C sudoku backtracking solver precheck function - c

Question

This is my first post on StackOverflow, so I apologize if I'm doing something wrong. I'm relatively new to C, so I'm sure my code is fairly ugly and hacky in places, however the majority of the code does what I expect it to. I'm having trouble with a precheck method that I'm using to check a sudoku board before I begin feeding it through my solver-logic. I'm redirecting input from a text file with strings that look like
4.....8.5.34.........7......2.....6.....8.4......1.......6.3.7.5..2.....1.4......
4.....8.5.3..........7......2.....6.....8.4......1.......6.3.7.5..2.....1.5......
4.....8.5.3..........7......2.....6.....8.4......1...x...6.3.7.5..2.....1.4......
417369825632158947958724316825437169791586432346912758289643571573291684164875293
417369825632158947958724316825437169791586432346912758289643.71573291684164875293
Each string is (ideally) 81 characters with just digits 1-9 and '.'s. I parse a string into a temp char array, and then use the method fillBoard to transfer the chars in the temp array into a 2d int array. Once this is complete, I call my precheck method. If the filled board doesn't pass the row, column, and box checks, the precheck method returns a one, indicating that the puzzle is not solvable (meaning an error message should be printed and that the program should move on to the next string). For some reason, my precheck method is returning one even for strings that should be solvable. I'm not sure why this is. Any help would be appreciated. Thanks.
#include <stdio.h>
#include <string.h>
int alphaError = 0;
struct Point findEmpty(int board[9][9]);
int usedInBox(int board[9][9], int boxStartRow, int boxStartCol, int num);
int positionSafe(int board[9][9], int row, int col, int num);
int usedInCol(int board[9][9], int col, int num);
int usedInRow(int board[9][9], int row, int num);
int solvePuzzle(int board[9][9]);
int precheck(int board[9][9]);
int main()
{
char c;
int charCount = 0;
int i = 0;
char tempStr[100000];
int board[9][9];
while((fscanf(stdin, "%c", &c)) != EOF)
{
printf("%c", c);
if(c != '\n')
{
if(isalpha(c))
{
alphaError = 1;
}
tempStr[i] = c;
i++;
charCount++;
}
else
{
if(charCount != 81 || alphaError == 1)
{
printf("Error\n\n");
i = 0;
charCount = 0;
alphaError = 0;
}
else
{
fillBoard(board, tempStr);
printBoard(board);
if(precheck(board) == 1)
{
printf("Error\n\n");
}
else
{
if(solvePuzzle(board) == 1)
{
printBoard(board);
}
else
{
printf("No solution\n\n");
}
}
i = 0;
charCount = 0;
}
}
}
return 0;
}
struct Point
{
int x;
int y;
} point;
struct Point findEmpty(int board[9][9])
{
struct Point point1;
point1.x = -1;
point1.y = -1;
int row, col;
for(row = 0; row < 9; row++)
{
for(col = 0; col < 9; col++)
{
if(board[row][col] == 0)
{
point1.x = col;
point1.y = row;
}
}
}
return point1;
}
int usedInBox(int board[9][9], int boxStartRow, int boxStartCol, int num)
{
int row, col;
for(row = 0; row < 3; row++)
{
for(col = 0; col < 3; col++)
{
if(board[row + boxStartRow][col + boxStartCol] == num)
{
return 1;
}
}
}
return 0;
}
int positionSafe(int board[9][9], int row, int col, int num)
{
if((usedInRow(board, row, num)) == 0 && (usedInCol(board, col, num)) == 0 && (usedInBox(board, (row-row%3), (col-col%3), num)) == 0)
{
return 1;
}
else
{
return 0;
}
}
int usedInCol(int board[9][9], int col, int num)
{
int row;
for(row = 0; row < 9; row++)
{
if(board[row][col] == num)
{
return 1;
}
}
return 0;
}
int usedInRow(int board[9][9], int row, int num)
{
int col;
for(col = 0; col < 9; col++)
{
if(board[row][col] == num)
{
return 1;
}
}
return 0;
}
int solvePuzzle(int board[9][9])
{
int num;
struct Point point2;
point2 = findEmpty(board);
if(point2.x == -1)
{
return 1;
}
for(num = 1; num <= 9; num++)
{
if(positionSafe(board, point2.y, point2.x, num) == 1)
{
board[point2.y][point2.x] = num;
if(solvePuzzle(board) == 1)
{
return 1;
}
board[point2.y][point2.x] = 0;
}
}
return 0;
}
void printBoard(int board[9][9])
{
int i, j;
for (i = 0; i < 9; i++)
{
for (j = 0; j < 9; j++)
{
printf("%d", board[i][j]);
}
}
printf("\n\n");
}
void fillBoard(int board[9][9], char tempStr[100000])
{
int i, j;
int k = 0;
for(i = 0; i < 9; i++)
{
for(j = 0; j < 9; j++)
{
if(tempStr[k] == '.')
{
board[i][j] = 0;
}
else
{
board[i][j] = (tempStr[k] - '0');
}
k++;
}
}
}
int precheck(int board[9][9])
{
int i, j, num;
for(i = 0; i < 9; i++)
{
for(j = 0; j < 9; j++)
{
if(board[i][j] != 0)
{
num = board[i][j];
if(positionSafe(board, i, j, num) == 0)
{
return 1;
}
}
}
}
return 0;
}

Answer 1

So you are using precheck on an already filled board? That might be the problem because usedInCol, usedInRow and usedInBlock will return 1 if the value is already present. So precheck should be used only while filling the board, not after. It will always return 1 if you check values you take from the already filled board.