UFL Specification and User Manual 0.3 - FEniCS Project

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UFL Specification and User Manual 0.3Martin S. Alnæs, Anders Loggpost traversalTODO: traversal.pypre traversalTODO: traversal.pywalkTODO: traversal.pytraverse terminalsTODO: traversal.py5.2.2 Extracting informationTODO: analysis.py5.2.3 Transforming expressionsSo far the algorithms presented has been about inspecting expressions in variousways. Some recurring patterns occur when writing algorithms to modifyexpressions, either to apply mathematical transformations or to change theirrepresentation. Usually, different expression node types need different treatment.To assist in such algorithms, UFL provides the Transformer class. This implementsa variant of the Visitor pattern to enable easy definition of trans-84
UFL Specification and User Manual 0.3Martin S. Alnæs, Anders Loggformation rules for the types you wish to handle.Shown here is maybe the simplest transformer possible:class Printer(Transformer):def __init__(self):Transformer.__init__(self)def expr(self, o, *operands):print "Visiting", str(o), "with operands:"print ", ".join(map(str,operands))return oelement = FiniteElement("CG", triangle, 1)v = TestFunction(element)u = TrialFunction(element)a = u*vp = Printer()p.visit(a)The call to visit will traverse a and call Printer.expr on all expressionnodes in post–order, with the argument operands holding the return valuesfrom visits to the operands of o. The output is:TODOImplementing expr above provides a default handler for any expression nodetype. For each subclass of Expr you can define a handler function to overridethe default by using the name of the type in underscore notation, e.g.basis function for BasisFunction. The constructor of Transformer andimplementation of Transformer.visit handles the mapping from type tohandler function automatically.Here is a simple example to show how to override default behaviour:85
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UFL Specification and User Manual 0
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ContentsAbout this manual 111 Intro
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2.8.10 skew . . . . . . . . . . . .
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4 Internal Representation Details 7
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B Installation 97B.1 Installing fro
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Page 93: Bibliography[1] M. Alnæs and K.-A.
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UFL Specification and User Manual 0.3 - FEniCS Project

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