Passing 2D-Array to a function C++

Question

I would like to pass a series of 2D-arrays to a function, which then saves it to a file. My "saving function" is as follows: (Question marks indicate I don't know what the hell I am doing.)

void saveArray(double* z(?),double* u1(?),double* u2 (?),double* theta (?), int row, 
int column)
{
ofstream output("data.csv");
for(int j=0;j<row;++j)
for(int i=0;i<column;i++)
{
output<<setprecision(32)<<z[j]<<","<<setprecision(32)<<u1[j][i]<<","
<<setprecision(32)
<<u2[j][i] <<","<<setprecision(32)<<theta[j][i]<<endl;
}

z is : z[30000],u1,u2 and theta are [101][30000] 2D-arrays.

Please let me know if this is confusing and I can post the entire code.

Why do you have question marks in your code? Use `/* comments */` instead — bobobobo, Dec 15 '11 at 18:16
possible duplicate of [How to pass a multidimensional array to a function in C and C++](http://stackoverflow.com/questions/2828648/how-to-pass-a-multidimensional-array-to-a-function-in-c-and-c) — Kos, Dec 15 '11 at 18:30
This needs a FAQ entry. Been discussed tens and tens of times because it's hard to answer. — Kos, Dec 15 '11 at 18:31

Benjamin Lindley · Accepted Answer · 2011-12-15T23:57:05.327

I find it's much easier to deal with 2d arrays by treating them as 1d arrays, at least when passing them as parameters.

void saveArray(double* z, double* u1, double* u2, double* theta, int row, int column)
{
    ofstream output("data.csv");
    for(int j=0; j<row; ++j)
        for(int i=0; i<column; i++)
        {
            int offset = j * column + i;
            output <<setprecision(32)<<z[j]<<","
                   <<setprecision(32)<<u1[offset]<<","
                   <<setprecision(32)<<u2[offset] <<","
                   <<setprecision(32)<<theta[offset]<<endl;
        }
}

When you call it, be sure to dereference your 2d arrays:

saveArray(z, *u1, *u2, *theta, 101, 30000);

You could also do it another way where you can pass the arrays by reference, and you don't need to pass the size. This will only work if you have the actual arrays, not just pointers to them:

template<size_t R, size_t C>
void saveArray(double (&z)[C], double (&u1)[R][C], double (&u2)[R][C], double (&theta)[R][C])
{
    ofstream output("data.csv");
    for(int r=0; r<R; ++r)
        for(int c=0; c<C; ++c)
        {
            output <<setprecision(32)<<z[c]<<","
                   <<setprecision(32)<<u1[r][c]<<","
                   <<setprecision(32)<<u2[r][c] <<","
                   <<setprecision(32)<<theta[r][c]<<endl;
        }
}

Then you would just call it like this:

saveArray(z, u1, u2, theta);

That's actually the correct way to do this (well at least in C :), in C++ you could wrap that with a class. Or use something already written (if your project has boost, then boost::multi_array is the best variant). — Kos, Dec 15 '11 at 18:32
Hi Benjamin: Thank you for helping me out. This seems to work. — Antillar Maximus, Dec 15 '11 at 18:54

Passing 2D-Array to a function C++

1 Answers1