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<?xml version="1.0" encoding="UTF-8"?>
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<!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>
<note>.xlsx format is suported since POI 3.5, make sure yoy upgraded to that version before experimenting with this
code. Users of all versions of POI may wish to make use of a recent
SVN checkout, as new functions are currently being added fairly frequently.
</note>
</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 is 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 < 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 formuals 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>
</body>
</document>
|
