.. _array.assignment:

Assignment
==========

Dask Array supports most of the NumPy assignment indexing syntax. In
particular, it supports combinations of the following:

* Indexing by integers: ``x[1] = y``
* Indexing by slices: ``x[2::-1] = y``
* Indexing by a list of integers: ``x[[0, -1, 1]] = y``
* Indexing by a 1-d :class:`numpy` array of integers: ``x[np.arange(3)] = y``
* Indexing by a 1-d :class:`~dask.array.Array` of integers: ``x[da.arange(3)] = y``, ``x[da.from_array([0, -1, 1])] = y``, ``x[da.where(np.array([1, 2, 3]) < 3)[0]] = y``
* Indexing by a list of booleans: ``x[[False, True, True]] = y``
* Indexing by a 1-d :class:`numpy` array of booleans: ``x[np.arange(3) > 0] = y``

It also supports:

* Indexing by one broadcastable :class:`~dask.array.Array` of
  booleans: ``x[x > 0] = y``.

However, it does not currently support the following:

* Indexing with lists in multiple axes: ``x[[1, 2, 3], [3, 1, 2]] = y``


.. _array.assignment.broadcasting:

Broadcasting
------------

The normal NumPy broadcasting rules apply:

.. code-block:: python

   >>> x = da.zeros((2, 6))
   >>> x[0] = 1
   >>> x[..., 1] = 2.0
   >>> x[:, 2] = [3, 4]
   >>> x[:, 5:2:-2] = [[6, 5]]
   >>> x.compute()
   array([[1., 2., 3., 5., 1., 6.],
          [0., 2., 4., 5., 0., 6.]])
   >>> x[1] = -x[0]
   >>> x.compute()
   array([[ 1.,  2.,  3.,  5.,  1.,  6.],
          [-1., -2., -3., -5., -1., -6.]])

.. _array.assignment.masking:

Masking
-------

Elements may be masked by assigning to the NumPy masked value, or to an
array with masked values:

.. code-block:: python

   >>> x = da.ones((2, 6))
   >>> x[0, [1, -2]] = np.ma.masked
   >>> x[1] = np.ma.array([0, 1, 2, 3, 4, 5], mask=[0, 1, 1, 0, 0, 0])
   >>> print(x.compute())
   [[1.0 -- 1.0 1.0 -- 1.0]
    [0.0 -- -- 3.0 4.0 5.0]]
   >>> x[:, 0] = x[:, 1]
   >>> print(x.compute())
   [[1.0 -- 1.0 1.0 -- 1.0]
    [0.0 -- -- 3.0 4.0 5.0]]
   >>> x[:, 0] = x[:, 1]
   >>> print(x.compute())
   [[-- -- 1.0 1.0 -- 1.0]
    [-- -- -- 3.0 4.0 5.0]]

If, and only if, a single broadcastable :class:`~dask.array.Array` of
booleans is provided then masked array assignment does not yet work as
expected. In this case the data underlying the mask are assigned:

.. code-block:: python

   >>> x = da.arange(12).reshape(2, 6)
   >>> x[x > 7] = np.ma.array(-99, mask=True)
   >>> print(x.compute())
   [[  0   1   2   3   4   5]
    [  6   7 -99 -99 -99 -99]]

Note that masked assignments do work when a boolean
:class:`~dask.array.Array` index used in a tuple, or implicit tuple,
of indices:

.. code-block:: python

   >>> x = da.arange(12).reshape(2, 6)
   >>> x[1, x[0] > 3] = np.ma.masked
   >>> print(x.compute())
   [[0 1 2 3 4 5]
    [6 7 8 9 -- --]]
   >>> x = da.arange(12).reshape(2, 6)
   >>> print(x.compute())
   [[ 0  1  2  3  4  5]
    [ 6  7  8  9 10 11]]
   >>> x[(x[:, 2] < 4,)] = np.ma.masked
   >>> print(x.compute())
   [[-- -- -- -- -- --]
    [6 7 8 9 10 11]]