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							<?xml version="1.0" encoding="UTF-8"?>
<!-- Copyright (C) 2005 The Apache Software Foundation. All rights reserved. -->
<!DOCTYPE document PUBLIC "-//APACHE//DTD Documentation V1.1//EN" "../dtd/document-v11.dtd">

<document>
    <header>
        <title>Formula Evaluation</title>
        <authors>
			<person email="amoweb@yahoo.com" name="Amol Deshmukh" id="AD"/>
        </authors>
    </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>
			<warning> This code currently lives in Bugzilla as 
				<link href="http://issues.apache.org/bugzilla/show_bug.cgi?id=34828"> 
				bug 34828 </link>. It is expected to land in POI CVS in the scratchpad 
				area soon.
			</warning>
		</section>
		<section><title>Status</title>
			<p>	The code currently provides implementations for all the arithmatic operators.
				It also provides implementations for about 30 built in 
				functions in Excel. The framework however makes is easy to add 
			 	implementation of new functions. See the <link href="eval-devguide.html"> Formula
				evaluation development guide</link> for details. </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 HSSFFormulaEvaluator 
				in the context of other POI excel reading code.
			</p>
			<p>There are two ways in which you can use the HSSFFormulaEvalutator API.</p>
			<section><title>Using HSSFFormulaEvaluator.<strong>evaluate</strong>(HSSFCell cell)</title>
				<source>
FileInputStream fis = new FileInputStream("c:/temp/test.xls");
HSSFWorkbook wb = new HSSFWorkbook(fis);
HSSFSheet sheet = wb.getSheetAt(0);
HSSFFormulaEvaluator evaluator = new HSSFFormulaEvaluator(sheet, wb);

// suppose your formula is in B3
CellReference cellReference = new CellReference("B3"); 
HSSFRow row = sheet.getRow(cellReference.getRow());
HSSFCell cell = row.getCell(cellReference.getCol()); 
String formulaString = c.getCellFormula();
HSSFFormulaEvaluator.CellValue cellValue = 
        evaluator.evaluate(formulaString);

switch (cellValue.getCellType()) {
	case HSSFCell.CELL_TYPE_BOOLEAN:
    	System.out.println(cellValue.getBooleanCellValue());
    	break;
	case HSSFCell.CELL_TYPE_NUMERIC:
    	System.out.println(cellValue.getNumberCellValue());
    	break;
	case HSSFCell.CELL_TYPE_STRING:
    	System.out.println(cellValue.getStringCellValue());
    	break;
	case HSSFCell.CELL_TYPE_BLANK:
    	break;
	case HSSFCell.CELL_TYPE_ERROR:
    	break;

	// CELL_TYPE_FORMULA will never happen
	case HSSFCell.CELL_TYPE_FORMULA: 
    	break;
}				
				</source>
				<p>Thus using the retrieved value (of type 
					HSSFFormulaEvaluator.CellValue - a nested class) returned 
					by HSSFFormulaEvaluator is similar to using a HSSFCell 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>
			<section><title>Using HSSFFormulaEvaluator.<strong>evaluateInCell</strong>(HSSFCell cell)
				</title>
				<source>
FileInputStream fis = new FileInputStream("c:/temp/test.xls");
HSSFWorkbook wb = new HSSFWorkbook(fis);
HSSFSheet sheet = wb.getSheetAt(0);
HSSFFormulaEvaluator evaluator = new HSSFFormulaEvaluator(sheet, wb);

// suppose your formula is in B3
CellReference cellReference = new CellReference("B3"); 
HSSFRow row = sheet.getRow(cellReference.getRow());
HSSFCell cell = row.getCell(cellReference.getCol()); 
String formulaString = c.getCellFormula();

if (cell!=null) {
	switch (<strong>evaluator.evaluateInCell</strong>(cell).getCellType()) {
		case HSSFCell.CELL_TYPE_BOOLEAN:
		    System.out.println(cell.getBooleanCellValue());
		    break;
		case HSSFCell.CELL_TYPE_NUMERIC:
		    System.out.println(cell.getNumberCellValue());
		    break;
		case HSSFCell.CELL_TYPE_STRING:
		    System.out.println(cell.getStringCellValue());
		    break;
		case HSSFCell.CELL_TYPE_BLANK:
		    break;
		case HSSFCell.CELL_TYPE_ERROR:
		    System.out.println(cell.getErrorCellValue());
		    break;
		
		// CELL_TYPE_FORMULA will never occur
		case HSSFCell.CELL_TYPE_FORMULA: 
		    break;
	}
}
					</source>

				</section>
		</section>
		
		<section><title></title>
			
		</section>
		
		<section><title>Performance Notes</title>
			<ul>
				<li>Generally you should have to create only one HSSFFormulaEvaluator 
					instance per sheet, but there really is no overhead in creating 
					multiple HSSFFormulaEvaluators per sheet other than that of the 
					HSSFFormulaEvaluator object creation. 
				</li>
				<li>Also note that HSSFFormulaEvaluator 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 
					HSSFFormulaEvaluator 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 
					HSSFCell or the sheet or workbook, so these can be long-lived 
					objects without any adverse effect on performance.
				</li>
			</ul>
		</section>
	</body>
</document>