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From: dwelly@saddle-lk.cs.UAlberta.CA (Andrew Dwelly)
Newsgroups: comp.lang.c++
Subject: Beginners problems (longish)
Message-ID: <1991Mar13.170912.4020@cs.UAlberta.CA>
Date: 13 Mar 91 17:09:12 GMT
Sender: news@cs.UAlberta.CA
Organization: University of Alberta, Edmonton, Canada
Lines: 162


I've just started using c++ and I have an example of code here that
I can't get to run, even though it seems to be legal c++. This is almost 
certainly a beginners mistake, but I can't for the life of me see what the
problem is.

The situation :

I've produced a class of dynamic matrices, with appropriate access, constructor
and destructor functions. Now I'm trying to get a simple matrix multiply to
work, overloading the * operator. In my main function, I create and print
out a matrix "m", then try to print out m*m;

What I think I'm doing :

I create an automatic matrix variable for my results, then perform
a simple multiply on the inputs, placing the result in the output
variable. Finally I return "out".

Since my function has been declared as returning a matrix, I understood that
a _copy_ of "out" was returned before the destructor was called on "out".

What actually happens :

Compiling with g++ on a Sparc produces the following results :-

saddle-lk% Matrix
1 2 
3 4 
 
2.24208e-44 2.16332e-40 
0 2.16355e-40 
Segmentation fault (core dumped)

Would someone take pity on me and tell me what I'm doing wrong ???

Andy Dwelly

-------------------------------------------------------------------------------

#include <stream.h>
#include <stddef.h>
#include <bool.h>

class matrix
{
    float **m;
    int   rows;
    int   cols;
public:

            matrix(int,int);
            ~matrix();
    float*& operator[](int);
    int     sizer();
    int     sizec();
    friend matrix 
            operator*(matrix&, matrix&);
};

// We want to be able to print it out

ostream& operator<<(ostream& s, matrix& m) // m must be a reference, why ?
{
    for (int i = 0; i < m.sizer(); i++)
    { 
        for (int j = 0; j < m.sizec(); j++)
        {
            s << m[i][j] << " ";
        }
        s << "\n";
    }
    return s;
}

matrix::matrix(int r, int c)
{
    rows = r;
    cols = c;
    m = new float*[r];
    for (int i = 0; i < r; i++)
    {
	m[i] = new float[c];
    }
}

matrix::~matrix()
{
    for (int i = 0; i < rows; i++)
    {
	delete m[i];
    }

    delete m;
}

float*&
matrix::operator[](int i)
{
    return m[i];
}

matrix::sizer()
{
    return rows;
}

matrix::sizec()
{
    return cols;
}

// Problems here ????

matrix
operator*(matrix& a, matrix& b)
{
    matrix out(a.sizer(), b.sizec());

    if (a.sizec() == b.sizer())
    {
	for (int i = 0; i < a.sizer(); i++)
	{
	    float *row = a.m[i];
	    float multsum;

	    for (int j = 0; j < b.sizec(); j++)
	    {
		multsum = 0.0;

		for (int k = 0; k < b.sizer(); k++)
		{
		    multsum += row[k] * b.m[k][j];
		}

		out.m[i][j] = multsum;
	    }
	}
    }
    return out;
}

main()
{
    matrix m(2,2);

    m[0][0] = 1.0;
    m[0][1] = 2.0;
    m[1][0] = 3.0;
    m[1][1] = 4.0;

    cout << m << "\n";

    cout << m * m << "\n";
}


--
******************************************************************************
Andy Dwelly : dwelly@cs.uablberta.ca, Tel: 403-492-7591

!@#$%$#, %^&*%, - Listen, who swears ? Christopher Robin has fallen downstairs.