Multidimensional variable size array in C++

Question

hi I want to do something like this:

int op(string s1, string s2){
    int x = s1.size();
    int y = s2.size();
    int matrix = new int[x][y]
    /* do stuff with matrix */
}

For some reason I get the following errors:

SuperString.cpp(69) : error C2540: non-constant expression as array bound
SuperString.cpp(69) : error C2440: 'initializing' : cannot convert from 'int (*)[1]' to 'int'
        This conversion requires a reinterpret_cast, a C-style cast or function-style cast
SuperString.cpp(71) : error C2109: subscript requires array or pointer type

Thanks!

Answer 1

You need to declare the matrix var as int* matrix, as a dynamic array is declared as a pointer. But you can't do a 2d array in one new with both dimensions being variable. You can do a 1D array and do the indexing math on your own.
int* matrix = new int[x*y];
// Set element x1,y1 to 5
matrix[x1+y1*x] = 5;

Answer 2

You cannot have a matrix of non-constant row size.

You may choose to have an "array of pointers to arrays" structure which can be indexed as pp[a][b] just as a matrix. You cannot allocate such a structure with a single new. You will have to build it manually within a buffer.

Answer 3

Here is a summary of how to build a 2d array in C++ using various techniques.

Static 2D Matrix:

const size_t N = 25; // the dimension of the matrix

int matrix[N][N]; // N must be known at compile-time.
// you can't change the size of N afterwards

for(size_t i = 0; i < N; ++i)
{
    for(size_t j = 0; j < N; ++j)
    {
     matrix[i][j] = /* random value! */;
    }
}

Dynamic 2d Matrix:

const size_t N = 25; // the dimension of the matrix
int** matrix = new int*[N]; // each element is a pointer to an array.

for(size_t i = 0; i < N; ++i)
    matrix[i] = new int[N]; // build rows

for(size_t i = 0; i < N; ++i)
{
    for(size_t j = 0; j < N; ++j)
    {
     matrix[i][j] = /* random value! */;
    }
}

// DON'T FORGET TO DELETE THE MATRIX!
for(size_t i = 0; i < N; ++i)
    delete matrix[i];

delete matrix;

Matrix using std::vector:

// Note: This has some additional overhead
// This overhead would be eliminated once C++0x becomes main-stream ;)
// I am talking about r-value references specifically.
typedef vector< vector<int> > Matrix;
typedef vector<int> Row;

const size_t N = 25; // the dimension of the matrix
Matrix matrix(N);

for(size_t i = 0; i < N; ++i)
{
    Row row(N);

    for(size_t j = 0; j < N; ++j)
    {
     row[j] = /* random value! */;
    }

    matrix.push_back(row); // push each row after you fill it
}

// Once you fill the matrix, you can use it like native arrays
for(size_t i = 0; i < N; ++i)
{
    for(size_t j = 0; j < N; ++j)
    {
     cout << matrix[i][j] << " ";
    }

    cout << endl;
}

3d matrix using boost::multi_array (taken from boost multi_array docs):

// Note that this is much more efficient than using std::vector!
int 
main () {
  // Create a 3D array that is 3 x 4 x 2
  typedef boost::multi_array<double, 3> array_type;
  typedef array_type::index index;
  array_type A(boost::extents[3][4][2]);

  // Assign values to the elements
  int values = 0;
  for(index i = 0; i != 3; ++i) 
    for(index j = 0; j != 4; ++j)
      for(index k = 0; k != 2; ++k)
        A[i][j][k] = values++;

  // Verify values
  int verify = 0;
  for(index i = 0; i != 3; ++i) 
    for(index j = 0; j != 4; ++j)
      for(index k = 0; k != 2; ++k)
        assert(A[i][j][k] == verify++);

  return 0;
}

Answer 4

Use boost::multi_array. See the doc and this question for details.

That will help you avoid a lot of errors.

Answer 5

If the size of matrix does not need to change through the function, you can declare the ints storing the string length as const. This allows you to create a multi-dimensional array that can vary in size for each function call, but retains a constant size for the duration of the function.

#include <iostream>
#include <string>

using namespace std;

int someFunc(string, string);

int someFunc(string s1, string s2)
{
    const int x = s1.length();
    const int y = s2.length();

    int matrix[x][y];
    int result=0;

    for(int i=0;i<x;i++)
     for(int j=0;j<y;j++)
      matrix[i][j]=i*j;

    for(int i=0;i<x;i++)
     for(int j=0;j<y;j++)
      result+=matrix[i][j];

    return result;
}

int main()
{
    string s1 = "fubar";
    string s2 = "somethingelse";

    cout<<someFunc(s1,s2)<<endl;
}

EDIT: On reading one of the other answers posted while I was writing mine, I suppose you should use const size_t instead of const int. Sorry, my C++ is just a little rusty.

Answer 6

It's more like a syntax problem.

Last check in gcc 4.4, int matrix[x][y]; seems to work as expected. If your array don't need resizing in the middle of the function. You may try this syntax and see if it works in your compiler.