[RESOLVED] Assignment on function

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:

Code:

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

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which forces us to write such statements as:

Code:

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

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

Code:

---

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

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which is more elegant?

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

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

Code:

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

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

Actual code:

Code:

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        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];
        }

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

well, that should work, so something else is wrong

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) ?

You need to make them member functions, of course:

Code:

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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];
  }
};

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Quote:

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Originally Posted by Zaccheus

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

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

Code:

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

---

Quote:

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Originally Posted by Zaccheus

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

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

Quote:

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Originally Posted by grahamr (work)

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

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Nope.

I thought column-major layout is

Code:

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

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G.

Oh I see.

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

Quote:

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Originally Posted by grahamr (work)

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

Code:

---

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

---

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

Quote:

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Originally Posted by Zaccheus

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

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Code:

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float m[16];

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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

Quote:

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Originally Posted by grahamr (work)

Code:

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float m[16];

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

Code:

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Array2D m(4, 4);

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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).