Sudoku: checking 3x3 grids for repeating values

Question

I have worked for a sudoku puzzle in C but I'm stuck in one problem: Checking every 3x3 grid for not having duplicate values. Here is my code:

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
using namespace std; 
int v[10][10];
//Matrix start from 1 and end with 9
//So everywhere it should be i=1;i<=9 not from 0 to i<9 !!!
//Display function ( Display the results when it have)
void afisare()
{
    for(int i=1;i<=9;i++){
    for(int j=1;j<=9;j++)
    printf("%2d",v[i][j]);
    printf("\n");
}
    printf("\n");
}
//Function to check the valability of value
int valid(int k, int ii, int jj)
{
    int i;
    //Check for Row/Column duplicate
    for(i = 1; i <= 9; ++i) {
        if (i != ii && v[i][jj] == k)
            return 0;
        if (i != jj && v[ii][i] == k)
            return 0;
    }
    //Returns 0 if duplicate found return 1 if no duplicate found.
    return 1;
}

void bt()
{
    int i,j,k,ok=0;
    //Backtracking function recursive
    for(i=1;i<=9;i++){
    for(j=1;j<=9;j++)
    if(v[i][j]==0)
    {
        ok=1;
        break;
    }

    if(ok)
    break;
    }

    if(!ok)
    {
        afisare();
        return;
    }

    for(k=1;k<=9;k++)
    if(valid(k,i,j))
    {
        v[i][j]=k;
        bt();
    }
    v[i][j]=0;
}
int main()
{
    //Reading from the file the Matrix blank boxes with 0
    int i,j;
    freopen("sudoku.in","r",stdin);
    for(i=1;i<=9;i++)
    for(j=1;j<=9;j++)
    scanf("%d",&v[i][j]);

    bt();
    system("pause");
    return 0;
}

I know in function Valid I should have the condition to check every 3x3 grid but I don't figure it out: I found those solution to create some variables start and end and every variable get something like this:

start = i/3*3;
finnish = j/3*3;

i and j in my case are ii and jj.

For example found something like this:

  for (int row = (i / 3) * 3; row < (i / 3) * 3 + 3; row++)
    for (int col = (j / 3) * 3; col < (j / 3) * 3 + 3; col++)
      if (row != i && col != j && grid[row][col] == grid[i][j])
        return false;

I tryed this code and it doesn't work.

I don't understand this: I have the next matrix for sudoku:

1-1 1-2 1-3 1-4 1-5 1-6
2-1 2-2 2-3 2-4 2-5 2-6
3-1 3-2 3-3 3-4 3-5 3-6

If my code put's a value on 3-2 how he check in his grid for duplicate value, that formula may work for 1-1 or 3-3 but for middle values doesn't work, understand ?

If my program get's to 2-5 matrix value It should check if this value is duplicate with 1-4 1-5 1-6 2-4 2-6 ... untill 3-6.

Answer 1

Since you are using index arrays starting with 1 and not zero, you have to correct for that when calculating the sub-grid indexes.

start = (i - 1) / 3 * 3 + 1;
finish = (j - 1) / 3 * 3 + 1;