重载<<运算符以在模板类中打印矩阵的元素

Question

why am I having an error on the operator '[]'. I wanted to print the contents of my matrix. If i can't use the brackets, what could i do then?

here's a sample of the code:

#include <iostream>
#include <vector>

template <typename T>
class Matrix {
    private:
    int m; int n;
    std::vector<T> x;
    std::vector<std::vector<int>> Mat;


    public:
    Matrix (const unsigned int &m, const unsigned int &n, std::vector<T> x);
    Matrix(const Matrix &M);
    Matrix<T> operator = (const Matrix &M);
    // Matrix<T> operator [](const int &index);

    friend std::ostream& operator << (std::ostream& os, const Matrix<T> &M) {
        os << "[";
        for (int i = 0; i< M.m; i++){
            for (int j = 0; j< M.n; j++){
                os << M[i][j] << ' ';
            }
            os << '\n';
        }
        os << "]\n";
        return os;
    }
};

I have fixed the errors. But it doesn't print my matrix. This is my main:

int main(){
    std::vector<int> x = {1,2,3,4};
    Matrix<int> A{2,2,x};
    Matrix<int> B{2,2,x};
    std::cout << A;
    std::cout << B;
    return 0;
}

And this is my constructor, I needed to make a matrix from a vector where I specify the rows and columns.

template <typename T>
    Matrix<T>::Matrix (const unsigned int &m, const unsigned int &n, std::vector<T> x){ //constructor
    this -> m = m;
    this -> n = n;
    this -> x = x;

    int index = 0;
    for (int i = 0; i<m; i++){
        for (int j = 0; j<n; j++){
             Mat[i][j] = x[index];
            index++;
        }
    }
}

Answer 1

The issue is this line:

os << M[i][j] << ' ';

Because M is of type Matrix<T> and you haven't defined [] operator, any attempts to use [] operator will give you error.

Instead, You should use data member Mat .

friend std::ostream& operator << (std::ostream& os, const Matrix<T> &M) {
    os << "[";
    for (int i = 0; i< M.m; i++){
        for (int j = 0; j< M.n; j++){
            os << M.Mat[i][j] << ' '; // using M.Mat instead of M
        }
        os << '\n';
    }
    os << "]\n";
    return os;
}

Edit:

As per your updated code, you may get a nice SIGSEGV error (Segmentation Fault) issue. Why? Simple. You are not resizing your data member Mat .

You have to do this:

template <typename T>
    Matrix<T>::Matrix (const unsigned int &m, const unsigned int &n, std::vector<T> x){ //constructor
    this -> m = m;
    this -> n = n;
    this -> x = x;

    int index = 0;

    // resizing Matrix according to our need i.e. m rows and n columns
    Mat.resize(m);
    for (unsigned int i = 0; i < Mat.size(); i++) {
        Mat[i].resize(n);
    }

    for (unsigned int i = 0; i<m; i++){
        for (unsigned int j = 0; j<n; j++){
             Mat[i][j] = x[index];
            index++;
        }
    }
}

Answer 2

You have different mistakes in your code:

1.- Your constructor:

 Matrix<T>::Matrix (const unsigned int &m, const unsigned int &n, std::vector<T> x)

it's not a good idea to pass a vector by value, its always better to pass it by reference, and never pass an integer by const reference, pass it by value:

 Matrix<T>::Matrix (unsigned int m, unsigned int n, const std::vector<T>& x)

2.- This is not an error, but you don't usually need to use "this->", and its better to construct an object like:

Matrix<T>::Matrix (unsigned int m0, unsigned int n0, const std::vector<T>& x0)
      : m{m0}, n{n0}, x{x0} {...}

3.- I don't understand why you make 2 copys of the same elements: one in your vector x, and other with M. Your wasting your memory. Maybe with x is enough?

4.- You define m and n as signed int, but in your constructor you use "unsigned" int. Which one do you want to use?

I propose this changes:

1.- If you want to access to an element (i,j), write the operator:

  Matrix& operator()(unsigned int i, unsigned int j);

its easy using your vector x.

2.- To write all the elements you can write something like:

std::ostream& operator << (std::ostream& os, const Matrix<T>& M) {
    os << "[";
    for (int i = 0; i< M.rows(); ++i){
        for (int j = 0; j< M.cols(); ++j){
            os << M(i, j) << ' ';
        }
        os << '\n';
    }
    os << "]\n";
    return os;
}

and this is not a friend function (if you define rows() and cols()).

Answer 3

your operator[] returns a Matrix. You should change it to your template value.