRow;
if(i-rowGap>=0 && board[row][i-rowGap]==0)
{
//left bottom diagonal position
board[row][i-rowGap]=-(1+currentRow);
}
if(i+rowGap<n && board[row][i+rowGap]==0)
{
//bottom right diagonal position
board[row][i+rowGap]=-(1+currentRow);
}
}
}
void RecoverBoard(int board[4][4],int currentRow,int n)
{
//recover is to check all remaining rows if index is higher than current row(setters)
//OR less than -currentRow(invalids)!
for(int row=currentRow+1;row<n;row++)
{
for(int col=0;col<n;col++)
{
if(board[row][col]>currentRow || board[row][col]< -currentRow)
board[row][col]=0;
}
}
}
int main()
{
int board[n][n];
printf("Number of settings:-> %d",GetQueenSettings(board,0,n));
return 0;
}
There are N queeens placed on a NxN chessboard without interfering with each other. when i run this code i get the answer as zero instead of 2 . also i cant figure out a way of passing array board to functions with variable size(size will be given by user).What am i doing wrong?!

Answer 1

You should initialize your board. As is, you start with a board full of garbage values. You are using a variable-length array as board. Such arrays cannot be initialized, so you have to set the board to all zero with a loop or with memset from <string.h>:
int board[n][n];
memset(board, 0, sizeof(board));
You can pass variable-length arrays to functions when the dimensions are passed in as earlier arguments, for example:
int GetQueenSettings(int n, int board[n][n], int currentRow) { ... }
Also fix the =/== switch in setInvalid:
if (board[row][i] == 0)
board[row][i] = -(1 + currentRow);
Finally make sure that all functions have proper prototypes when they are called.

Answer 2

A loooooooong time ago I developed an algorithm like the one you have, maybe it can help you.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
long Solutions;
void AllocBoard(int *** Board, int Queens)
{
int i;
*Board = (int **)malloc(sizeof(int *) * Queens);
for(i = 0; i < Queens; i++)
{
(*Board)[i] = (int *)malloc(sizeof(int) * Queens);
memset((*Board)[i], 0, sizeof(int) * Queens);
}
}
void DeallocBoard(int *** Board, int Queens)
{
int i;
for(i = 0; i < Queens; i++)
free((*Board)[i]);
free(*Board);
}
void SavePosition(int *** Board, int Queens, int Col, int Row, int Inc)
{
int i, j;
for(i = 0; i < Queens; i++)
{
if((*Board)[Col][i] >= 0) (*Board)[Col][i] += Inc;
if((*Board)[i][Row] >= 0) (*Board)[i][Row] += Inc;
}
for(i = Col, j = Row; j < Queens && i < Queens; i++, j++)
if((*Board)[i][j] >= 0) (*Board)[i][j] += Inc;
for(i = Col, j = Row; j >= 0 && i >= Col; i--, j--)
if((*Board)[i][j] >= 0) (*Board)[Col][j] += Inc;
for(i = Col, j = Row; j >= 0 && i < Queens; i++, j--)
if((*Board)[i][j] >= 0) (*Board)[i][j] += Inc;
for(i = Col, j = Row; j < Queens && i >= Col; i--, j++)
if((*Board)[i][j] >= 0) (*Board)[i][j] += Inc;
}
void FindSolutions(int *** Board, int Queens, int Col)
{
int i, j;
for(i = 0; i < Queens; i++)
{
if((*Board)[Col][i] != 0) continue;
(*Board)[Col][i] = -1;
if(Col + 1 == Queens)
Solutions++;
else
{
SavePosition(Board, Queens, Col, i, 1);
FindSolutions(Board, Queens, Col + 1);
SavePosition(Board, Queens, Col, i, -1);
}
(*Board)[Col][i] = 0;
}
}
void main(int argc, char **argv)
{
int Queens, ** Board = NULL;
clock_t Start, End;
clrscr();
if(argc < 2)
Queens = 8;
else
Queens = atoi(argv[1]);
Solutions = 0;
Start = clock();
AllocBoard(&Board, Queens);
FindSolutions(&Board, Queens, 0);
DeallocBoard(&Board, Queens);
End = clock();
printf("Solutions %ld\n", Solutions);
printf("Estimated time: %f", (End - Start) / CLK_TCK);
getch();
}
Hope this helps

Answer 3

Asking "why it doesn't work" about short and simple programs is not very well. Try debug it yourself with any debugger or simply using printing in important places/on each step. Actually, C has no arrays, instead it has pointers. You can work with them very alike, that is why all you need is to change int board[4][4] to int **board and add one argument: int N(count of cells) and that's all. All your other code shouldn't be changed.