##### https://github.com/tensorly/tensorly
Tip revision: 0fa8ab5
_tucker.py
``````import tensorly as tl
from ..base import unfold
from ..tenalg import multi_mode_dot, mode_dot
from ..tucker_tensor import tucker_to_tensor
from ..random import check_random_state
from math import sqrt

import warnings

# Author: Jean Kossaifi <jean.kossaifi+tensors@gmail.com>

def partial_tucker(tensor, modes, rank=None, n_iter_max=100, init='svd', tol=10e-5,
svd='numpy_svd', random_state=None, verbose=False, ranks=None):
"""Partial tucker decomposition via Higher Order Orthogonal Iteration (HOI)

Decomposes `tensor` into a Tucker decomposition exclusively along the provided modes.

Parameters
----------
tensor : ndarray
modes : int list
list of the modes on which to perform the decomposition
ranks : None or int list
size of the core tensor, ``(len(ranks) == len(modes))``
n_iter_max : int
maximum number of iteration
init : {'svd', 'random'}, optional
svd : str, default is 'numpy_svd'
function to use to compute the SVD,
acceptable values in tensorly.SVD_FUNS
tol : float, optional
tolerance: the algorithm stops when the variation in
the reconstruction error is less than the tolerance
random_state : {None, int, np.random.RandomState}
verbose : int, optional
level of verbosity

Returns
-------
core : ndarray
core tensor of the Tucker decomposition
factors : ndarray list
list of factors of the Tucker decomposition.
with ``core.shape[i] == (tensor.shape[i], ranks[i]) for i in modes``

"""
if ranks is not None:
warnings.warn(message, DeprecationWarning)
rank = ranks

if rank is None:
message = "No value given for 'rank'. The decomposition will preserve the original size."
warnings.warn(message, Warning)
rank = [tl.shape(tensor)[mode] for mode in modes]
elif isinstance(rank, int):
message = "Given only one int for 'rank' intead of a list of {} modes. Using this rank for all modes.".format(len(modes))
warnings.warn(message, Warning)
rank = [rank for _ in modes]

try:
svd_fun = tl.SVD_FUNS[svd]
except KeyError:
message = 'Got svd={}. However, for the current backend ({}), the possible choices are {}'.format(
svd, tl.get_backend(), tl.SVD_FUNS)
raise ValueError(message)

# SVD init
if init == 'svd':
factors = []
for index, mode in enumerate(modes):
eigenvecs, _, _ = svd_fun(unfold(tensor, mode), n_eigenvecs=rank[index])
factors.append(eigenvecs)
else:
rng = check_random_state(random_state)
core = tl.tensor(rng.random_sample(rank), **tl.context(tensor))
factors = [tl.tensor(rng.random_sample((tl.shape(tensor)[mode], rank[index])), **tl.context(tensor)) for (index, mode) in enumerate(modes)]

rec_errors = []
norm_tensor = tl.norm(tensor, 2)

for iteration in range(n_iter_max):
for index, mode in enumerate(modes):
core_approximation = multi_mode_dot(tensor, factors, modes=modes, skip=index, transpose=True)
eigenvecs, _, _ = svd_fun(unfold(core_approximation, mode), n_eigenvecs=rank[index])
factors[index] = eigenvecs

core = multi_mode_dot(tensor, factors, modes=modes, transpose=True)

# The factors are orthonormal and therefore do not affect the reconstructed tensor's norm
rec_error = sqrt(abs(norm_tensor**2 - tl.norm(core, 2)**2)) / norm_tensor
rec_errors.append(rec_error)

if iteration > 1:
if verbose:
print('reconsturction error={}, variation={}.'.format(
rec_errors[-1], rec_errors[-2] - rec_errors[-1]))

if tol and abs(rec_errors[-2] - rec_errors[-1]) < tol:
if verbose:
print('converged in {} iterations.'.format(iteration))
break

return core, factors

def tucker(tensor, rank=None, ranks=None, n_iter_max=100, init='svd',
svd='numpy_svd', tol=10e-5, random_state=None, verbose=False):
"""Tucker decomposition via Higher Order Orthogonal Iteration (HOI)

Decomposes `tensor` into a Tucker decomposition:
``tensor = [| core; factors[0], ...factors[-1] |]`` [1]_

Parameters
----------
tensor : ndarray
ranks : None or int list
size of the core tensor, ``(len(ranks) == tensor.ndim)``
n_iter_max : int
maximum number of iteration
init : {'svd', 'random'}, optional
svd : str, default is 'numpy_svd'
function to use to compute the SVD,
acceptable values in tensorly.SVD_FUNS
tol : float, optional
tolerance: the algorithm stops when the variation in
the reconstruction error is less than the tolerance
random_state : {None, int, np.random.RandomState}
verbose : int, optional
level of verbosity

Returns
-------
core : ndarray of size `ranks`
core tensor of the Tucker decomposition
factors : ndarray list
list of factors of the Tucker decomposition.
Its ``i``-th element is of shape ``(tensor.shape[i], ranks[i])``

References
----------
.. [1] tl.G.Kolda and B.W.Bader, "Tensor Decompositions and Applications",
SIAM REVIEW, vol. 51, n. 3, pp. 455-500, 2009.
"""
modes = list(range(tl.ndim(tensor)))
return partial_tucker(tensor, modes, rank=rank, ranks=ranks, n_iter_max=n_iter_max, init=init,
svd=svd, tol=tol, random_state=random_state, verbose=verbose)

def non_negative_tucker(tensor, rank, n_iter_max=10, init='svd', tol=10e-5,
random_state=None, verbose=False, ranks=None):
"""Non-negative Tucker decomposition

Iterative multiplicative update, see [2]_

Parameters
----------
tensor : ``ndarray``
rank   : int
number of components
n_iter_max : int
maximum number of iteration
init : {'svd', 'random'}
random_state : {None, int, np.random.RandomState}

Returns
-------
core : ndarray
positive core of the Tucker decomposition
has shape `ranks`
factors : ndarray list
list of factors of the CP decomposition
element `i` is of shape ``(tensor.shape[i], rank)``

References
----------
.. [2] Yong-Deok Kim and Seungjin Choi,
"Nonnegative tucker decomposition",
IEEE Conference on Computer Vision and Pattern Recognition s(CVPR),
pp 1-8, 2007
"""
if ranks is not None:
warnings.warn(message, DeprecationWarning)
rank = ranks

if rank is None:
rank = [tl.shape(tensor)[mode] for mode in range(tl.ndim(tensor))]

elif isinstance(rank, int):
n_mode = tl.ndim(tensor)
message = "Given only one int for 'rank' for decomposition a tensor of order {}. Using this rank for all modes.".format(n_mode)
warnings.warn(message, RuntimeWarning)
rank = [rank]*n_mode

epsilon = 10e-12

# Initialisation
if init == 'svd':
core, factors = tucker(tensor, rank)
nn_factors = [tl.abs(f) for f in factors]
nn_core = tl.abs(core)
else:
rng = check_random_state(random_state)
core = tl.tensor(rng.random_sample(rank) + 0.01, **tl.context(tensor))  # Check this
factors = [tl.tensor(rng.random_sample(s), **tl.context(tensor)) for s in zip(tl.shape(tensor), rank)]
nn_factors = [tl.abs(f) for f in factors]
nn_core = tl.abs(core)

norm_tensor = tl.norm(tensor, 2)
rec_errors = []

for iteration in range(n_iter_max):
for mode in range(tl.ndim(tensor)):
B = tucker_to_tensor((nn_core, nn_factors), skip_factor=mode)
B = tl.transpose(unfold(B, mode))

numerator = tl.dot(unfold(tensor, mode), B)
numerator = tl.clip(numerator, a_min=epsilon, a_max=None)
denominator = tl.dot(nn_factors[mode], tl.dot(tl.transpose(B), B))
denominator = tl.clip(denominator, a_min=epsilon, a_max=None)
nn_factors[mode] *= numerator / denominator

numerator = tucker_to_tensor((tensor, nn_factors), transpose_factors=True)
numerator = tl.clip(numerator, a_min=epsilon, a_max=None)
for i, f in enumerate(nn_factors):
if i:
denominator = mode_dot(denominator, tl.dot(tl.transpose(f), f), i)
else:
denominator = mode_dot(nn_core, tl.dot(tl.transpose(f), f), i)
denominator = tl.clip(denominator, a_min=epsilon, a_max=None)
nn_core *= numerator / denominator

rec_error = tl.norm(tensor - tucker_to_tensor((nn_core, nn_factors)), 2) / norm_tensor
rec_errors.append(rec_error)
if iteration > 1 and verbose:
print('reconsturction error={}, variation={}.'.format(
rec_errors[-1], rec_errors[-2] - rec_errors[-1]))

if iteration > 1 and abs(rec_errors[-2] - rec_errors[-1]) < tol:
if verbose:
print('converged in {} iterations.'.format(iteration))
break

return nn_core, nn_factors
``````