CODEWITHSUNDEEP
c++c++17
user222061
user222061

Reputation: 1

unable to create 2d matrix inside a class

The error is : "member matrix::rows is not a type name" and "member matrix::init is not a type name". If it compiles successfully I shall have a matrix rows = ROWS and columns = COLS. What am i doing wrong here:

#include <vector>
class matrix
{
    int ROWS{}, COLS{}, init{-1};
    std::vector<std::vector<int>> table(ROWS, std::vector<int>(COLS, init));
};

Upvotes: 0

Views: 128

Answers (3)

John Zwinck
John Zwinck

Reputation: 249173

You can write it like this:

class matrix
{
    static constexpr int ROWS{}; // TODO: change to non-zero
    static constexpr int COLS{}; // TODO: change to non-zero
    static constexpr int init{-1};
    std::vector<std::vector<int>> table{ROWS, std::vector<int>(COLS, init)};
};

Upvotes: 1

Sergey Strashko
Sergey Strashko

Reputation: 63

I don't understand the usage of the vector of vectors for this case. For such a case, I would propose to use a plain array.

template<int N, int M, typename D> struct Matrix {
public:
    D get(int i, int j) {
        return _data[index(i,j)];
    }
    void set(int i, int j, D v) {
        _data[index(i,j)] = v;
    }

private:
    int index(int i, int j){
        return i*N + j;
    }
    D _data[N*M];
};

So you don't really need std::vector. Just add some checks and you are good.

Upvotes: 0

Jeffrey
Jeffrey

Reputation: 11410

As John Zwinck points out, you need curly brackets. Why? Because, otherwise, the compiler takes

std::vector<std::vector<int>> table(ROWS, std::vector<int>(COLS, init));

as the definition of a member function, called table, returning a std::vector<std::vector<int>> and taking a first (unnamed) parameter of type ROWS. At which point the compiler goes "Ugh, I don't know the type ROWS, let me throw a compilation error".

You can google Most Vexing Parse. https://en.wikipedia.org/wiki/Most_vexing_parse

Upvotes: 2

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