Dynamic Multidimensional Arrays

Hallo Gurus

I've done the following. It works.


float *array = new float[m_numbRows];




What I like to do is the following:

float *array = new float[m_numbRows][m_numbRows];




But it doesn't work.


I've got the following help from a friend. We think there should be an easier way.

float** pvfArray;

.....later....

pvfArray = new float*[iNumRows];
for(int i = 0; i < iNumRows; i++)
pvfArray[i] = new float[iNumCols];

pvfArray[iR][iC] = 3.9;




Thanks for the help.

M

Have you tried:

float *array = new float[m_numbCol*m_numbRows];

array[7][9] = 25; //or
array[(7*m_numbRows) + 9] = 25;

...or something similar?

[meintjesb]

Thank you very much for your help.

I got everything to work, except for


array[7][9] = 25;



Thanks again for your help.

Regards
Meintjes

[Yangghi Min]

When you are to allocate a multi-dimensional array with the new operator, you can do it
only if all dimensions except for the leftmost array dimension are constant expression.

Here are some examples:

#define NUM_ROWS 10
#define NUM_COLS 10

float (*array2)[NUM_ROWS];
float (*array3)[NUM_ROWS][NUM_COLS];

int first_rows;

// after somehow the value of the first_rows is determined.

array2 = new float [first_rows][NUM_ROWS];
array3 = new float [first_rows][NUM_ROWS][NUM_COLS];

// When the array buffers are no longer needed, you can use delete operator like the followings.
delete [] array2;
delete [] array3;

If you are to allocate a dynamic multidimensional array without any fixed dimension specifier,
the advice of your friend is a standard way of allocating a dynamic multidimensional array.

Regards,
Yangghi.

[sally]

array[7][9] = 25; won't work

array is a single dimensional array SIMULATING a multi dimensional array

index = 7 * numRows + 9;
array[index] = 25;

There's a forumla you can use on a raw block of memroy to treat it as a multi-dimensional array.

You can also take a raw block of memory and initialize it as a multi-dimensional array. Uses more memory but allows you to keep the array[][] schemantics.

And of course, the easiest route is to use STL.

I think someone already posted the formula for option one, so here's option two and three...


static const int ROWS = 10;
static const int COLS = 10;

int STORAGE = ( (ROWS * COLS) * (sizeof(float)/sizeof(DWORD)) ) + ROWS;

float** ppFloat = (float**)new DWORD[STORAGE];

::memset(ppFloat,0,STORAGE * sizeof(DWORD));

for(int index = 0;index < ROWS;++index)
{
ppFloat[index] = (float*)((DWORD*)ppFloat + ROWS) +
index * ((sizeof(float)/sizeof(DWORD)) * ROWS);
}


float val = 1.5;


for(int r = 0;r < ROWS;++r)
for(int c = 0;c < COLS;++c)
ppFloat[r][c] = val++;

for(r = 0;r < ROWS;++r)
for(int c = 0;c < COLS;++c)
std::cout << '[' << r << "][" << c << "]: "
<< ppFloat[r][c] << std::endl;



For STL it helps to create a custom function object to help initializing the array.


typedef std::vector<float> tFloatCol;
typedef std::vector<tFloatCol> t2dFloatVec;


class setdim
{
public:
setdim(const int dim = 1):
m_dim(dim)
{}

void operator()(tFloatCol& col)
{
col.resize(m_dim,0);
}
int m_dim;
};




Then using it is...


static const int ROWS = 10;
static const int COLS = 10;

t2dFloatVec vec;

vec.assign(ROWS);
std::for_each(vec.begin(),vec.end(),setdim(COLS));

float val = 1.5;

for(int r = 0;r < ROWS;++r)
for(int c = 0;c < COLS;++c)
vec[r][c] = val++;

for(r = 0;r < ROWS;++r)
for(int c = 0;c < COLS;++c)
std::cout << '[' << r << "][" << c << "]: " << vec[r][c] << std::endl;




Hope it helps...

Mike