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JEP is a Java library for parsing and evaluating mathematical expressions.
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package org.lsmp.djep.matrixJep.function;

import java.util.*;
import org.nfunk.jep.*;
import org.nfunk.jep.function.*;
import org.lsmp.djep.matrixJep.*;
import org.lsmp.djep.matrixJep.nodeTypes.*;
import org.lsmp.djep.vectorJep.values.*;
import org.lsmp.djep.vectorJep.*;

/**
 * A matrix enabled assignment function.
 * The lhs of an assignment must be a variable.
 * 
 * @author Rich Morris
 * Created on 23-Feb-2004
 */
public class MAssign extends Assign implements MatrixSpecialEvaluationI,SpecialPreProcessorI
{
	public MAssign()
	{
		numberOfParameters = 2;
	}

	/** The run method should not be called. 
	 * Use {@link #evaluate} instead.
	 */	
	public void run(Stack s) throws ParseException 
	{
		throw new ParseException("Eval should not be called by Evaluator"); 
	}

	/**
	 * A special methods for evaluating an assignment.
	 * When an assignment is encountered, first
	 * evaluate the rhs. Then set the value 
	 * of the lhs to the result.
	 */
	public MatrixValueI evaluate(MatrixNodeI node,MatrixEvaluator visitor,MatrixJep j) throws ParseException
	{
		if(node.jjtGetNumChildren()!=2)
			throw new ParseException("Assignment operator must have 2 operators.");

		// evaluate the value of the right-hand side. Left on top of stack
		
		MatrixValueI rhsVal = (MatrixValueI) node.jjtGetChild(1).jjtAccept(visitor,null);	

		// Set the value of the variable on the lhs. 
		Node lhsNode = node.jjtGetChild(0);
		if(lhsNode instanceof ASTMVarNode)
		{
			ASTMVarNode vn = (ASTMVarNode) lhsNode;
			MatrixVariableI var = (MatrixVariableI) vn.getVar();
			var.setMValue(rhsVal);
			return rhsVal;
		}
		else if(lhsNode instanceof ASTMFunNode && ((ASTMFunNode) lhsNode).getPFMC() instanceof LValueI)
		{
			((LValueI) ((ASTMFunNode) lhsNode).getPFMC()).set(visitor,lhsNode,rhsVal);
			return rhsVal;
		}

		throw new ParseException("Assignment should have a variable for the lhs.");
	}

	public MatrixNodeI preprocess(
		ASTFunNode node,
		MatrixPreprocessor visitor,
		MatrixJep mjep,
		MatrixNodeFactory nf)
		throws ParseException
	{
		MatrixNodeI children[] = visitor.visitChildrenAsArray(node,null);

		if(node.jjtGetNumChildren()!=2) throw new ParseException("Operator "+node.getOperator().getName()+" must have two elements, it has "+children.length);
		Dimensions rhsDim = children[1].getDim();
		if(children[0] instanceof ASTVarNode)
		{
			MatrixVariable var = (MatrixVariable) ((ASTVarNode) children[0]).getVar();
			var.setDimensions(rhsDim);
			Node copy =mjep.deepCopy(children[1]);
			Node simp = mjep.simplify(copy);
			//Node preproc = (Node) simp.jjtAccept(this,data);
			var.setEquation(simp);
		}
		//TODO cope with ArrayAccess. Should really set the array access equations
		return (ASTMFunNode) nf.buildOperatorNode(node.getOperator(),children,rhsDim);
	}

}