Passing 2D array by reference

[HazySmoke)345]

Well, yes. I know how to pass a 1D array by reference. I can just use a pointer for the first element and I'm still able to refer to its elements by using the [] symbol.

Now... how do you pass 2D arrays? I tried to have two stars before the variable names and I tried to use its elements with square brackets, like this.

code:

void someFunction(int** variable)   { variable[3][5] = 1; }

And the program crashed.

[wtd]

Was the array you passed in properly allocated?

[OneOffDriveByPoster]

HazySmoke)345 wrote:

Well, yes. I know how to pass a 1D array by reference. I can just use a pointer for the first element and I'm still able to refer to its elements by using the [] symbol.

Now... how do you pass 2D arrays? I tried to have two stars before the variable names and I tried to use its elements with square brackets, like this.

code:

void someFunction(int** variable)   { variable[3][5] = 1; }

And the program crashed.

Use int **var with dynamically allocated 2-D arrays (where it is a pointer-to pointer-to-int).
I suspect the 2-D array you are passing is declared like so: int v[X][Y]; (something else).

Properly-allocated multidimenional arrays of both types will give unexpected results if you pass them wrong.

[wtd]

Problems with multi-dimensional arrays will eventually teach you to love code like the following.

code:

#include <iostream> #include <sstream> using namespace std; template <typename T> class Matrix2D {    private:       size_t m_rows, m_cols;       T *m_data;                      size_t max_string_length() const;    public:       explicit Matrix2D();       explicit Matrix2D(size_t rows, size_t cols);       explicit Matrix2D(size_t rows, size_t cols, T initial_value);           Matrix2D(const Matrix2D<T>& m);           Matrix2D<T>& operator=(const Matrix2D<T>& m);                  virtual ~Matrix2D()       {          delete [] m_data;       }            T &operator()(size_t row, size_t col);       T operator()(size_t row, size_t col) const;                      template <typename _T>           friend ostream& operator<<(ostream& out, const Matrix2D<_T>& m);                      size_t rows() const;           size_t cols() const; }; int main() {    Matrix2D<int> foo(10, 10, 0);        foo(8, 3) = 42;    foo(9, 9) = 523;        cout << foo << endl;    return 0; } template <typename T> Matrix2D<T>::Matrix2D() : m_rows(0), m_cols(0), m_data(new T[0]) { } template <typename T> Matrix2D<T>::Matrix2D(size_t rows, size_t cols) : m_rows(rows), m_cols(cols), m_data(new T[rows * cols]) { } template <typename T> Matrix2D<T>::Matrix2D(size_t rows, size_t cols, T initial_value) : m_rows(rows), m_cols(cols), m_data(new T[rows * cols]) {    for (T *start = m_data; start != m_data + m_rows * m_rows; start++)    {       *start = initial_value;    } } template <typename T> Matrix2D<T>::Matrix2D(const Matrix2D<T>& m) : m_rows(m.m_rows), m_cols(m.m_cols),   m_data(new T[m.m_rows * m.m_cols]) {    for (size_t i = 0; i < m.m_rows * m.m_cols; i++)    {       m_data[i] = m.m_data[i];     } }         template <typename T>    Matrix2D<T>& Matrix2D<T>::operator=(const Matrix2D<T>& m) {    delete [] m_data;        m_rows = m.m_rows;    m_cols = m.m_cols;    m_data = new T[m_rows * m_cols];        for (size_t i = 0; i < m_rows * m_cols; i++)    {       m_data[i] = m.m_data[i];     }        return *this; } template <typename T> T &Matrix2D<T>::operator()(size_t row, size_t col) {    return m_data[row * m_cols + col]; } template <typename T> T Matrix2D<T>::operator()(size_t row, size_t col) const {    return m_data[row * m_cols + col]; } template <typename T> ostream& operator<<(ostream& out, const Matrix2D<T>& m) {    size_t max_len = m.max_string_length();    for (int r = 0; r < m.m_rows; r++)    {       for (int c = 0; c < m.m_cols; c++)           {                    stringstream ss;                                   ss << m(r, c);                                   size_t cur_len = ss.str().length();                         for (int i = 0; i < max_len - cur_len; i++)                  {                     out << " ";                  }                         out << m(r, c);                                   if (c == m.m_cols - 1)                  {                     if (r != m.m_rows - 1)                         {                        out << endl;                         }                  }                  else                  {                     out << " | ";                  }           }    }        return out; } template <typename T> size_t Matrix2D<T>::max_string_length() const {    size_t len = 0;        for (int r = 0; r < m_rows; r++)    {       for (int c = 0; c < m_cols; c++)           {              stringstream ss;                                   ss << (*this)(r, c);                                   size_t cur_len = ss.str().length();                                   if (cur_len > len)                  {                     len = cur_len;                  }           }    }        return len; } template <typename T> size_t Matrix2D<T>::rows() const {    return m_rows; } template <typename T> size_t Matrix2D<T>::cols() const {    return m_cols; }

[md]

Did you write that yourself wtd? And if so... mind if I make a copy of the code in case of need?

[wtd]

Cornflake wrote:

Did you write that yourself wtd?

Yes, I did.

[HazySmoke)345]

Quote:

Was the array you passed in properly allocated?

No, it wasn't...

code:

Use int **var with dynamically allocated 2-D arrays (where it is a pointer-to pointer-to-int). I suspect the 2-D array you are passing is declared like so: int v[X][Y]; (something else). Properly-allocated multidimenional arrays of both types will give unexpected results if you pass them wrong.

I see... thanks.

Too bad I don't have too many bits to gave away I gave them all to one of the guys in the VB boards, but I'll just distribute whatever's left.