Strange pass by reference issue in a simple program - c++

Question

I am implementing Sudoku solver and using 2D vector and passing it around using reference but still, when at the end of the main I try to print the 2D vector it prints the initial 2D vector.
#include <iostream>
#include <vector>
#include <map>
using namespace std;
void display(vector<vector<int>>& _board) {
for (auto row: _board) {
for (auto col: row) {
cout << col << " ";
}
cout << endl;
}
}
bool isBoardSolved(vector<vector<int>>& _board) {
for (auto row: _board) {
for (auto col: row) { //style[1] of 2D vector traversal
if (col == 0) {
return false;
}
}
}
return true;
}
map<int, int> findOptions(vector<vector<int>>& _board, int _row, int _col) {
map<int, int> options;
int x, y;
for (int digit = 1; digit < 10; ++digit) {
options[digit] = 0; //state 0 means available as options
}
//col in a row
for (y = 0; y < 9; ++y) {
if (_board[_row][y] != 0) {
options[_board[_row][y]] = 1;
}
}
//row in a col
for (x = 0; x < 9; ++x) {
if (_board[x][_col] != 0) {
options[_board[x][_col]] = 1;
}
}
//in a rectangular 3*3 matrix
if (_row <= 2)
x = 0;
else if (_row > 2 && _row <= 5)
x = 3;
else
x = 6;
if (_col <= 2)
y = 0;
else if (_col > 2 && _col <= 5)
y = 3;
else
y = 6;
for (int i = x; i < x + 3; ++i) {
for (int j = y; j < y + 3; ++j) {
if (_board[i][j] != 0) {
options[_board[i][j]] = 1;
}
}
}
return options;
}
void solveBoard(vector<vector<int>>& _board) {
int row = 0;
int col = 0;
bool flag = false;
if (isBoardSolved(_board)) {
// cout << "Solved Sudoku Board" << endl;
// display(_board); //gives correct answer when I print it here
return;
}
else {
for (int x = 0; x < 9; ++x) { //not using the style[1] because I need explicit index of empty slot
flag = false;
for (int y = 0; y < 9; ++y) {
if (_board[x][y] == 0) {
row = x;
col = y;
flag = true;
break;
}
}
if (flag)
break;
}
}
auto options = findOptions(_board, row, col);
for (auto digit: options) {
if (digit.second != 1) {
_board[row][col] = digit.first;
solveBoard(_board);
}
}
_board[row][col] = 0;
}
int main(int argc, char *argv[]) {
vector<vector<int>> board(9, vector<int>(9, 0));
board[0][3] = 3;
board[0][6] = 2;
board[2][1] = 7;
board[2][2] = 8;
board[2][3] = 0;
board[2][4] = 6;
board[2][6] = 3;
board[2][7] = 4;
board[3][1] = 4;
board[3][2] = 2;
board[3][3] = 5;
board[3][4] = 1;
board[4][0] = 1;
board[4][1] = 0;
board[4][2] = 6;
board[4][6] = 4;
board[4][7] = 0;
board[4][8] = 9;
board[5][4] = 8;
board[5][5] = 6;
board[5][6] = 1;
board[5][7] = 5;
board[6][1] = 3;
board[6][2] = 5;
board[6][4] = 9;
board[6][6] = 7;
board[6][7] = 6;
board[7][3] = 7;
board[8][2] = 9;
board[8][5] = 5;
cout << "Given Sudoku Board" << endl;
display(board);
solveBoard(board);
cout << "Solved Sudoku Board" << endl;
display(board); //gives unchanged answer when i print it here
}
What is wrong I am doing and how to correct it.
When I try this :
void change(vector<vector<int>>& _b){
_b[0][1] = 99;
}
int main(){
vector<vector<int>> b(1, vector<int>(9, 1));
cout<<b[0][1]<<endl;
change(b);
cout<<b[0][1];
return 0;
}
This displays the changed value for 2D vecor b.

Answer 1

You are not exiting the recursion call stack correctly in solveBoard(). Note the new bool function signature for solveBoard() being returned to signal up the invocation chain to exit early. Also note the three different return points now depending on where you are.
bool solveBoard(vector<vector<int>>& _board) {
int row = 0;
int col = 0;
bool flag;
if (isBoardSolved(_board)) {
// cout << "Solved Sudoku Board" << endl;
// display(_board); //gives correct answer when I print it here
return true;
}
else {
for (int x = 0; x < 9; ++x) { //not using the style[1] because I need explicit index of empty slot
flag = false;
for (int y = 0; y < 9; ++y) {
if (_board[x][y] == 0) {
row = x;
col = y;
flag = true;
break;
}
}
if (flag)
break;
}
}
auto options = findOptions(_board, row, col);
for (auto digit : options) {
if (digit.second != 1) {
_board[row][col] = digit.first;
if (solveBoard(_board))
return true;
}
}
_board[row][col] = 0;
return false;
}