def graph_implementation(self,
arg_objs,
shape: Tuple[int, ...],
data=None) -> Tuple[lo.LinOp, List[Constraint]]:
"""Index/slice into the expression.
Parameters
----------
arg_objs : list
LinExpr for each argument.
shape : tuple
The shape of the resulting expression.
data : tuple
A tuple of slices.
Returns
-------
tuple
(LinOp, [constraints])
"""
select_mat = self._select_mat
final_shape = self._select_mat.shape
select_vec = np.reshape(select_mat, select_mat.size, order='F')
# Select the chosen entries from expr.
arg = arg_objs[0]
identity = sp.eye(self.args[0].size).tocsc()
vec_arg = lu.reshape(arg, (self.args[0].size, ))
mul_mat = identity[select_vec]
mul_const = lu.create_const(mul_mat, mul_mat.shape, sparse=True)
mul_expr = lu.mul_expr(mul_const, vec_arg, (mul_mat.shape[0], ))
obj = lu.reshape(mul_expr, final_shape)
return (obj, [])
def graph_implementation(self, arg_objs, shape: Tuple[int, ...], data=None):
"""Reshape
Parameters
----------
arg_objs : list
LinExpr for each argument.
shape : tuple
The shape of the resulting expression.
data :
Additional data required by the atom.
Returns
-------
tuple
(LinOp for objective, list of constraints)
"""
arg = arg_objs[0]
if data[1] == 'F':
return (lu.reshape(arg, shape), [])
else: # 'C':
arg = lu.transpose(arg)
if len(shape) <= 1:
return (lu.reshape(arg, shape), [])
else:
result = lu.reshape(arg, (shape[1], shape[0]))
return (lu.transpose(result), [])
def gm(t, x, y):
length = t.size[0] * t.size[1]
return SOC_Axis(
lu.reshape(lu.sum_expr([x, y]), (length, 1)),
lu.vstack([
lu.reshape(lu.sub_expr(x, y), (1, length)),
lu.reshape(lu.mul_expr(two, t, t.size), (1, length))
], (2, length)), 0)
def gm(t, x, y):
length = t.size[0]*t.size[1]
return SOC_Axis(lu.reshape(lu.sum_expr([x, y]), (length, 1)),
lu.vstack([
lu.reshape(lu.sub_expr(x, y), (1, length)),
lu.reshape(lu.mul_expr(two, t, t.size), (1, length))
], (2, length)),
0)
def _scaled_lower_tri(self):
"""Returns a LinOp representing the lower triangular entries.
Scales the strictly lower triangular entries by
sqrt(2) as required by SCS.
"""
rows = cols = self.size[0]
entries = rows*(cols + 1)//2
val_arr = []
row_arr = []
col_arr = []
count = 0
for j in range(cols):
for i in range(rows):
if j <= i:
# Index in the original matrix.
col_arr.append(j*rows + i)
# Index in the extracted vector.
row_arr.append(count)
if j == i:
val_arr.append(1.0)
else:
val_arr.append(np.sqrt(2))
count += 1
size = (entries, rows*cols)
coeff = sp.coo_matrix((val_arr, (row_arr, col_arr)), size).tocsc()
coeff = lu.create_const(coeff, size, sparse=True)
vect = lu.reshape(self.A, (rows*cols, 1))
return lu.mul_expr(coeff, vect, (entries, 1))
def format_axis(t, X, axis):
"""Formats all the row/column cones for the solver.
Parameters
----------
t: The scalar part of the second-order constraint.
X: A matrix whose rows/columns are each a cone.
axis: Slice by column 0 or row 1.
Returns
-------
list
A list of LinLeqConstr that represent all the elementwise cones.
"""
# Reduce to norms of columns.
if axis == 1:
X = lu.transpose(X)
# Create matrices Tmat, Xmat such that Tmat*t + Xmat*X
# gives the format for the elementwise cone constraints.
cone_size = 1 + X.shape[0]
terms = []
# Make t_mat
mat_shape = (cone_size, 1)
t_mat = sp.coo_matrix(([1.0], ([0], [0])), mat_shape).tocsc()
t_mat = lu.create_const(t_mat, mat_shape, sparse=True)
t_vec = t
if not t.shape:
# t is scalar
t_vec = lu.reshape(t, (1, 1))
else:
# t is 1D
t_vec = lu.reshape(t, (1, t.shape[0]))
mul_shape = (cone_size, t_vec.shape[1])
terms += [lu.mul_expr(t_mat, t_vec, mul_shape)]
# Make X_mat
if len(X.shape) == 1:
X = lu.reshape(X, (X.shape[0], 1))
mat_shape = (cone_size, X.shape[0])
val_arr = (cone_size - 1) * [1.0]
row_arr = list(range(1, cone_size))
col_arr = list(range(cone_size - 1))
X_mat = sp.coo_matrix((val_arr, (row_arr, col_arr)), mat_shape).tocsc()
X_mat = lu.create_const(X_mat, mat_shape, sparse=True)
mul_shape = (cone_size, X.shape[1])
terms += [lu.mul_expr(X_mat, X, mul_shape)]
return [lu.create_geq(lu.sum_expr(terms))]
def canonicalize(self):
"""Variable must be semidefinite and symmetric.
"""
upper_tri = lu.create_var((self.size[0], 1), self.id)
fill_coeff = upper_tri_to_full(self.n)
fill_coeff = lu.create_const(fill_coeff, (self.n*self.n, self.size[0]),
sparse=True)
full_mat = lu.mul_expr(fill_coeff, upper_tri, (self.n*self.n, 1))
full_mat = lu.reshape(full_mat, (self.n, self.n))
return (upper_tri, [SDP(full_mat, enforce_sym=False)])
def graph_implementation(arg_objs, size, data=None):
"""Convolve two vectors.
Parameters
----------
arg_objs : list
LinExpr for each argument.
size : tuple
The size of the resulting expression.
data :
Additional data required by the atom.
Returns
-------
tuple
(LinOp for objective, list of constraints)
"""
return (lu.reshape(arg_objs[0], size), [])
def graph_implementation(arg_objs, size, data=None):
"""Convolve two vectors.
Parameters
----------
arg_objs : list
LinExpr for each argument.
size : tuple
The size of the resulting expression.
data :
Additional data required by the atom.
Returns
-------
tuple
(LinOp for objective, list of constraints)
"""
return (lu.reshape(arg_objs[0], size), [])
def combine_lin_ops(operators):
"""Combines the LinOps by stacking their output into a vector.
Parameters
----------
operators : list
A list of LinOps.
Returns
-------
LinOp
The combined LinOp.
"""
# First vectorize all the LinOp outputs.
vect_lin_ops = []
total_length = 0
for lin_op in operators:
if lin_op.size[1] != 1:
new_size = (lin_op.size[0] * lin_op.size[1], 1)
lin_op = lu.reshape(lin_op, new_size)
vect_lin_ops.append(lin_op)
total_length += lin_op.size[0]
# Stack all the outputs into a single vector.
return lu.vstack(vect_lin_ops, (total_length, 1))
def graph_implementation(arg_objs, size, data=None):
r"""Reduces the atom to an affine expression and list of constraints.
Parameters
----------
arg_objs : list
LinExpr for each argument.
size : tuple
The size of the resulting expression.
data :
Additional data required by the atom.
Returns
-------
tuple
(LinOp for objective, list of constraints)
Notes
-----
Implementation notes.
- For general :math:`p \geq 1`, the inequality :math:`\|x\|_p \leq t`
is equivalent to the following convex inequalities:
.. math::
|x_i| &\leq r_i^{1/p} t^{1 - 1/p}\\
\sum_i r_i &= t.
These inequalities happen to also be correct for :math:`p = +\infty`,
if we interpret :math:`1/\infty` as :math:`0`.
- For general :math:`0 < p < 1`, the inequality :math:`\|x\|_p \geq t`
is equivalent to the following convex inequalities:
.. math::
r_i &\leq x_i^{p} t^{1 - p}\\
\sum_i r_i &= t.
- For general :math:`p < 0`, the inequality :math:`\|x\|_p \geq t`
is equivalent to the following convex inequalities:
.. math::
t &\leq x_i^{-p/(1-p)} r_i^{1/(1 - p)}\\
\sum_i r_i &= t.
Although the inequalities above are correct, for a few special cases,
we can represent the p-norm more efficiently and with fewer variables and inequalities.
- For :math:`p = 1`, we use the representation
.. math::
x_i &\leq r_i\\
-x_i &\leq r_i\\
\sum_i r_i &= t
- For :math:`p = \infty`, we use the representation
.. math::
x_i &\leq t\\
-x_i &\leq t
Note that we don't need the :math:`r` variable or the sum inequality.
- For :math:`p = 2`, we use the natural second-order cone representation
.. math::
\|x\|_2 \leq t
Note that we could have used the set of inequalities given above if we wanted
an alternate decomposition of a large second-order cone into into several
smaller inequalities.
"""
p = data[0]
axis = data[1]
x = arg_objs[0]
t = lu.create_var((1, 1))
constraints = []
# first, take care of the special cases of p = 2, inf, and 1
if p == 2:
if axis is None:
return t, [SOC(t, [x])]
else:
t = lu.create_var(size)
return t, [
SOC_Axis(lu.reshape(t, (t.size[0] * t.size[1], 1)), x,
axis)
]
if p == np.inf:
t_ = lu.promote(t, x.size)
return t, [
lu.create_leq(x, t_),
lu.create_geq(lu.sum_expr([x, t_]))
]
# we need an absolute value constraint for the symmetric convex branches (p >= 1)
# we alias |x| as x from this point forward to make the code pretty :)
if p >= 1:
absx = lu.create_var(x.size)
constraints += [
lu.create_leq(x, absx),
lu.create_geq(lu.sum_expr([x, absx]))
]
x = absx
if p == 1:
return lu.sum_entries(x), constraints
# now, we take care of the remaining convex and concave branches
# to create the rational powers, we need a new variable, r, and
# the constraint sum(r) == t
r = lu.create_var(x.size)
t_ = lu.promote(t, x.size)
constraints += [lu.create_eq(lu.sum_entries(r), t)]
# make p a fraction so that the input weight to gm_constrs
# is a nice tuple of fractions.
p = Fraction(p)
if p < 0:
constraints += gm_constrs(t_, [x, r], (-p / (1 - p), 1 / (1 - p)))
if 0 < p < 1:
constraints += gm_constrs(r, [x, t_], (p, 1 - p))
if p > 1:
constraints += gm_constrs(x, [r, t_], (1 / p, 1 - 1 / p))
return t, constraints
def graph_implementation(arg_objs, size, data=None):
r"""Reduces the atom to an affine expression and list of constraints.
Parameters
----------
arg_objs : list
LinExpr for each argument.
size : tuple
The size of the resulting expression.
data :
Additional data required by the atom.
Returns
-------
tuple
(LinOp for objective, list of constraints)
Notes
-----
Implementation notes.
- For general :math:`p \geq 1`, the inequality :math:`\|x\|_p \leq t`
is equivalent to the following convex inequalities:
.. math::
|x_i| &\leq r_i^{1/p} t^{1 - 1/p}\\
\sum_i r_i &= t.
These inequalities happen to also be correct for :math:`p = +\infty`,
if we interpret :math:`1/\infty` as :math:`0`.
- For general :math:`0 < p < 1`, the inequality :math:`\|x\|_p \geq t`
is equivalent to the following convex inequalities:
.. math::
r_i &\leq x_i^{p} t^{1 - p}\\
\sum_i r_i &= t.
- For general :math:`p < 0`, the inequality :math:`\|x\|_p \geq t`
is equivalent to the following convex inequalities:
.. math::
t &\leq x_i^{-p/(1-p)} r_i^{1/(1 - p)}\\
\sum_i r_i &= t.
Although the inequalities above are correct, for a few special cases,
we can represent the p-norm more efficiently and with fewer variables and inequalities.
- For :math:`p = 1`, we use the representation
.. math::
x_i &\leq r_i\\
-x_i &\leq r_i\\
\sum_i r_i &= t
- For :math:`p = \infty`, we use the representation
.. math::
x_i &\leq t\\
-x_i &\leq t
Note that we don't need the :math:`r` variable or the sum inequality.
- For :math:`p = 2`, we use the natural second-order cone representation
.. math::
\|x\|_2 \leq t
Note that we could have used the set of inequalities given above if we wanted
an alternate decomposition of a large second-order cone into into several
smaller inequalities.
"""
p = data[0]
axis = data[1]
x = arg_objs[0]
t = lu.create_var((1, 1))
constraints = []
# first, take care of the special cases of p = 2, inf, and 1
if p == 2:
if axis is None:
return t, [SOC(t, [x])]
else:
t = lu.create_var(size)
return t, [SOC_Axis(lu.reshape(t, (t.size[0]*t.size[1], 1)),
x, axis)]
if p == np.inf:
t_ = lu.promote(t, x.size)
return t, [lu.create_leq(x, t_), lu.create_geq(lu.sum_expr([x, t_]))]
# we need an absolute value constraint for the symmetric convex branches (p >= 1)
# we alias |x| as x from this point forward to make the code pretty :)
if p >= 1:
absx = lu.create_var(x.size)
constraints += [lu.create_leq(x, absx), lu.create_geq(lu.sum_expr([x, absx]))]
x = absx
if p == 1:
return lu.sum_entries(x), constraints
# now, we take care of the remaining convex and concave branches
# to create the rational powers, we need a new variable, r, and
# the constraint sum(r) == t
r = lu.create_var(x.size)
t_ = lu.promote(t, x.size)
constraints += [lu.create_eq(lu.sum_entries(r), t)]
# make p a fraction so that the input weight to gm_constrs
# is a nice tuple of fractions.
p = Fraction(p)
if p < 0:
constraints += gm_constrs(t_, [x, r], (-p/(1-p), 1/(1-p)))
if 0 < p < 1:
constraints += gm_constrs(r, [x, t_], (p, 1-p))
if p > 1:
constraints += gm_constrs(x, [r, t_], (1/p, 1-1/p))
return t, constraints
