Use assignment operator with subscript operator to assign value to std::map

Question

I have a Matrix class

template <typename T>
class Matrix
{
public:
  const size_t rows;
  const size_t cols;
  const std::map<std::array<int, 2>, T> data;

  Matrix(int a, int b) : rows(a), cols(b)
  {
  }
};

which is initialized like this:

Matrix<double> M(5,5);

creating a 5x5 matrix.

I want to assign values to the map like so:

M[{1,2}] = 1;

How would I go about doing that in the most readable way? I'm not sure how to get the subscript and assignment operator working together.

"How would I go about doing that in the most readable way?" first of all by making data non const. — Slava, Jan 25 '18 at 15:50
[Read this](https://stackoverflow.com/questions/6969881/operator-overload) or [this](https://stackoverflow.com/q/27221482/5267751). — user202729, Jan 25 '18 at 15:50
@manni66 not according to my literature when dealing with sparse matrices — Nick, Jan 25 '18 at 15:56
@FrançoisAndrieux it's on my scientific computing class' slides and in the reader, so I'll stick with that for now. I know there are far superior methods and I don't like reinventing the wheel either, but it's what I have to deal with now. — Nick, Jan 25 '18 at 16:00
@FrançoisAndrieux it's overkill for `M(5,5)`, but is more obviously useful for `M(500000,500000)` — Caleth, Jan 25 '18 at 16:05
@Caleth I don't see how. Calculating an index in a flat vector and accessing the element is constant time. Accessing an element in a `std::map` has logarithmic time complexity. Meaning that `std::vector` becomes a more attractive solution as the size of the matrix grows. — François Andrieux, Jan 25 '18 at 16:09
I actually think OP is fine with `std::array` here, all they need is something that can be list initialized with 2 indexes — Caleth, Jan 25 '18 at 16:56

score 6 · Accepted Answer · answered Jan 25 '18 at 16:25

Let's add some helper aliases to Matrix while we are at it

template <typename T>
class Matrix
{   
    // rather than stoping people changing the members via const
    // make them private
    size_t rows;
    size_t cols;
    map_type data;
public:
    using key_type = std::array<size_t, 2>;
    using mapped_type = T;
    using map_type = std::map<key_type, mapped_type>;

    Matrix(size_t r, size_t c) : rows(r), cols(c) { }

    const T& operator [](key_type key) const { return data[key]; }
          T& operator [](key_type key)       { return data[key]; }

    // other operations as appropriate
};

score 3 · Answer 2 · edited Jan 25 '18 at 16:13

3

You have to provide members:

const T& operator [](std::pair<std::size_t, std::size_t>) const;
      T& operator [](std::pair<std::size_t, std::size_t>);

edited Jan 25 '18 at 16:13

bolov

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answered Jan 25 '18 at 15:52

Jarod42

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I'm not sure how to go from here, could you be a bit more specific? – Nick Jan 25 '18 at 15:58

Use assignment operator with subscript operator to assign value to std::map

2 Answers2