How to print all the diagonal elements of a 2d vector

Question

I'm looking for a C++ way using vector to get all the diagonals of a (square) matrix, represented as a 2d vector.

matrix = [
[1,2,3,4],
[5,1,2,3],
[9,5,1,2]]

But I have trouble coming up with a way to generate all the diagonals. The output I'm looking for is:

"[9]", "[5, 5]", "[1, 1, 1]", "[2, 2, 2]", "[3, 3]", "[4]".

I found answers but all of them are in python. I need it in C++

Can you show the program you've already written, and explain how exactly your program doesn't work or doesn't produce the expected results? You have to show your work first; it must meet all requirements for a [mre]; and it must be a good-faith real attempt to implement your program ***and not a few token lines of code*** (like what you showed), before asking for help on stackoverflow.com. We don't write entire programs for other people, here. For more information, see [ask] questions, take the [tour], and read the [help]. — Sam Varshavchik, Mar 28 '20 at 00:57

A M · Accepted Answer · 2020-03-28T07:27:03.067

The solution is rather straigt forward. I will first show the code and then explain it.

#include <iostream>
#include <vector>

using Matrix = std::vector<std::vector<int>>;

int main() {

    Matrix matrix{
        {1,2,3,4},
        {5,1,2,3},
        {9,5,1,2}
    };
    // Shortcut for the width and height of the matrix
    const size_t width{ matrix.at(0).size() };
    const size_t height{ matrix.size() };

    // Set start row and start column
    size_t startRow{ height-1 };
    size_t startColumn{ 0 };
    size_t column{};

    // for all possible start positions
    do {
        // set the row and column values to the start values
        size_t row{ startRow };
        column = startColumn;

        //Now go through the diagonal
        do {
            // Show current value
            std::cout << matrix[row][column] << ' ';

            // Set next position in the diagonal
            ++row;      // Go one row down
            ++column;   // Go one column to the right

          // As long as we do not cross the border
        } while ((row < height) && (column < width));

        std::cout << '\n';

        // Calculate new start row and start column
        // If possible
        if (startRow > 0) {
            // Go up
            --startRow;
        }
        else {
            // Else go right
            ++startColumn;
        }
    } while (startColumn < width);

    return 0;
}

So, we need to play a little bit with indices.

The indices for a diagonal are very simple. Simply start at a startPosition,then we increment the row and the column in each step, until we hit the border.

For the start positions of the diagonal, we will start at the low left border. Then we will keep the column and decrement the row. If the row is 0 then we go to the right, until we hit the border.

All this can be formulated very easily in C++.

While you generally want to ensure the OP has put effort into solving the problem on his own and isn't using SO as a coding-service (or to get out of doing homework), you have put effort into your answer and it is worth the UV. Give a man a fish -- feed him for a day, teach a man to fish -- feed him for life `:)` — David C. Rankin, Mar 28 '20 at 07:54

How to print all the diagonal elements of a 2d vector

1 Answers1