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ConditionalExpression.java

/*************************************************************************
*                                                                        *
*   1) This source code file, in unmodified form, and compiled classes   *
*      derived from it can be used and distributed without restriction,  *
*      including for commercial use.  (Attribution is not required       *
*      but is appreciated.)                                              *
*                                                                        *
*    2) Modified versions of this file can be made and distributed       *
*       provided:  the modified versions are put into a Java package     *
*       different from the original package, edu.hws;  modified          *
*       versions are distributed under the same terms as the original;   *
*       and the modifications are documented in comments.  (Modification *
*       here does not include simply making subclasses that belong to    *
*       a package other than edu.hws, which can be done without any      *
*       restriction.)                                                    *
*                                                                        *
*   David J. Eck                                                         *
*   Department of Mathematics and Computer Science                       *
*   Hobart and William Smith Colleges                                    *
*   Geneva, New York 14456,   USA                                        *
*   Email: eck@hws.edu          WWW: http://math.hws.edu/eck/            *
*                                                                        *
*************************************************************************/

package edu.hws.jcm.data;

/**
 * A "conditional expression" is an expression using the "?" operator, such as "(x > 0)? x : -x"
 * or "x <> 3 ? 1 / (x - 3)".  Note that the second case, which follows the ":", is optional.
 * If not present, the expression has the value Double.NaN when the boolean condition evaluates
 * to false.  A ConditionalExpression object is generated by a parser when it encounters a 
 * "?" operator in the string it is parsing (provided the BOOLEANS option is turned on in that parser).
 *    <p>A ConditionalExpression object holds the two expressions that are the cases in a conditional expression.
 * Note that the boolean condition is NOT stored in the ConditionalExpression object; it is part of
 * the ExpressionProgram for the expression in which the conditional expression occurs.
 * A ConditionalExpression is an ExpressionCommand, meaning that it can occur in an ExpressionProgram.
 * When the apply() method in this class is called, the boolean condition has already been evaluated,
 * and the result is on the top of the stack.  The ConditionalExpression will look at this result
 * and replace it with the value of one of the two expressions that it contains.
 *   <p>It is unlikely that there will be any reason for anyone to use or understand this class,
 * except possibly as an example of an ExpressionCommand.
 */
00043 public class ConditionalExpression implements ExpressionCommand {

   private ExpressionProgram trueCase;    // Expression to be evaluated if condition is true.
   
   private ExpressionProgram falseCase;   // Expression to be evaluated if condition is false.
                                          // This can be null, representing the value Double.NaN.
   

   /**   
    * Create a ConditionalExpression object containing the two given expressions.
    * trueCase must not be null, but falseCase can be null.
    */
00055    public ConditionalExpression(ExpressionProgram trueCase, ExpressionProgram falseCase) {
      this.trueCase = trueCase;
      this.falseCase = falseCase;
   }
   

   // ---------------- Methods from the ExpressionCommand interface -------------------------

   /**
    * Apply this ConditionalExpression to the stack.  
    * (Get the top item from the stack.  If it is non-zero (representing a boolean
    * value of true), evaluate trueCase and put it's value on the stack.  Otherwise,
    * evaluate falseCase and put its value on the stack.  If cases is non-null, record
    * case values for the boolean condition and for the expression that is evaluated.)
    */
00070    public void apply(StackOfDouble stack, Cases cases) {
      double test = stack.pop();
      if (cases != null)
         cases.addCase((int)test);
      if (test != 0)
         stack.push(trueCase.getValueWithCases(cases));
      else if (falseCase != null)
         stack.push(falseCase.getValueWithCases(cases));
      else
         stack.push(Double.NaN);
   }
   
   /**
    * Add commands to deriv that evaluate the derivative of this conditional expression with
    * respect to the variable wrt.  Assume that the ConditionalExpression Object
    * occurs in the program prog at index myIndex.
    *    (The derivative of a conditional expression is another conditional expression.
    * The boolean test for the derivative is the same as the test for this expression,
    * so copy that test onto deriv.  Than add a new ConditionalExpression object to deriv
    * whose trueCase is the derivative of this.trueCase and whose falseCase is the derivative of 
    * this.falseCase.)
    */
00092    public void compileDerivative(ExpressionProgram prog, int myIndex, ExpressionProgram deriv, Variable wrt) {
      prog.copyExpression(myIndex-1,deriv);
      ExpressionProgram trueDeriv = (ExpressionProgram)trueCase.derivative(wrt);
      ExpressionProgram falseDeriv = (falseCase == null)? null : (ExpressionProgram)falseCase.derivative(wrt);
      deriv.addCommandObject( new ConditionalExpression( trueDeriv, falseDeriv ) );
   }

   /**   
    * Assume that this ConditionalExpression object occurs in prog at index myIndex.
    * Compute the total number of commands in prog used by the conditional expression,
    * including the boolean test, which occurs in prog at position myIndex-1.
    *    (The number of commands in prog used by the conditional expression is 1 (for the
    * ConditionalExpression object itself) plus the number of commands in the
    * boolean condition.)
    */
00107    public int extent(ExpressionProgram prog, int myIndex) {
      return 1 + prog.extent(myIndex-1);
   }
   
   /**
    * Returns true if x occurs in either the trueCase or the falseCase expression.
    */
00114    public boolean dependsOn(Variable x) {
      return trueCase.dependsOn(x) || (falseCase != null && falseCase.dependsOn(x));
   }

   /**
    * Append the string representation of the expression (including the boolean
    * condition) to the buffer.  Assume that this ConditionalExpression occurs as
    * a command in prog at index myIndex (so the boolean condition starts at index myIndex-1).
    */
00123    public void appendOutputString(ExpressionProgram prog, int myIndex, StringBuffer buffer) {
      buffer.append('(');
      prog.appendOutputString(myIndex-1,buffer);
      buffer.append(") ? (");
      buffer.append(trueCase.toString());
      buffer.append(')');
      if (falseCase != null) {
         buffer.append(" : (");
         buffer.append(falseCase.toString());
         buffer.append(')');
      }
   }

} // end class ConditionalExpression


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