I am in the process of building a matrix class (uni project, with my choice of programming language) and appear to have run into a problem that I'm not too sure how to fix.

My class uses templates to define the shape of the matrix, although I'm not sure if that matters. As part of my class, I included a opEquals, but when I try to assert that a matrix created by multiplying with the identity matrix matches the output (I checked with liberal writeln()s to make sure they do) but my opEquals() function (I moved it out of the class as I kept getting errors saying it was supposed to be non-const, and couldn't figure out how to fix that either) just isn't called.

Matrix code (the only bit of the class used by opEquals):

class Matrix(size_t X, size_t Y = X){
    const size_t x_length = X;
    const size_t y_length = Y;

    double[X*Y] data;

    ...
}

The entirety of opEquals:

/// both matrices have to have an identical shape to compare
/// each element must be identical to match
bool opEquals(size_t X, size_t Y)(const Matrix!(X,Y) m1, const Matrix!(X,Y) m2) {
    for (int i = 0; i < m1.data.length; ++i) if (m1.data[i] != m2.data[i]) return false;
    return true;
}

and the code that excepts:

    void main() {
        auto m = new Matrix!(2)();
        m.data = [1.0,0.0,0.0,1.0].dup;
        auto n = new Matrix!(2)();
        n.data = [2.0,3.0,4.0,5.0].dup;

        auto u = mult(m,n);
        writeln("u.data vs n.data:");
        for (int i = 0; i < u.data.length; ++i)
            writeln(u.data[i], "\t", n.data[i]);


        // using opEquals() directly is working, but it doesn't seem to be being used
        //assert(opEquals(u,n),"\"opEquals(u, n)\" is failing."); // this works fine
        assert(u == n,"\"u == n\" is failing."); // this fails. Why?
    }

Any help would be very much appreciated. A way to figure out what the '==' is converting its arguments to would also be great, but I'm not sure it exists (beyond just sitting down and trying to work it out myself, of course)

On Tuesday, 18 October 2022 at 18:59:37 UTC, rikki cattermole wrote:

Hi Rikki, thanks for the rapid reply.

I'm not so sure it isn't a type issue, tbh. The only reason being that the opEquals() works whenever I call it directly (ignoring the comparison of proximity to a value for the moment)
but doesn't work when I use "=="

stripped down to just the two lines that are causing issues:

//assert(opEquals(u,n)); // this works fine
assert(u == n); // this fails. Why?

I guess to put it more bluntly, I should ask "why is opEquals working when I call it directly, but when I try to use the equality check, which should be CALLING opEquals, it doesn't appear to do so?"

On Tuesday, 18 October 2022 at 18:53:41 UTC, Matthew Rushworth wrote:

/// both matrices have to have an identical shape to compare
/// each element must be identical to match
bool opEquals(size_t X, size_t Y)(const Matrix!(X,Y) m1, const Matrix!(X,Y) m2) {
    for (int i = 0; i < m1.data.length; ++i) if (m1.data[i] != m2.data[i]) return false;
    return true;
}

A free-standing opEquals won't work. It needs to be a method of the class. So:

class Matrix(size_t X, size_t Y = X)
{
    /* ... */

    bool opEquals(const Matrix m2)
    {
        for (int i = 0; i < this.data.length; ++i)
            if (this.data[i] != m2.data[i]) return false;
        return true;
    }
}

On Tuesday, 18 October 2022 at 20:02:02 UTC, ag0aep6g wrote:

/// both matrices have to have an identical shape to compare
/// each element must be identical to match
bool opEquals(size_t X, size_t Y)(const Matrix!(X,Y) m1, const Matrix!(X,Y) m2) {
    for (int i = 0; i < m1.data.length; ++i) if (m1.data[i] != m2.data[i]) return false;
    return true;
}

class Matrix(size_t X, size_t Y = X)
{
    /* ... */

    bool opEquals(const Matrix m2)
    {
        for (int i = 0; i < this.data.length; ++i)
            if (this.data[i] != m2.data[i]) return false;
        return true;
    }
}

Thank you, that worked perfectly, not sure exactly what I did wrong, I'm assuming I forgot to make the parameter a const variable

On Tuesday, 18 October 2022 at 18:53:41 UTC, Matthew Rushworth wrote:

class Matrix(size_t X, size_t Y = X){
    const size_t x_length = X;
    const size_t y_length = Y;

    double[X*Y] data;

    ...
}

/// both matrices have to have an identical shape to compare
/// each element must be identical to match
bool opEquals(size_t X, size_t Y)(const Matrix!(X,Y) m1, const Matrix!(X,Y) m2) {
    for (int i = 0; i < m1.data.length; ++i) if (m1.data[i] != m2.data[i]) return false;
    return true;
}

    void main() {
        auto m = new Matrix!(2)();
        m.data = [1.0,0.0,0.0,1.0].dup;
        auto n = new Matrix!(2)();
        n.data = [2.0,3.0,4.0,5.0].dup;

        auto u = mult(m,n);
        writeln("u.data vs n.data:");
        for (int i = 0; i < u.data.length; ++i)
            writeln(u.data[i], "\t", n.data[i]);


        // using opEquals() directly is working, but it doesn't seem to be being used
        //assert(opEquals(u,n),"\"opEquals(u, n)\" is failing."); // this works fine
        assert(u == n,"\"u == n\" is failing."); // this fails. Why?
    }

What's the reason for it being a class? struct makes more sense here, plus it'll be faster

If it were a struct, you'd not need any of that, and you can also have operator overloading for your mult while also being able to check how many row/col at compile time, here an example:

import std;

struct Matrix(size_t X, size_t Y = X)
{
    enum x_length = X; // enum so it's available only at compile time
    enum y_length = Y;
    double[X * Y] data;

    // operator overloading
    Matrix opBinary(string op)(Matrix rhs)
    {
        static if (op == "*")
        {
            //
            // ** ADD YOUR MULT OPERATION HERE **
            //
            static if (X == 2)
            {
                return Matrix();
            }
            else
                static assert("not supported");
        }
        else
            static assert(0, "Operator " ~ op ~ " not implemented");
    }
}

void main()
{
    // check equality by value:
    auto a = Matrix!(2)();
    a.data = 0;
    auto b = Matrix!(2)();
    b.data = 0;

    writeln("are they the same?: ", a == b); // it'll check for equality by value

    auto m = Matrix!(2)();
    m.data = [1.0, 0.0, 0.0, 1.0];
    auto n = Matrix!(2)();
    n.data = [2.0, 3.0, 4.0, 5.0];

    // here we can use the * operator!
    auto u = m * n;

    writeln("u.data vs n.data:");

    for (int i = 0; i < u.data.length; ++i)
        writeln(u.data[i], "\t", n.data[i]);
}