/// <summary>
/// Extract a diagonal or construct a diagonal array.
/// </summary>
/// <param name="v">input array</param>
/// <param name="k">Diagonal in question. The default is 0. Use `k>0` for diagonals above the main diagonal, and `k<0` for diagonals below the main diagonal.</param>
/// <returns></returns>
public static ndarray diag(ndarray v, int k = 0)
{
/*
* Extract a diagonal or construct a diagonal array.
*
* See the more detailed documentation for ``numpy.diagonal`` if you use this
* function to extract a diagonal and wish to write to the resulting array;
* whether it returns a copy or a view depends on what version of numpy you
* are using.
*
* Parameters
* ----------
* v : array_like
* If `v` is a 2-D array, return a copy of its `k`-th diagonal.
* If `v` is a 1-D array, return a 2-D array with `v` on the `k`-th
* diagonal.
* k : int, optional
* Diagonal in question. The default is 0. Use `k>0` for diagonals
* above the main diagonal, and `k<0` for diagonals below the main
* diagonal.
*
* Returns
* -------
* out : ndarray
* The extracted diagonal or constructed diagonal array.
*
* See Also
* --------
* diagonal : Return specified diagonals.
* diagflat : Create a 2-D array with the flattened input as a diagonal.
* trace : Sum along diagonals.
* triu : Upper triangle of an array.
* tril : Lower triangle of an array.
*
* Examples
* --------
* >>> x = np.arange(9).reshape((3,3))
* >>> x
* array([[0, 1, 2],
* [3, 4, 5],
* [6, 7, 8]])
*
* >>> np.diag(x)
* array([0, 4, 8])
* >>> np.diag(x, k=1)
* array([1, 5])
* >>> np.diag(x, k=-1)
* array([3, 7])
*
* >>> np.diag(np.diag(x))
* array([[0, 0, 0],
* [0, 4, 0],
* [0, 0, 8]])
*/
int i;
v = asanyarray(v);
var s = v.dims;
if (len(s) == 1)
{
int n = (int)(s[0] + Math.Abs(k));
var res = zeros(new shape(n, n), dtype: v.Dtype);
if (k >= 0)
{
i = k;
}
else
{
i = (-k) * n;
}
res.A(":" + (n - k).ToString()).Flat[i.ToString() + "::" + (n + 1).ToString()] = v;
return(res);
}
else if (len(s) == 2)
{
return(v.diagonal(k));
}
else
{
throw new ValueError("Input must be 1- or 2-d.");
}
}