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# numpy.diag

`numpy.``diag`(v, k=0)

  Extract a diagonal or construct a diagonal array.

  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 triange 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]])

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