How do i create an array of unequal length arrays?

Question

Hello everyone,

I need to create a sort of 2D array in which each one of the secondary arrays are of different length. I have a 1D array of known length (which defines the number of arrays to be formed) with each element having a number that denotes the length of the secondary array in that position.

Each one of the arrays are fairly large so i don't want to create a one-size-fits-all "fake" 2D heap array to cover everything.

How would i go about doing this? Any 2D array I have made before are always rectangular.

I'm trying to do this so that i can create some code to dynamically generate threads to split up some workload.

Thanks,

-Faken

Answer 1

Use a std::vector of std::vectors - for integers:

#include <vector>
std::vector <std::vector <int> > v2d;

Answer 2

Some sort of sparse array might be helpful here. There's this question at stack overflow or this site.

Good luck.

Answer 3

Or if you're thinking more along the lines of C (there are zillions of people writing C code but compiling it with C++, that mostly works):

Declare not a two dimensional array: int arr[rows][cols]

but a one dimensional array of pointers to arrays: int (arr*)[rows]

Not quite sure about that syntax, someone correct me if I'm wrong please!

Answer 4

You can do something like following :

int **array = new int*[3];
array[0] = new int[5];
array[1] = new int[2];
array[2] = new int[11];

Do not forget to deallocate properly after you are done with using array.

EDIT :

You can put secondary pointer initialization in the loop in the following way :

int cells[] = {5,2,11};
int **array = new int*[3];
for (int i = 0; i < 3; i++){
  array[i] = new int[cells[i]];
}

PS : I personally prefer to use Boost.MultiArray when I need multidimensional arrays. You might also want to have a look at it.