C++ matrix size depending input doesn't compile - c++

Question

there is a topic about this subject which is working with arrays but I can't make it work with matrices.
(Topic: C++ array size dependent on function parameter causes compile errors)
In summary:
long rows, columns;
cin >> rows >> columns;
char *a = new char [rows];
compiles great in visual studio, but:
char **a = new char[rows][columns];
or
char *a[] = new char[rows][columns];
or
char *a[] = new char[rows][columns]();
or
char **a = new char[rows][columns]();
don't compile at all.
Any help? Thank you

Answer 1

The array-new operator only allocates 1-dimensional arrays. There are different solutions, depending on what sort of array structure you want. For dimensions only known at runtime, you can either create an array of arrays:
char **a = new char*[rows];
for (int i = 0; i < rows; ++i) {
a[i] = new char[columns];
}
or an array of elements with an associated array of pointers to the first element of each row (requiring just two hits to the memory allocator):
char *a = new char[rows*columns];
char **a = new char*[rows];
for (int i = 0; i < rows; ++i) {
a[i] = a + i*columns;
}
Either one will let you access matrix elements via a[row][column].
An alternate solution is to just use the one-dimensional array and generate indexes by hand:
char *a = new char[rows*columns];
...
a[columns*row + column]
This is probably faster than the double-indirection required in the first two solutions.
You could of course wrap this in a class to preserve a semblance of 2D indexing syntax:
class MatrixWrapper {
...
char& operator()(int row, int column) { return a_[columns*row + column]; }
...
};
...
a(row, column)

Answer 2

A n-dimensional array can not be directly allocated as you are trying to. Here is a way to do it.
int main(){
unsigned int **p;
unsigned int rows = 10;
unsigned int cols = 20;
p = new unsigned int* [rows];
for(size_t row = 0; row < rows; row++){
p[row] = new unsigned int [cols];
}
for(size_t row = 0; row < rows; row++){
delete [] p[row];
}
delete [] p;
}

Answer 3

If you're going to use c-arrays in c++ (instead of std::vector), you're stuck doing something like this:
long rows = 0;
long columns = 0;
cin >> rows >> columns;
// declaration
long** a = new long* [rows];
for(long i = 0; i < rows; ++i)
{
a[i] = new long[cols];
}
// initialization
for(long j = 0; j < rows; ++j)
{
for(long i = 0; i < rows; i++)
{
a[i][j] = 0;
}
}
// delete sub-arrays
for(long i = 0; i < rows; ++i)
{
delete[] a[i];
}
// delete array
delete[] a;