Question

Like the question says, I am trying to pass multi-dimensional arrays into a function to print it to a file for an engineering project. The format for which the data is inputted CANNOT be changed, so please don’t suggest I just input it as a different datatype.

This particular function anticipates a two-dimensional array (although I have others with three dimensions after this one), where nothing is known about the size of the array until run-time. I know I must use pointers to point to each row of the array separately, but I have NO idea what the syntax is for passing it to the function. In the following code, the array in question is ‘block’. The main function is just a little testing example I made to try to make it work:

#include<fstream>
using namespace std;

void of_write_blocks(string filename, string block_type[], int **block,
            int grid[][3], string grade_type[], int grade[][3], int n_blocks, int m[])
{
    ofstream file_out(filename.c_str(),ios::app);
    file_out<<"\nblocks\n(\n";

    for(int i=0;i<n_blocks;++i) {
            file_out<<"   "<<block_type[i]<<" ( ";
                    for(int j=0;j<m[i];++j)
                            file_out<<block[i][j]<<" ";
            file_out<<") ( ";
            file_out<<grid[i][0]<<' '<<grid[i][1]<<' '<<grid[i][2]<<" ) ";
            file_out<<grade_type[i]<<" ( ";
            file_out<<grade[i][0]<<' '<<grade[i][1]<<' '<<grade[i][2]<<" )\n";
    }
    file_out<<");\n";
}


//testing example:
int main()
{       
    int block[6][9];
    for(int i=0; i<6;++i) 
            for(int j=0; i<9;++j)
                    block[i][j] = i*j;

    int grid[6][3];
    for(int i=0; i<6;++i)
            for(int j=0; i<3;++j)
                    block[i][j] = i*j;


    int grade[6][3];
    for(int i=0; i<6;++i) 
            for(int j=0; i<3;++j)
                    block[i][j] = i*j;

    string grade_type[6] = {"simpleGrading"};
    string block_type[6] = {"hex"};
    int m[6] = {8};
    int n_blocks = 6;

    of_write_blocks("name",block_type,block,grid,grade_type,grade,n_blocks,m);
}       

any help is appreciated!

Answer 1

You can’t. Multidimensional arrays are syntactic sugar, and are compiled directly into the code that does manipulations on the array, which is a single memory block. The dimensions are not passed into the function as parameters or anything like that as part of the array, as things are done in e.g. Java or C#.

If you need the dimensions of the array in your function, you’ll need to just accept a pointer to the first element of the array, and the dimensions, and do the multiplies and adds to get the right index yourself.

Alternately, use something like a std::vector<std::vector<block>>, which pass the dimensions as part of the object, rather than a built in array.