code: ///////////////////////////////////////////////////////////////////////////////
  // For information as to what this class does, see the Javadoc, below.       //
  // Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006,       //
  // 2007, 2008, 2009, 2010, 2014, 2015 by Peter Spirtes, Richard Scheines, Joseph   //
  // Ramsey, and Clark Glymour.                                                //
  //                                                                           //
  // This program is free software; you can redistribute it and/or modify      //
  // it under the terms of the GNU General Public License as published by      //
  // the Free Software Foundation; either version 2 of the License, or         //
  // (at your option) any later version.                                       //
  //                                                                           //
  // This program is distributed in the hope that it will be useful,           //
  // but WITHOUT ANY WARRANTY; without even the implied warranty of            //
  // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the             //
  // GNU General Public License for more details.                              //
  //                                                                           //
  // You should have received a copy of the GNU General Public License         //
  // along with this program; if not, write to the Free Software               //
  // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA //
  ///////////////////////////////////////////////////////////////////////////////
  
  package edu.cmu.tetrad.calculator.expression;
  
  import org.apache.commons.math3.distribution.IntegerDistribution;
  import org.apache.commons.math3.distribution.RealDistribution;
  
  import java.util.Collections;
  import java.util.List;
  
  /**
   * An equation expression.
   *
   * @author Tyler Gibson
   */
  public class EvaluationExpression implements Expression {
      static final long serialVersionUID = 23L;
  
      /**
       * The variable part of the expression.
       */
      private VariableExpression variable;
  
  
      /**
       * The string you are testing the variable against.
       */
      private String string;
  
  
      public EvaluationExpression(VariableExpression exp, String s) {
          if (exp == null) {
              throw new NullPointerException("Variable must not be null.");
          }
          if (s == null) {
              throw new NullPointerException("String must not be null.");
          }
          this.variable = exp;
          this.string = s;
      }
  
      public static EvaluationExpression serializableInstance() {
          return new EvaluationExpression(VariableExpression.serializableInstance(), "a");
      }
  
      public String toString() {
          return super.toString();
      }
  
      public double evaluate(Context context) {
          Object o = variable.evaluateGeneric(context);
          if (o != null && string.equals(o.toString())) {
              return 1.0;
          }
          return 0.0;
      }
  
      public String getToken() {
          return "Eval";
      }
  
      public ExpressionDescriptor.Position getPosition() {
          return ExpressionDescriptor.Position.NEITHER;
      }
  
      public List<Expression> getExpressions() {
          return Collections.singletonList((Expression) variable);
      }
  
      @Override
      public RealDistribution getRealDistribution(Context context) {
          return null;
      }
  
      public IntegerDistribution getIntegerDistribution(Context context) {
          return null;
      }
  }
  
  
  
  
  