What is wrong with my sudoku generate code in c? - c

Question

That's my homework, making a sudoku game. I have done my algorithm but it's entering infinite loop. I didn't understand why.
I am trying create a random number and control it for find true number. Checking all columns and rows for find same number as like as our random number if it is, it's changing test number and if test has changed trying find another number for true number. Simple sudoku logic.
#include <stdio.h>
#include <stdlib.h>
int main() {
srand(time(NULL));
int num, col, row, row2, col2, test = 0;
int sudo[9][9] = {{0,0,0,0,0,0,0,0,0},{0,0,0,0,0,0,0,0,0},{0,0,0,0,0,0,0,0,0},{0,0,0,0,0,0,0,0,0},{0,0,0,0,0,0,0,0,0},{0,0,0,0,0,0,0,0,0},{0,0,0,0,0,0,0,0,0},{0,0,0,0,0,0,0,0,0},{0,0,0,0,0,0,0,0,0}};
for (row = 0; row <= 8; row++) {
for (col = 0; col <= 8; col++) {
do {
test = 0;
num = rand() % 9 + 1;
//control
for (col2 = 0; col2 <= 8; col2++) {
if (num == sudo[col2][row]) {
test++;
}
}
for (row2 = 0; row2 <= 8; row2++) {
if (num == sudo[col][row2]) {
test++;
}
}
} while (test > 0);
sudo[col][row] = num;
}
}
//print
for (row = 0; row <= 8; row++) {
for (col = 0; col <= 8; col++) {
printf(" %d ", sudo[col][row]);
if (col == 2 || col == 5) {
printf(" | ");
}
}
if (row == 2 || row == 5) {
printf("\n---------------------------------");
}
printf("\n");
}
}

Answer 1

Your algorithm is broken, and I can demonstrate why. If it were possible to fill in a sudoku puzzle this way, it would also be trivial to solve a sudoku puzzle this way, which it is not.
Essentially your code boils down to the following. I've added early exits on the inner for-loops to stop searching once we find the number already in the current row or column (and actually made sense of what 99.9% of the world thinks of concerning "rows" and "columns" in a NxN matrix):
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define NSIZE 9
void print_matrix(int const ar[][NSIZE])
{
for (size_t i=0; i<NSIZE; ++i)
{
for (size_t j=0; j<NSIZE; ++j)
{
fputc('0' + ar[i][j], stdout);
fputc(' ', stdout);
}
fputc('\n', stdout);
}
}
int main()
{
srand((unsigned)time(NULL));
int sudo[NSIZE][NSIZE] = {{0}};
int row, col;
for(row=0;row<NSIZE;++row)
{
for(col=0;col<NSIZE;++col)
{
int row2 = 0, col2 = 0, num;
printf("Trying ");
do
{
num = rand()%9+1;
printf("%d ", num);
for(row2=0; row2<NSIZE && num!=sudo[row2][col]; ++row2);
for(col2=0; col2<NSIZE && num!=sudo[row][col2]; ++col2);
}
while (row2 < NSIZE || col2 < NSIZE);
fputc('\n', stdout);
sudo[row][col] = num;
printf("sudo[%d][%d] = %d\n", row, col, num);
print_matrix(sudo);
}
}
}
As the loops progress, we report what number we're trying, and what the matrix looks like upon placement of a keeper. For example, a test run of the above initially can look like this:
Trying 8
sudo[0][0] = 8
8 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
Trying 1
sudo[0][1] = 1
8 1 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
Trying 9
sudo[0][2] = 9
8 1 9 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
Trying 6
sudo[0][3] = 6
8 1 9 6 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
Trying 3
sudo[0][4] = 3
8 1 9 6 3 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
Trying 4
sudo[0][5] = 4
8 1 9 6 3 4 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
Trying 4 6 7
sudo[0][6] = 7
8 1 9 6 3 4 7 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
Trying 1 3 1 3 4 1 3 8 4 9 3 8 1 4 7 9 3 8 8 8 4 9 6 5
sudo[0][7] = 5
8 1 9 6 3 4 7 5 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
and this continues for perhaps a while. But eventually, unless you get extraordinarily lucky, the following is bound to happen (and this one went pretty deep before the wheels fell off):
Trying 1 6 3 4
sudo[6][6] = 4
8 1 9 6 3 4 7 5 2
1 3 5 4 8 6 2 7 9
3 6 4 8 7 9 5 2 1
7 9 1 2 4 5 3 8 6
4 7 3 9 2 8 6 1 5
5 4 2 3 6 1 8 9 7
6 8 7 1 9 3 4 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
Note we're about to try and populate sudo[6][7]. To do that we must find a number that is not in the sudo[r][7] column already, nor the sudo[6][c] row. But looking at the numbers already in those positions.
sudo[r][7] : {5,7,2,8,1,9}
sudo[6][c] : {6,8,7,1,9,3,4}
Therefore we're looking for a number from 1..9 that is NOT in: {1,2,3,4,5,6,7,8,9}, which we're NEVER going to find.
The algorithm is broken. There is a reason backtracking is used for tasks like this.