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| author | Jörg Frings-Fürst <debian@jff-webhosting.net> | 2014-08-02 08:43:31 +0200 |
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| committer | Jörg Frings-Fürst <debian@jff-webhosting.net> | 2014-08-02 08:43:31 +0200 |
| commit | daf17154bf13139d9375f48525d19d6aaba08155 (patch) | |
| tree | e3c08b6c49dc8a8e83f03327591310546675b43d /html/xbc5.htm | |
Imported Upstream version 3.1.2upstream/3.1.2
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diff --git a/html/xbc5.htm b/html/xbc5.htm new file mode 100755 index 0000000..377129f --- /dev/null +++ b/html/xbc5.htm @@ -0,0 +1,156 @@ +<!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> |