.. testsetup::

   import numpy as np
   import theano
   import theano.tensor as tt

.. _tutbroadcasting:

============
Broadcasting
============

Broadcasting is a mechanism which allows tensors with
different numbers of dimensions to be added or multiplied
together by (virtually) replicating the smaller tensor along
the dimensions that it is lacking.

Broadcasting is the mechanism by which a scalar
may be added to a matrix, a vector to a matrix or a scalar to
a vector.

.. figure:: bcast.png

Broadcasting a row matrix. T and F respectively stand for
True and False and indicate along which dimensions we allow
broadcasting.

If the second argument were a vector, its shape would be
``(2,)`` and its broadcastable pattern ``(False,)``. They would
be automatically expanded to the **left** to match the
dimensions of the matrix (adding ``1`` to the shape and ``True``
to the pattern), resulting in ``(1, 2)`` and ``(True, False)``.
It would then behave just like the example above.

Unlike numpy which does broadcasting dynamically, Theano needs
to know, for any operation which supports broadcasting, which
dimensions will need to be broadcasted. When applicable, this
information is given in the :ref:`type` of a *Variable*.

The following code illustrates how rows and columns are broadcasted in order to perform an addition operation with a matrix:

>>> r = tt.row()
>>> r.broadcastable
(True, False)
>>> mtr = tt.matrix()
>>> mtr.broadcastable
(False, False)
>>> f_row = theano.function([r, mtr], [r + mtr])
>>> R = np.arange(3).reshape(1, 3)
>>> R
array([[0, 1, 2]])
>>> M = np.arange(9).reshape(3, 3)
>>> M
array([[0, 1, 2],
       [3, 4, 5],
       [6, 7, 8]])
>>> f_row(R, M)
[array([[  0.,   2.,   4.],
       [  3.,   5.,   7.],
       [  6.,   8.,  10.]])]
>>> c = tt.col()
>>> c.broadcastable
(False, True)
>>> f_col = theano.function([c, mtr], [c + mtr])
>>> C = np.arange(3).reshape(3, 1)
>>> C
array([[0],
       [1],
       [2]])
>>> M = np.arange(9).reshape(3, 3)
>>> f_col(C, M)
[array([[  0.,   1.,   2.],
       [  4.,   5.,   6.],
       [  8.,   9.,  10.]])]

In these examples, we can see that both the row vector and the column vector are broadcasted in order to be be added to the matrix.

See also:

* `SciPy documentation about numpy's broadcasting <http://www.scipy.org/EricsBroadcastingDoc>`_

* `OnLamp article about numpy's broadcasting <http://www.onlamp.com/pub/a/python/2000/09/27/numerically.html>`_
