jax.numpy.block

jax.numpy.block(arrays)[source]

Assemble an nd-array from nested lists of blocks.

LAX-backend implementation of block(). Original docstring below.

Blocks in the innermost lists are concatenated (see concatenate) along the last dimension (-1), then these are concatenated along the second-last dimension (-2), and so on until the outermost list is reached.

Blocks can be of any dimension, but will not be broadcasted using the normal rules. Instead, leading axes of size 1 are inserted, to make block.ndim the same for all blocks. This is primarily useful for working with scalars, and means that code like np.block([v, 1]) is valid, where v.ndim == 1.

When the nested list is two levels deep, this allows block matrices to be constructed from their components.

New in version 1.13.0.

Returns

block_array – The array assembled from the given blocks.

The dimensionality of the output is equal to the greatest of: * the dimensionality of all the inputs * the depth to which the input list is nested

Return type

ndarray

Raises

ValueError

  • If list depths are mismatched - for instance, [[a, b], c] is illegal, and should be spelt [[a, b], [c]] * If lists are empty - for instance, [[a, b], []]

See also

concatenate()

Join a sequence of arrays together.

stack()

Stack arrays in sequence along a new dimension.

hstack()

Stack arrays in sequence horizontally (column wise).

vstack()

Stack arrays in sequence vertically (row wise).

dstack()

Stack arrays in sequence depth wise (along third dimension).

vsplit()

Split array into a list of multiple sub-arrays vertically.

Notes

When called with only scalars, np.block is equivalent to an ndarray call. So np.block([[1, 2], [3, 4]]) is equivalent to np.array([[1, 2], [3, 4]]).

This function does not enforce that the blocks lie on a fixed grid. np.block([[a, b], [c, d]]) is not restricted to arrays of the form:

AAAbb
AAAbb
cccDD

But is also allowed to produce, for some a, b, c, d:

AAAbb
AAAbb
cDDDD

Since concatenation happens along the last axis first, block is _not_ capable of producing the following directly:

AAAbb
cccbb
cccDD

Matlab’s “square bracket stacking”, [A, B, ...; p, q, ...], is equivalent to np.block([[A, B, ...], [p, q, ...]]).

Examples

The most common use of this function is to build a block matrix

>>> A = np.eye(2) * 2
>>> B = np.eye(3) * 3
>>> np.block([
...     [A,               np.zeros((2, 3))],
...     [np.ones((3, 2)), B               ]
... ])
array([[2., 0., 0., 0., 0.],
       [0., 2., 0., 0., 0.],
       [1., 1., 3., 0., 0.],
       [1., 1., 0., 3., 0.],
       [1., 1., 0., 0., 3.]])

With a list of depth 1, block can be used as hstack

>>> np.block([1, 2, 3])              # hstack([1, 2, 3])
array([1, 2, 3])
>>> a = np.array([1, 2, 3])
>>> b = np.array([2, 3, 4])
>>> np.block([a, b, 10])             # hstack([a, b, 10])
array([ 1,  2,  3,  2,  3,  4, 10])
>>> A = np.ones((2, 2), int)
>>> B = 2 * A
>>> np.block([A, B])                 # hstack([A, B])
array([[1, 1, 2, 2],
       [1, 1, 2, 2]])

With a list of depth 2, block can be used in place of vstack:

>>> a = np.array([1, 2, 3])
>>> b = np.array([2, 3, 4])
>>> np.block([[a], [b]])             # vstack([a, b])
array([[1, 2, 3],
       [2, 3, 4]])
>>> A = np.ones((2, 2), int)
>>> B = 2 * A
>>> np.block([[A], [B]])             # vstack([A, B])
array([[1, 1],
       [1, 1],
       [2, 2],
       [2, 2]])

It can also be used in places of atleast_1d and atleast_2d

>>> a = np.array(0)
>>> b = np.array([1])
>>> np.block([a])                    # atleast_1d(a)
array([0])
>>> np.block([b])                    # atleast_1d(b)
array([1])
>>> np.block([[a]])                  # atleast_2d(a)
array([[0]])
>>> np.block([[b]])                  # atleast_2d(b)
array([[1]])