Assume we have the following class:
#include <vector>
class Matrix {
private:
std::vector<std::vector<int>> data;
};
First of all I would like suggest you to implement a default constructor:
#include <vector>
class Matrix {
public:
Matrix(): data({}) {}
private:
std::vector<std::vector<int>> data;
};
At this time we can create Matrix instance as follows:
Matrix one;
The next strategic step is to implement a Reset method, which takes two integer parameters that specify the new number of rows and columns of the matrix, respectively:
#include <vector>
class Matrix {
public:
Matrix(): data({}) {}
Matrix(const int &rows, const int &cols) {
Reset(rows, cols);
}
void Reset(const int &rows, const int &cols) {
if (rows == 0 || cols == 0) {
data.assign(0, std::vector<int>(0));
} else {
data.assign(rows, std::vector<int>(cols));
}
}
private:
std::vector<std::vector<int>> data;
};
At this time the Reset method changes the dimensions of the 2D-matrix to the given ones and resets all its elements. Let me show you a bit later why we may need this.
Well, we can create and initialize our matrix:
Matrix two(3, 5);
Lets add info methods for our matrix:
#include <vector>
class Matrix {
public:
Matrix(): data({}) {}
Matrix(const int &rows, const int &cols) {
Reset(rows, cols);
}
void Reset(const int &rows, const int &cols) {
data.resize(rows);
for (int i = 0; i < rows; ++i) {
data.at(i).resize(cols);
}
}
int GetNumRows() const {
return data.size();
}
int GetNumColumns() const {
if (GetNumRows() > 0) {
return data[0].size();
}
return 0;
}
private:
std::vector<std::vector<int>> data;
};
At this time we can get some trivial matrix debug info:
#include <iostream>
void MatrixInfo(const Matrix& m) {
std::cout << "{ \"rows\": " << m.GetNumRows()
<< ", \"cols\": " << m.GetNumColumns() << " }" << std::endl;
}
int main() {
Matrix three(3, 4);
MatrixInfo(three);
}
The second class method we need at this time is At. A sort of getter for our private data:
#include <vector>
class Matrix {
public:
Matrix(): data({}) {}
Matrix(const int &rows, const int &cols) {
Reset(rows, cols);
}
void Reset(const int &rows, const int &cols) {
data.resize(rows);
for (int i = 0; i < rows; ++i) {
data.at(i).resize(cols);
}
}
int At(const int &row, const int &col) const {
return data.at(row).at(col);
}
int& At(const int &row, const int &col) {
return data.at(row).at(col);
}
int GetNumRows() const {
return data.size();
}
int GetNumColumns() const {
if (GetNumRows() > 0) {
return data[0].size();
}
return 0;
}
private:
std::vector<std::vector<int>> data;
};
The constant At method takes the row number and column number and returns the value in the corresponding matrix cell:
#include <iostream>
int main() {
Matrix three(3, 4);
std::cout << three.At(1, 2); // 0 at this time
}
The second, non-constant At method with the same parameters returns a reference to the value in the corresponding matrix cell:
#include <iostream>
int main() {
Matrix three(3, 4);
three.At(1, 2) = 8;
std::cout << three.At(1, 2); // 8
}
Finally lets implement >> operator:
#include <iostream>
std::istream& operator>>(std::istream& stream, Matrix &matrix) {
int row = 0, col = 0;
stream >> row >> col;
matrix.Reset(row, col);
for (int r = 0; r < row; ++r) {
for (int c = 0; c < col; ++c) {
stream >> matrix.At(r, c);
}
}
return stream;
}
And test it:
#include <iostream>
int main() {
Matrix four; // An empty matrix
MatrixInfo(four);
// Example output:
//
// { "rows": 0, "cols": 0 }
std::cin >> four;
// Example input
//
// 2 3
// 4 -1 10
// 8 7 13
MatrixInfo(four);
// Example output:
//
// { "rows": 2, "cols": 3 }
}
Feel free to add out of range check. I hope this example helps you :)
