output

The row string is 396240001
The col string is 000003000
The box string is 396240001
The all string is 396240001000003000396240001
The row string is 39624�001
The col string is 003000080
The box string is 39624�001
The all string is 39624�00100300008039624�001
The row string is 39624�501
The col string is 009401530
The box string is 39624�501
The all string is 39624�50100940153039624�501
The row string is 9624�5711
The col string is 900030150
The box string is 39624�571
The all string is 9624�571190003015039624�571
The row string is 9624�5711
The col string is 604705093
The box string is 39624�571
The all string is 9624�571160470509339624�571
The row string is 9624�5711
The col string is 208900605
The box string is 39624�571
The all string is 9624�571120890060539624�571
The board string is 39624�571188060004504810390007950043030080000405023018010630059059070830003590007

sudoku.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>


// look into memcpy docs for stuff
void printString(char *string, char *descrip);

char *getRowValues(int index, char *board)
{
    int row = index / 9;
    char *rowValues = malloc(10 * sizeof(char));
    memcpy(rowValues, &board[row], 9);
    rowValues[9] = '\0';
    printString(rowValues, "row");

    return rowValues;

}

char *getColValues(int index, char *board)
{
    int col = index % 9;
    char *colValues = malloc(10 * sizeof(char));
    for(int i = 0; i < 9; i++) {
        int value = 9 * i + col;
        memcpy(colValues+i, &board[value], 1);
    }
    colValues[9] = '\0';
    printString(colValues, "col");

    return colValues;

}

char *getBoxValues(int index, char *board)
{
    int boxIndices[9][9] = {0, 1, 2, 9, 10, 11, 18, 19, 20, 3, 4, 5, 12, 13, 14, 21, 22, 23, 6, 7, 8, 15, 16, 17, 24, 25, 26, 27, 28, 29, 36, 37, 38, 45, 46, 47, 30, 31, 32, 39, 40, 41, 48, 49, 50, 33, 34, 35, 42, 43, 44, 51, 52, 53, 54, 55, 56, 63, 64, 65, 72, 73, 74, 57, 58, 59, 66, 67, 68, 75, 76, 77, 60, 61, 62, 69, 70, 71, 78, 79, 80};
    int boxNumber = ((( index / 9) / 3) * 3) + (( index % 9 ) / 3);
    char *boxValues = malloc(10 * sizeof(char));
    for(int i = 0; i < 9; i++) {
        int value = boxIndices[boxNumber][i];
        memcpy(boxValues+i, &board[i], 1);
    }
    printString(boxValues, "box");

    return boxValues;
}

void printString(char *string, char *descrip)
{
    printf("The %s string is %s\n",descrip, string);
}

void printInt(int zero)
{
    printf("The integer is %d\n", zero);
}

int getNextZero(char *board)
{
    int zero = strcspn(board, "0");

    return zero;
}

char *getValues(int index, char *board)
{
    char *all_values = malloc(28 * sizeof(char));
    char *row_values = getRowValues(index, board);
    char *col_values = getColValues(index, board);
    char *box_values = getBoxValues(index, board);

    strncpy(all_values, row_values, 9);
    strncat(all_values, col_values, 9);
    strncat(all_values, box_values, 9);
    all_values[27] = '\0';
    printString(all_values, "all");

    return all_values;
}

char *getPossibilities(int index, char *board)
{
    char *numbers = "123456789";
    char *possibilities = malloc(10 * sizeof(char));
    char *values = getValues(index, board);

    for(int i = 0; i < strlen(numbers); i++) {
        char *checkInValues = strchr(values, numbers[i]);

        if(checkInValues == NULL) {
            possibilities[strlen(possibilities)] = numbers[i];
        }
    }

    possibilities[strlen(possibilities)] = '\0';

    return possibilities;

}

char *solve(char *board)
{
    int zero = strcspn(board, "0");
    if(zero == 81) {
        return board;
    } else {
        char *possibilities = getPossibilities(zero, board);
        if(possibilities != '\0') {
            for(int i = 0; i < strlen(possibilities); i++) {
                char *new_string = malloc(strlen(board) + 1);
                memcpy(new_string, board, strlen(board) + 1);
                new_string[zero] = possibilities[i];
                return solve(new_string);
            }
        }
    }

}


int main(int argc, char *argv[])
{

    char *the_board = "396240001100060004504810390007950043030080000405023018010630059059070830003590007";

    // printString(getRowValues(4, the_board));
    // printString(getColValues(4, the_board));
    // printString(getBoxValues(4, the_board));

    // printf("Full values below\n");
    // printString(getPossibilities(4, the_board));
    // char * poss = getPossibilities(4, the_board);
    // printf("Possibilities length is %ld\n", strlen(poss));

    // printInt(getNextZero(the_board));
    printString(solve(the_board), "board");

    return 0;

}