Equation¶
User API to specify equations.

class
devito.equation.
Eq
[source]¶ Bases:
sympy.core.relational.Equality
,devito.tools.abc.Evaluable
An equal relation between two objects, the lefthand side and the righthand side.
The lefthand side may be a Function or a SparseFunction. The righthand side may be any arbitrary expressions with numbers, Dimensions, Constants, Functions and SparseFunctions as operands.
 Parameters
lhs (Function or SparseFunction) – The lefthand side.
rhs (exprlike, optional) – The righthand side. Defaults to 0.
subdomain (SubDomain, optional) – To restrict the computation of the Eq to a particular subregion in the computational domain.
coefficients (Substitutions, optional) – Can be used to replace symbolic finite difference weights with user defined weights.
implicit_dims (Dimension or list of Dimension, optional) – An ordered list of Dimensions that do not explicitly appear in either the lefthand side or in the righthand side, but that should be honored when constructing an Operator.
Examples
>>> from devito import Grid, Function, Eq >>> grid = Grid(shape=(4, 4)) >>> f = Function(name='f', grid=grid) >>> Eq(f, f + 1) Eq(f(x, y), f(x, y) + 1)
Any SymPy expressions may be used in the righthand side.
>>> from sympy import sin >>> Eq(f, sin(f.dx)**2) Eq(f(x, y), sin(Derivative(f(x, y), x))**2)
Notes
An Eq can be thought of as an assignment in an imperative programming language (e.g.,
a[i] = b[i]*c
).
evaluate
¶ Evaluate the Equation or system of Equations. The rhs of the Equation is evaluated at the indices of the lhs if required.

property
subdomain
¶ The SubDomain in which the Eq is defined.

xreplace
(rules)[source]¶ Replace occurrences of objects within the expression.
 Parameters
rule (dictlike) – Expresses a replacement rule
 Returns
xreplace
 Return type
the result of the replacement
Examples
>>> from sympy import symbols, pi, exp >>> x, y, z = symbols('x y z') >>> (1 + x*y).xreplace({x: pi}) pi*y + 1 >>> (1 + x*y).xreplace({x: pi, y: 2}) 1 + 2*pi
Replacements occur only if an entire node in the expression tree is matched:
>>> (x*y + z).xreplace({x*y: pi}) z + pi >>> (x*y*z).xreplace({x*y: pi}) x*y*z >>> (2*x).xreplace({2*x: y, x: z}) y >>> (2*2*x).xreplace({2*x: y, x: z}) 4*z >>> (x + y + 2).xreplace({x + y: 2}) x + y + 2 >>> (x + 2 + exp(x + 2)).xreplace({x + 2: y}) x + exp(y) + 2
xreplace doesn’t differentiate between free and bound symbols. In the following, subs(x, y) would not change x since it is a bound symbol, but xreplace does:
>>> from sympy import Integral >>> Integral(x, (x, 1, 2*x)).xreplace({x: y}) Integral(y, (y, 1, 2*y))
Trying to replace x with an expression raises an error:
>>> Integral(x, (x, 1, 2*x)).xreplace({x: 2*y}) ValueError: Invalid limits given: ((2*y, 1, 4*y),)
See also
replace()
replacement capable of doing wildcardlike matching, parsing of match, and conditional replacements
subs()
substitution of subexpressions as defined by the objects themselves.

class
devito.equation.
Inc
[source]¶ Bases:
devito.equation.Eq
An increment relation between two objects, the lefthand side and the righthand side.
 Parameters
lhs (Function or SparseFunction) – The lefthand side.
rhs (exprlike) – The righthand side.
subdomain (SubDomain, optional) – To restrict the computation of the Eq to a particular subregion in the computational domain.
coefficients (Substitutions, optional) – Can be used to replace symbolic finite difference weights with user defined weights.
implicit_dims (Dimension or list of Dimension, optional) – An ordered list of Dimensions that do not explicitly appear in either the lefthand side or in the righthand side, but that should be honored when constructing an Operator.
Examples
Inc may be used to express tensor contractions. Below, a summation along the userdefined Dimension
i
.>>> from devito import Grid, Dimension, Function, Inc >>> grid = Grid(shape=(4, 4)) >>> x, y = grid.dimensions >>> i = Dimension(name='i') >>> f = Function(name='f', grid=grid) >>> g = Function(name='g', shape=(10, 4, 4), dimensions=(i, x, y)) >>> Inc(f, g) Inc(f(x, y), g(i, x, y))
Notes
An Inc can be thought of as the augmented assignment ‘+=’ in an imperative programming language (e.g.,
a[i] += c
).

devito.equation.
solve
(eq, target, **kwargs)[source]¶ Algebraically rearrange an Eq w.r.t. a given symbol.
This is a wrapper around
sympy.solve
. Parameters
eq (exprlike) – The equation to be rearranged.
target (symbol) – The symbol w.r.t. which the equation is rearranged. May be a Function or any other symbolic object.
**kwargs – Symbolic optimizations applied while rearranging the equation. For more information. refer to
sympy.solve.__doc__
.