[RESOLVED] Assignment on function

[grahamr (work)]

Since you can't use [][] to access a matrix you are suggested to use () operator instead.

However a limitation of that is you can't assign:

m[col][row] = x; // valid
m(col, row) = x; // invalid

which forces us to write such statements as:

m[( col * cols ) + row] = rhs( col, row );

Is there any way of working around that limitation so you can use

m( col, row ) = rhs( col, row );

which is more elegant?

It's mainly an academic exercise as I've finished the matrix class and just having a wonder over lunch.

[souldog]

you can do an assign with the m(x,y) notation.

There should be two implementations of this function. I will assume double
for the example

double operator()(const size_t row, const size_t col) const;
double& operator()(const size_t row, const size_t col);

[grahamr (work)]

Well apart from the fact I use float and index in using int's ours are identical.

Actual code:

	float& operator()( const int col, const int row )
	{
		return m[( col * cols ) + row];
	}
	
	float operator()( const int col, const int row ) const
	{
		return m[( col * cols ) + row];
	}

However I get the error telling me that the "term does not evaluate to a function".

[souldog]

well, that should work, so something else is wrong

[Zaccheus]

You'll have to show us the full code so we can see WHAT isn't evaluating to a function.

BTW, shouldn't it be col + (row * cols) ?

[GNiewerth]

You need to make them member functions, of course:

template<typename T>
class Array2D
{
   std::vector<T> Data_;
   unsigned int Row_;
   unsigned int Cols_;
   
public:
   Array2D( unsigned int Rows, unsigned int Cols ) :
      Rows_( Rows ),
      Cols_( Cols ),
      Data_( Rows * Cols )
   {
   }

   const T& operator()( unsigned int Row,  unsigned int Col ) const
   {
      return Data[Row * Cols_ + Col];
   }

   T& operator()( unsigned int Row,  unsigned int Col )
   {
      return Data[Row * Cols_ + Col];
   }
};

[grahamr (work)]

You'll have to show us the full code so we can see WHAT isn't evaluating to a function.

Okay. How about setting up and identity matrix in the constructor?

	for ( int col=0; col<cols; col++ ) 
	{
		for ( int row=0; row<rows; row++ ) 
			m( col, row ) = 0.0f;
		m( col, row ) = 1.0f;
	}

BTW, shouldn't it be col + (row * cols) ?

No, I'm pretty sure column-major is (col * cols) + row.

[souldog]

No, I'm pretty sure column-major is (col * cols) + row.

Nope.

[grahamr (work)]

I thought column-major layout is

[0  4  8 12]
[1  5  9 13]
[2  6 10 14]
[3  7 11 15]

G.

[Zaccheus]

Oh I see.

I think it is better for each dimension to be contiguous memory.

[Zaccheus]

Okay. How about setting up and identity matrix in the constructor?

	for ( int col=0; col<cols; col++ ) 
	{
		for ( int row=0; row<rows; row++ ) 
			m( col, row ) = 0.0f;
		m( col, row ) = 1.0f;
	}

What is 'm' ? How is it defined/created?

[grahamr (work)]

What is 'm' ? How is it defined/created?

float m[16];

[souldog]

if for whatever reason you want to do column major layout (C uses row major layout in memory) then you would do

col*rows + row

[JohnW@Wessex]

float m[16];

'm' should be a 2D matrix class such as in GNiewerth's post.

 Array2D m(4, 4);

[grahamr (work)]

Does it matter what it is? It doesn't change the fact that using () to index into the array doesn't allow assignment (or I can't use assignment).