/usr/share/doc/libxbase-dev/html/xbc5.htm

<!DOCTYPE HTML PUBLIC>
<html>
<title>Xbase DBMS Chapter 5</title>
<body bgcolor=#FFFFFF>
<h1><p align="center">Expression Handling<br></h1>
<p align="center">Chapter Updated 3/12/04</p><hr>

<h3>Overview</h3>

The main objective of this chapter is to provide information regarding the
basic concepts of using the Xbase Expression module.<br><br>

Beginning with release 1.7.4, the Xbase library includes an expression parsing
routine which assists application programmers by providing a high level data 
manipulation tool and also allows for building complex index keys.

The functions included were derived from dBASE III Plus, but not all 
dBASE III Plus functions have been implemented yet.
<br><br>
Release 3.0 brought some upgrades to the expression module of the library
including bug fixes, additional operators and new functions.  The expression
module is usable and functional, but has some room for improvement. 
A future release of Xbase will include more comprehensive expresion logic.

<br><br>

<h3>Internal fuctioning</h3>
The expression module works in two phases.  Firstly, method 
<em>ParseExpression</em> is called and builds an expression tree from 
all the components of the expression. The tree is made up of indivdual 
nodes. The expression is checked for valid field names, literals, 
operands and functions.  Any field references are resolved.  If fields 
are used in an expression and the database name for the field is not 
included in the name with the -> operand, the routines assume the 
associated database has been successfully opened. 
<br><br>
Secondly, method <em>ProcessExpression</em> is called to process the 
expression tree created by ParseExpression().  The routine parses each 
node in the expression tree, executing functions, processing operands 
and manipulating data to produce the desired result.<br><br>

If an expression will be processed repeatedely, it is best to pre-parse the
tree using <em>ParseExpression</em>, then for each new call to the expression, 
execute method <em>ProcessExpression</em> which processes the tree. 


<h3>Expression Return Types</h3>
Expressions will return a type of CHAR *, NUMERIC or LOGICAL.<br><br> 

An expression return type can be determined with method <em>
GetExpressionResultType</em> after parsing it.<br><br>

Expressions returning a return type of CHAR are limited to a 200 byte internal 
buffer.  There is also a 100 byte limit for NDX index key support.  If 
the 200 byte limit is not large enough for your application, adjust field
<em>enum { WorkBufMaxLen = 200 };</em> in file <em>exp.h</em>.

<h3>Expression Functions</h3>
Each expression function also has a corresponding C++ function.  It is
slightly more efficient to call the C++ functions directly, rather than 
execute the expression parsing routines.

<h3>Expression Components</h3>
Expressions are made up of one or more tokens.  A token is one of literal, 
database field, operand or function.  Literals are either numeric or character.
Character literals are enclosed in 'single' or "double" quotes.  numeric 
literals are a series of one or more contiguous numerals, ".", "+" or "-'".
<br><br>
A field is simply a field name in the default database, or is in the form
of database->fieldname.  

<br><br>
<hr>
<h3>Sample Program Demonstrating Expression Processing</h3>
<xmp>

/* expressn.cpp */

#ifdef DOS 
extern unsigned _stklen = 40000;
#endif

#include <xbase/xbase.h>
xbSchema MyRecord[] =
{
  { "FIELD1", 'C', 20, 0 },
  { "FIELD2", 'C', 20, 0 },
  { "NAME",   'C', 10, 0 },
  { "DOUB1",  'N',  7, 2 },
  { "FLOAT1", 'F',  8, 3 },
  { "DATE1",  'D',  8, 0 },
};
xbShort rc;
xbXBase x;
xbDbf d( &x );
XB_EXPRESSION *e;

/****************************************************************************/
xbShort MyExpressionProcessor( char * Expression )
{
   xbShort rc;
   char type;

   if(( rc = x.ParseExpression( Expression, &d )) != 0 )
   {
      cout << "\nParse Error " << rc;
      return rc;
   }
   e = x.GetExpressionHandle();

   if(( rc = x.ProcessExpression( e )) != 0 )
   {
      cout << "\nError processing expression rc = " << rc;
      return rc;
   }
   type = x.GetExpressionResultType( e );
   cout << "\nExpression " << Expression << " produced result = ";
   if( type == 'C' )
      cout << x.GetStringResult();
   else if( type == 'N' )
      cout << x.GetDoubleResult();
   else if( type == 'L' )
      cout << x.GetIntResult();
   else
      cout << "\nUnknown result type " << type;
   return 0;
}
/****************************************************************************/
main()
{
   e = NULL;

   d.CreateDatabase( "TEST", MyRecord, OVERLAY );
   d.BlankRecord();
   d.PutField( d.GetFieldNo( "FIELD1" ), "TESTA " );
   d.PutField( d.GetFieldNo( "FIELD2" ), " testb" );
   d.PutField( d.GetFieldNo( "DOUB1" ),  "200.33" );
   d.PutField( d.GetFieldNo( "FLOAT1" ), "100.00" );
   d.PutField( d.GetFieldNo( "DATE1" ),  "19980101" );
   d.AppendRecord();

   /* process 3 simple expressions */
   MyExpressionProcessor( "FIELD1+FIELD2" );
   MyExpressionProcessor( "FIELD1-UPPER(FIELD2)" );
   MyExpressionProcessor( "5+TEST->DOUB1" );
 
   d.CloseDatabase();
   return 1;
}

</xmp>

<hr>
<p><img src="xbase.jpg"><br><hr>
</BODY>
</HTML>
libxbase64-dev 3.1.2-0ubutu1 / usr / share / doc / libxbase-dev / html / xbc5.htm
