Setting a nested array in c++

I'm new to C++, and I have a question that should be easy, but it's making me crazy. I'm trying to set up a 2D array. The array is declared in Solver.h like so:

 protected:
    static const int gridSize = 9;
    int theGrid[gridSize][gridSize]
    int *boxes[gridSize][gridSize];
...

and I'm trying to initialize it in Solver::Solver() like so:

boxes[0] ={ &theGrid[0][0],&theGrid[0][1],&theGrid[0][2],
            &theGrid[1][0],&theGrid[1][1],&theGrid[1][2],
            &theGrid[2][0],&theGrid[2][1],&theGrid[2][2] 
          };
...

But the error I'm getting says "expression must be a modifiable lvalue" and "error: expected an expression". This didn't make sense to me, because I was under the impression that boxes[0] was a modifiable lvalue.

So I wrote a tiny little (non OOP) program that just has the following in it's main():

int test[2][2];
test[0]= {1,2};
cout<<test[0][1];   //outputs "2" as expected.

And now I'm stuck and confused. What is wrong with my assignment routine in the Solver class?

To answer some questions:

I do want a multidimensional array, because eventually I'll be setting up boxes[1] with another array of pointers to another set of data inside the theGrid, a third set in boxes[2] and so on. The idea is to take a 9x9 grid and split it into 3x3 squares (sound familiar? "Solver" should solve sudoku sets simply.) so I can check (and modify) all the values in those 3x3 squares independently. I'm trying to get this working in VS2010. I'm sure there's a good algorithm to define which array members are part of each 3x3 "box" based on that boxes number but I haven't hit it yet, and it seems less wasteful to code it once than to have the program re-create those boxes in loops every time it checks an answer. For the scope of this program the size of theGrid is inalterable开发者_如何学Go, so my conscience will let me get away with a little hard coding.

The problem is that { ... } is not an expression, it is an initializer. Some compilers have an extension to allow expressions to be formed using {}, and C++0x adds several new meanings to the braces, but I'm going to keep this to standard C++.

I think the best general solution is to code a loop. You don't really want to write all that redundancy out, right?

Solver::Solver() {
    for ( int boxnum = 0; boxnum < 9; ++ boxnum ) {
        for ( int cellnum = 0; cellnum < 9; ++ cellnum ) {
            boxes[ boxnum ][ cellnum ]
                = &grid[ boxnum/3*3 + cellnum/3 ][ boxnum%3*3 + cellnum%3 ];
        }
    }
}

Checked code… I believe this is the pattern you're going for.

I think it wants the lvalue to be:

&boxes[0][0] = ...

See that you have

 int *boxes[gridSize][gridSize];

Just make a test and change it to:

 int boxes[gridSize][gridSize];

You can do this for example (I'm using Visual C++ Console Application to test this):

 int boxes[gridSize][gridSize] = {
 {1, 3, 5},      /* y[0][0], y[0][1], y[0][2] */
 {2, 4, 6},      /* y[1][0], y[1][1], y[1][2] */
 {3, 5, 7}       /* y[2][0], y[2][1], y[2][2] */

};

Visual C++ doesn't accept this:

int test[2][2];
test[0]= { 1, 2 }; // error C2059: syntax error : '{'   
cout << test[0][1];

You declare boxes as boxes[9][9] so in order to initialize it, you would need to specify boxes[0][0]. Well at least initialize that memory block.

I may be mistaken but it looks like you didn't want to make boxes a multi-dimensional array.

I am not sure about the example code given by you works. Atleast my compiler is not able to digest it (gcc 4.2). The reason for that is {...} are used while initialization and what you are doing is assignment.

u can try this

for (unsigned i=0; i < gridSize; ++i) {
     for (unsigned j=0; j < gridSize; ++j) {
            boxes[i][j] = &theGrid[i][j];
      }
}

int main(){
   int buf[2][2];
   cout << typeid(buf[0]).name();
}

This pseudo program prints the equivalent of int [2]. This means that the type of buf[0] is an array of 2 integers.

In C++ an array is a non modifiable Lvalue. It can be initalized but not assigned to.

Your code attempts to assign to boxes[0] and this is ill-formed. The solution of course is as suggested by Potatoswatter and many of the above posts