Question

I’m creating a sudoku generator, using a ‘brute-force’ randomity approach. I have been able to check the x / y axis for duplicate numbers just fine using the code:

for(l=0; l<9; l++){//Makes all vertical work.
                   if(sudoku[l][j] == temp){
                       isUsed=true;
                   }
                }                  
                for(m=0; m<9; m++){//makes all horizontal work
                   if(sudoku[i][m] == temp){
                       isUsed=true;
                   }
                }

I decided to implement the ‘box’ or ‘region’ checking (where you check every 3×3 square from the origin) and I just can’t seem to wrap my head around the code. Here’s what I’ve done so far. I just can’t quite figure out where my logic error lies (for the record the program will run with this code, but will not check regions properly)

rowbase = i-(i%3);
                if(i==2  || i==5 || i==8 ){
                    if(rowbase == 0 || rowbase == 3 || rowbase == 6){
                       isUsed= RegionCheck.RegCheck(rowbase, sudoku);
                    }
                }

Contents of regionCheck.java:

       boolean okay = false;
    int[] regionUsed = new int[9];
    int i=0, j=0, regionTester=0, counter=0, numcount;
    for (i=regionTester; i<regionTester+3; i++){
        for (; j<3; j++){
           regionUsed[counter]=sudoku[i][j];
           counter++;
        }
    }
    for(i=0; i<9; i++){
        numcount=regionUsed[i];
        for(j=0; j<9; j++){
            if(j==i){
                //null
            }
            else if(numcount == regionUsed[j]){
                okay=false;
            }
        }
    }

    return okay;

Somewhere along the way I’m just getting lost and not understanding how to ‘select’ a region and iterate through regions.

Full source here: http://ideone.com/FYLwm

Any help on simply how to ‘select’ a region for testing and then iterate through it would be greatly appreciated as I’m really out of ideas.

Answer 1

I cannot understand what you mean testing a region,
I assume test means each region has every number from 1 to 9 without duplication.

We can make may implementations:

1. use for loops for vertical, horizontal, and 3×3 regions.
2. use table which have location to each region, including horizontal or vertical one.

My recommend is 2nd one.
If you have a table, the loop which access regions is implemented once.
On the other hand, 1st one requires to implement 3 similar loops for vertical/horizontal/3×3.

Here is a code to generate the table:

for(int i=0, k=0; i<9; i++) {
    // generate vertical regions
    for(int j=0; j<9; j++)
        table[k][j] = new table_t(i, j);
    k++;
    // generate horizontal regions
    for(int j=0; j<9; j++)
        table[k][j] = new table_t(j, i);
    k++;
    // generate 3x3 regions
    for(int j=0; j<9; j++)
        table[k][j] = new table_t((i/3)*3+j/3, (i%3)*3+j%3);
    k++;
}

The code to generate vertical or horizontal regions is easy to read.
Though 3×3 region generation should be described.
The variable i is taken from 0 to 8, ((i/3)*3, (i%3)*3) points the corner of a 3×3 region each. And (+j/3, +j%3) moves each box in the region.

And you can test the matrix sudoku comply or not by following code:

boolean okey = true;
for(int i=0; i<27; i++) {
    int [] counter = new int[10];
    for(int j=0; i<10; i++)
        counter[i]=0;
    for(int j=0; j<9; j++)
        counter[ sudoku[table[i][j].x][table[i][j].y] ] ++;
    boolean ok = true;
    for(int j=0; j<9; j++)
        if(counter[j+1]!=1)
            ok = false;
    if(!ok)
        okey = false;
}

The array counter counts number of appearance for each digit
(I assume 0 is some special meaning and digits between 1 and 9 is valid).