Question

I’m pretty new to C++ and got an assignment to make a matrix using only STL containers. I’ve used a vector (rows) of vectors (columns). The problem I’m having is in the ‘write’ operation – for which I may only use an iterator-based implementation. Problem is, quite simply: it writes nothing.

I’ve tested with a matrix filled with different values, and while the iterator ends up on exactly the right spot, it doesn’t change the value.

Here’s my code:

void write(matrix mat, int row, int col, int input)
{
    assert(row>=0 && col>=0);
    assert(row<=mat.R && col<=mat.C);

    //I set up the iterators.
    vector<vector<int> >::iterator rowit;
    vector<int>::iterator colit;
    rowit = mat.rows.begin();

    //I go to the row.
    for(int i = 0; i<row-1; ++i)
    {   
        ++rowit;
    }

    colit = rowit->begin();

    //I go to the column.
    for(int j = 0; j<col-1; ++j)
    {
        ++colit;
    }

    *colit = input; //Does nothing.
}

What am I overlooking?
Thanks.

Answer 1

matrix mat is a parameter by value, it copies the matrix and hence you are writing to a copy.

You should pass the matrix by reference instead, like matrix & mat.

---

But wait… You are passing the matrix every time as the first parameter, this is a bad sign!

This usually indicates that the parameter should be turned into an object on which you can run the methods; that way, you don’t need to pass the parameter at all. So, create a Matrix class instead.

---

Please note that there is std::vector::operator[].

So, you could just do it like this:

void write(matrix & mat, int row, int col, int input)
{
    assert(row>=0 && col>=0);
    assert(row<=mat.R && col<=mat.C);

    mat[row][col] = input;
}