# [SOLVED] How to get reverse diagonal from a certain point in a 2d numpy array

## Issue

Let’s say I have a n x m numpy array. For example:

``````array([[ 1,  2,  3,  4,  5],
[ 6,  7,  8,  9, 10],
[11, 12, 13, 14, 15],
[16, 17, 18, 19, 20]])
``````

Now I want both diagonals intersection with a certain point (for example (1,2) which is 8). I already know that I can get the diagonal from top to bottom like so:

``````row = 1
col = 2
a = np.array(
[[1, 2, 3, 4, 5], [6, 7, 8, 9, 10], [11, 12, 13, 14, 15], [16, 17, 18, 19, 20]]
)
diagonal_1 = a.diagonal(col - row)
``````

Where the result of `col - row` gives the offset for the diagonal.

Now I want to also have the reverse diagonal (from bottom to top) intersecting with the first diagonal in a random point (in this case (1,2), but it can be any point). But for this example it would be:

``````[16, 12, 8, 4]
``````

I already tried a bunch with rotating and flipping the matrix. But I can’t get a hold on the offset which I should use after rotating or flipping the matrix.

## Solution

You can use `np.eye` to create a diagnal line of 1’s, and use that as a mask:

``````x, y = np.nonzero(a == 8)

k = y - a.shape + x + 1
nums = a[np.eye(*a.shape, k=k)[::-1].astype(bool)][::-1]
``````

Output:

``````>>> nums
array([16, 12,  8,  4])
``````

If you need to move the position of the line, increment/decrement the `k` parameter passed to `np.eye`.