<?xml version="1.0" encoding="UTF-8"?>
<!--
   ====================================================================
   Licensed to the Apache Software Foundation (ASF) under one or more
   contributor license agreements.  See the NOTICE file distributed with
   this work for additional information regarding copyright ownership.
   The ASF licenses this file to You under the Apache License, Version 2.0
   (the "License"); you may not use this file except in compliance with
   the License.  You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.
   ====================================================================
-->
<!DOCTYPE document PUBLIC "-//APACHE//DTD Documentation V1.3//EN" "../dtd/document-v13.dtd">

<document>
  <header>
    <title>Formula Evaluation</title>
  </header>
  <body>
    <section><title>Introduction</title>
      <p>The POI formula evaluation code enables you to calculate the result of 
         formulas in Excels sheets read-in, or created in POI. This document explains
         how to use the API to evaluate your formulas. 
      </p>
    </section>

    <anchor id="WhyEvaluate"/>
    <section><title>Why do I need to evaluate formulas?</title>
      <p>The Excel file format (both .xls and .xlsx) stores a "cached" result for
         every formula along with the formula itself. This means that when the file
         is opened, it can be quickly displayed, without needing to spend a long
         time calculating all of the formula results. It also means that when reading
         a file through Apache POI, the result is quickly available to you too!
      </p>
      <p>After making changes with Apache POI to either Formula Cells themselves,
         or those that they depend on, you should normally perform a Formula
         Evaluation to have these "cached" results updated. This is normally done
         after all changes have been performed, but before you write the file out.
         If you don't do this, there's a good chance that when you open the file in
         Excel, until you go to the cell and hit enter or F9, you will either see 
         the old value or '#VALUE!' for the cell. (Sometimes Excel will notice
         itself, and trigger a recalculation on load, but unless you know you are
         using volatile functions it's generally best to trigger a Recalulation
         through POI)
      </p>
    </section>

    <anchor id="Status"/>
    <section><title>Status</title>
			<p>The code currently provides implementations for all the arithmatic operators.
				It also provides implementations for approx. 140 built in
				functions in Excel. The framework however makes it easy to add 
			 	implementation of new functions. See the <link href="eval-devguide.html"> Formula
        evaluation development guide</link>  and <link href="../apidocs/org/apache/poi/hssf/record/formula/functions/package-summary.html">javadocs</link> 
        for details. </p>
            <p> Both HSSFWorkbook and XSSFWorkbook are supported, so you can 
                evaluate formulas on both .xls and .xlsx files.</p>
			<p> Note that user-defined functions are not supported, and is not likely to done
				any time soon... at least, not till there is a VB implementation in Java!
			</p>
		</section>
		<section><title>User API How-TO</title>
			<p>The following code demonstrates how to use the FormulaEvaluator 
				in the context of other POI excel reading code.
			</p>
			<p>There are several ways in which you can use the FormulaEvalutator API.</p>

			<anchor id="Evaluate"/>
			<section><title>Using FormulaEvaluator.<strong>evaluate</strong>(Cell cell)</title>
				<p>This evaluates a given cell, and returns the new value,
				without affecting the cell</p>
				<source>
FileInputStream fis = new FileInputStream("c:/temp/test.xls");
Workbook wb = new HSSFWorkbook(fis); //or new XSSFWorkbook("c:/temp/test.xls")
Sheet sheet = wb.getSheetAt(0);
FormulaEvaluator evaluator = wb.getCreationHelper().createFormulaEvaluator();

// suppose your formula is in B3
CellReference cellReference = new CellReference("B3"); 
Row row = sheet.getRow(cellReference.getRow());
Cell cell = row.getCell(cellReference.getCol()); 

CellValue cellValue = evaluator.evaluate(cell);

switch (cellValue.getCellType()) {
    case Cell.CELL_TYPE_BOOLEAN:
        System.out.println(cellValue.getBooleanValue());
        break;
    case Cell.CELL_TYPE_NUMERIC:
        System.out.println(cellValue.getNumberValue());
        break;
    case Cell.CELL_TYPE_STRING:
        System.out.println(cellValue.getStringValue());
        break;
    case Cell.CELL_TYPE_BLANK:
        break;
    case Cell.CELL_TYPE_ERROR:
        break;

    // CELL_TYPE_FORMULA will never happen
    case Cell.CELL_TYPE_FORMULA: 
        break;
}				
        </source>
				<p>Thus using the retrieved value (of type 
					FormulaEvaluator.CellValue - a nested class) returned 
					by FormulaEvaluator is similar to using a Cell object 
					containing the value of the formula evaluation. CellValue is 
					a simple value object and does not maintain reference 
					to the original cell.
				</p>
			</section>

			<anchor id="EvaluateFormulaCell"/>
			<section><title>Using FormulaEvaluator.<strong>evaluateFormulaCell</strong>(Cell cell)</title>
				<p><strong>evaluateFormulaCell</strong>(Cell cell) 
				will check to see if the supplied cell is a formula cell. 
				If it isn't, then no changes will be made to it. If it is, 
				then the formula is evaluated. The value for the formula
				is saved alongside it, to be displayed in excel. The
				formula remains in the cell, just with a new value</p>
				<p>The return of the function is the type of the
				formula result, such as Cell.CELL_TYPE_BOOLEAN</p>
        <source>
FileInputStream fis = new FileInputStream("/somepath/test.xls");
Workbook wb = new HSSFWorkbook(fis); //or new XSSFWorkbook("/somepath/test.xls")
Sheet sheet = wb.getSheetAt(0);
FormulaEvaluator evaluator = wb.getCreationHelper().createFormulaEvaluator();

// suppose your formula is in B3
CellReference cellReference = new CellReference("B3"); 
Row row = sheet.getRow(cellReference.getRow());
Cell cell = row.getCell(cellReference.getCol()); 

if (cell!=null) {
    switch (evaluator.evaluateFormulaCell(cell)) {
        case Cell.CELL_TYPE_BOOLEAN:
            System.out.println(cell.getBooleanCellValue());
            break;
        case Cell.CELL_TYPE_NUMERIC:
            System.out.println(cell.getNumericCellValue());
            break;
        case Cell.CELL_TYPE_STRING:
            System.out.println(cell.getStringCellValue());
            break;
        case Cell.CELL_TYPE_BLANK:
            break;
        case Cell.CELL_TYPE_ERROR:
            System.out.println(cell.getErrorCellValue());
            break;

        // CELL_TYPE_FORMULA will never occur
        case Cell.CELL_TYPE_FORMULA: 
            break;
    }
}
				</source>
			</section>

			<anchor id="EvaluateInCell"/>
			<section><title>Using FormulaEvaluator.<strong>evaluateInCell</strong>(Cell cell)</title>
				<p><strong>evaluateInCell</strong>(Cell cell) will check to
				see if the supplied cell is a formula cell. If it isn't,
				then no changes will be made to it. If it is, then the
				formula is evaluated, and the new value saved into the cell,
				in place of the old formula.</p>
				<source>
FileInputStream fis = new FileInputStream("/somepath/test.xls");
Workbook wb = new HSSFWorkbook(fis); //or new XSSFWorkbook("/somepath/test.xls")
Sheet sheet = wb.getSheetAt(0);
FormulaEvaluator evaluator = wb.getCreationHelper().createFormulaEvaluator();

// suppose your formula is in B3
CellReference cellReference = new CellReference("B3");
Row row = sheet.getRow(cellReference.getRow());
Cell cell = row.getCell(cellReference.getCol()); 

if (cell!=null) {
    switch (evaluator.<strong>evaluateInCell</strong>(cell).getCellType()) {
        case Cell.CELL_TYPE_BOOLEAN:
            System.out.println(cell.getBooleanCellValue());
            break;
        case Cell.CELL_TYPE_NUMERIC:
            System.out.println(cell.getNumericCellValue());
            break;
        case Cell.CELL_TYPE_STRING:
            System.out.println(cell.getStringCellValue());
            break;
        case Cell.CELL_TYPE_BLANK:
            break;
        case Cell.CELL_TYPE_ERROR:
            System.out.println(cell.getErrorCellValue());
            break;

        // CELL_TYPE_FORMULA will never occur
        case Cell.CELL_TYPE_FORMULA:
            break;
    }
}

        </source>
			</section>

			<anchor id="EvaluateAll"/>
			<section><title>Re-calculating all formulas in a Workbook</title>
				<source>
FileInputStream fis = new FileInputStream("/somepath/test.xls");
Workbook wb = new HSSFWorkbook(fis); //or new XSSFWorkbook("/somepath/test.xls")
FormulaEvaluator evaluator = wb.getCreationHelper().createFormulaEvaluator();
for(int sheetNum = 0; sheetNum &lt; wb.getNumberOfSheets(); sheetNum++) {
    Sheet sheet = wb.getSheetAt(sheetNum);
    for(Row r : sheet) {
        for(Cell c : r) {
            if(c.getCellType() == Cell.CELL_TYPE_FORMULA) {
                evaluator.evaluateFormulaCell(c);
            }
        }
    }
}
        </source>

           <p>Alternately, if you know which of HSSF or XSSF you're working
            with, then you can call the static 
            <strong>evaluateAllFormulaCells</strong> method on the appropriate
            HSSFFormulaEvaluator or XSSFFormulaEvaluator class.</p>
			</section>
		</section>
		
		<anchor id="recalculation"/>
		<section><title>Recalculation of Formulas</title>
      <p>
        In certain cases  you may want to force Excel to re-calculate formulas when the workbook is opened.
        Consider the following example:
      </p>  
      <p>
        Open Excel and create a new workbook. On the first sheet set A1=1, B1=1, C1=A1+B1. 
        Excel automatically calculates formulas and the value in C1 is 2. So far so good.
      </p>  
      <p>
        Now modify the workbook with POI:
      </p>
      <source>
  Workbook wb = WorkbookFactory.create(new FileInputStream("workbook.xls"));

  Sheet sh = wb.getSheetAt(0);
  sh.getRow(0).getCell(0).setCellValue(2);  // set A1=2

  FileOutputStream out = new FileOutputStream("workbook2.xls");
  wb.write(out);
  out.close();
      </source>      
      <p>
        Now open workbook2.xls in Excel and the value in C1 is still 2 while you expected 3. Wrong? No!
        The point is that Excel caches previously calculated results and you need to trigger recalculation to updated them. 
        It is not an issue when you are creating new workbooks from scratch, but important to remember when you are modifing 
        existing workbooks with formulas. This can be done in two ways:  
      </p>   
      <p>
        1. Re-evaluate  formulas with POI's FormulaEvaluator:
      </p>   
        <source>
  Workbook wb = WorkbookFactory.create(new FileInputStream("workbook.xls"));

  Sheet sh = wb.getSheetAt(0);
  sh.getRow(0).getCell(0).setCellValue(2);  // set A1=2

  wb.getCreationHelper().createFormulaEvaluator().evaluateAll();
        </source>        
      <p>
        2. Delegate re-calculation to Excel. The application will perform a full recalculation when the workbook is opened:
      </p>           
        <source>
  Workbook wb = WorkbookFactory.create(new FileInputStream("workbook.xls"));

  Sheet sh = wb.getSheetAt(0);
  sh.getRow(0).getCell(0).setCellValue(2);  // set A1=2

  wb.setForceFormulaRecalculation(true);
        </source>        
		</section>

    <anchor id="Performance"/>
    <section><title>Performance Notes</title>
      <ul>
        <li>Generally you should have to create only one FormulaEvaluator 
          instance per Workbook. The FormulaEvaluator will cache 
          evaluations of dependent cells, so if you have multiple
          formulas all depending on a cell then subsequent evaluations
          will be faster.
        </li>
        <li>You should normally perform all of your updates to cells,
          before triggering the evaluation, rather than doing one
          cell at a time. By waiting until all the updates/sets are
          performed, you'll be able to take best advantage of the caching
          for complex formulas.
        </li>
        <li>If you do end up making changes to cells part way through
          evaluation, you should call <em>notifySetFormula</em> or
          <em>notifyUpdateCell</em> to trigger suitable cache clearance.
          Alternately, you could instantiate a new FormulaEvaluator,
          which will start with empty caches.
        </li>
        <li>Also note that FormulaEvaluator maintains a reference to 
          the sheet and workbook, so ensure that the evaluator instance 
          is available for garbage collection when you are done with it 
          (in other words don't maintain long lived reference to 
          FormulaEvaluator if you don't really need to - unless 
          all references to the sheet and workbook are removed, these 
          don't get garbage collected and continue to occupy potentially 
          large amounts of memory). 
        </li>	
        <li>CellValue instances however do not maintain reference to the 
          Cell or the sheet or workbook, so these can be long-lived 
          objects without any adverse effect on performance.
        </li>
      </ul>
    </section>
    <section><title>Formula Evaluation Debugging</title>
		<p>POI is not perfect and you may stumble across formula evaluation problems (Java exceptions
		or just different results) in your special use case. To support an easy detailed analysis, a special
		logging of the full evaluation is provided.</p>
		<p>The output of this logging may be very large (depends on your EXCEL), so this logging has to be explicitly enabled
		for each single formula evaluation. Should not be used in production - only for specific development use.</p>
		<p>Example use:</p>
		<source>
	// activate logging to console
	System.setProperty("org.apache.poi.util.POILogger", "org.apache.poi.util.SystemOutLogger");
	System.setProperty("poi.log.level", POILogger.INFO + "");

	// open your file
	Workbook wb = new HSSFWorkbook(new FileInputStream("foobar.xls"));
    FormulaEvaluator evaluator = wb.getCreationHelper().createFormulaEvaluator();

	// get your cell
	Cell cell = wb.getSheet(0).getRow(0).getCell(0);		// just a dummy example

	// perform debug output for the next evaluate-call only
    evaluator.setDebugEvaluationOutputForNextEval(true);
	evaluator.evaluateFormulaCell(cell);
	evaluator.evaluateFormulaCell(cell);		// no logging performed for this next evaluate-call
		</source>
		<p>The special Logger called "POI.FormulaEval" is used (useful if you use the CommonsLogger and a detailed logging configuration).
		The used log levels are WARN and INFO (for detailed parameter info and results) - the level are so high to allow this
		special logging without beeing disturbed by the bunch of DEBUG log entries from other classes.</p>
	</section>
  </body>
</document>
