Output dynamic array from a function pointer

Question

I'm very new to pointers so please bear with me...

My code defines a function for the multiplication of two matrices (matrixMultiplication). I have then defined a function pointer to this function.

#include <iostream>

void matrixMultiplication (const double A[3][3], const double B[3][3], double output[3][3])
{
    int i, j, k; 
    for (i=0;i<3;i++) 
    {
        for(j=0;j<3;j++)
        {
            for(k=0;k<3;k++)
            {
                output[i][j]+=A[i][k]*B[k][j];
            }
        }
    }
}

double (*matrixMultiplication (const double (*left)[3], const double (*right)[3]))[3]
{
    double output[3][3];
    matrixMultiplication(left, right, output);
}

int main ()
{
    using namespace std;

    double A[3][3]={{1,1,1},{1,1,1},{1,1,1}};
    double B[3][3]={{1,1,1},{1,1,1},{1,1,1}};

    cout<<"The function returns..."<<endl;

    double print[3][3]=matrixMultiplication(A,B);

    int i, j;
    for (i=0;i<3;i++)
    {
        for (j=0;j<3;j++)
        {
            cout<<print[i][j]<<"\t";
        }
        cout<<"\n";
    }
    return 0;
}

What I want to do is output the array given by the pointer function, *matrixMultiplication, using a for loop (just for aesthetic purposes). I have played around with the code and ended up with initialiser or segmentation (11) errors. I feel like I'm missing something blatantly obvious given I'm new to C++...

Any ideas would be most welcome!

is this the exact code you used to run? because the given code doesn't even compile for me. — codeling
– codeling, Commented Nov 15, 2013 at 12:57
Yes, sorry this doesn't compile. The problem like is in the main when introducing 'double print[3][3]'. I have put the for loop into the matrixMultiplication function itself to see if the function pointer produces anything, which it does. — psweens
– psweens, Commented Nov 15, 2013 at 13:03
Is there any particular reason you're trying to tackle two different problems -- 2D arrays and function pointers -- at once? Pick one to do first, it'll be easier for everyone. — Beta
– Beta, Commented Nov 15, 2013 at 13:05
Yeh, it's an assignment I've been given. The first being to create functions for 2D arrays and now the second to use function pointers as an illustration of "code reuse" — psweens
– psweens, Commented Nov 15, 2013 at 13:09

kenba · Accepted Answer · 2013-11-15 16:31:03Z

1

The problem is with:

double (*matrixMultiplication (const double (*left)[3], const double (*right)[3]))[3]
{
  double output[3][3];
  matrixMultiplication(left, right, output);
}

I don't know what it is and neither does my compiler! ;)

Using functional, a matrixMultiplication function type can be defined and used, like so:

#include <functional> // or <tr1/functional>

// type
typedef function<void (const double[3][3], const double[3][3], double[3][3])> MatrixFunction;

// instance
MatrixFunction matrixFunctionPtr(&matrixMultiplication);

// call
matrixFunctionPtr(A,B,print);

Note: you also need to declare your output array double print[3][3]; * before* you call the matrixMultiplication function...

answered Nov 15, 2013 at 16:31

kenba

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Comments

Beta · Accepted Answer · 2013-11-16 03:41:30Z

You have a function:

void matrixMultiplication (const double A[3][3], const double B[3][3], double output[3][3])
{
  ...
}

This function works. It takes three arrays as arguments (which is to say it takes three pointers-- this is a subtle point, and I don't think it's a good exercise for a beginner because it clouds the distinction between passing by value and passing by reference -- but never mind that for now) and returns void (i.e. nothing), and . Now you want to construct a function pointer that points to this function. But this:

double (*matrixMultiplication (const double (*left)[3], const double (*right)[3]))[3]
{
  ...
}

is not a function pointer; it's a function that returns a pointer to an array of double, but it has some internal errors (and don't even worry about what it takes as arguments for now).

Let's do a simpler example first:

double foo(int n)      // function
{
  return(3);
}

int main()
{
  double (*bar)(int);  // function pointer
  bar = &foo;
  double z = (*bar)(5);
  cout << z << endl;
  return(0);
}

Now that we see how function pointers work, we apply one to matrixMultiplication:

void (*matFP)(const double A[3][3], const double B[3][3], double output[3][3]);
matFP = &matrixMultiplication;
double C[3][3];
(*matFP)(A,B,C);

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