Thread: Why is this happening?

Code:

#include <iostream>

int main()
{
    int lines;
    int columns;
    
    std::cout << "lines: ";
    std::cin >> lines;
    
    std::cout << "columns: ";
    std::cin >> columns;
    
    int *matrix1 = new int[lines * columns]; 
    
    for (int i = 0; i < lines; i++)
        for (int j = 0; j < columns; j++) {
            std::cout << "matrix1[" << i << "][" << j << "]: ";
            std::cin >> matrix1[i * j + j];
        }
    
    for (int i = 0; i < lines; i++)
        for (int j = 0; j < columns; j++)
            std::cout << matrix1[i * j + j] << std::endl;
    
    delete [] matrix1;
}

If I type
1
2
3
4
5
6
7
8
9

I get
7
2
5
7
5
6
7
8
9