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Expression Handling

Chapter Updated 2/1/98

### Overview

The main objective of this chapter is to provide information regarding the basic concepts of using the Xbase Expression module.
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 are derived from dBASE III Plus, but not all dBASE III Plus functions have been implemented yet.

### Internal fuctioning

The expression module works in two phases. Firstly method ParseExpression is called and builds an expression tree from all the components of the expression. 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.
Secondly, method ProcessExpression is called to process the expression tree which was already created. The routine parses each node in the expression tree, executing functions, processing operands and manipulating data to produce the desired result.
If an expression will be processed repeatedely, it is best to pre-parse the tree using ParseExpression, then for each new call to the expression, execute method ProcessExpression which processes the tree.

### Expression Return Types

Expressions will return a type of CHAR *, NUMERIC or LOGICAL.
An expression return type can be determined with method GetExpressionResultType after parsing it.
Expressions returning a return type of CHAR are limited to a 200 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, you can adjust the the enum { WorkBufMaxLen = 200 }; in file exp.h.

### Expression Functions

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. All functions which return CHAR * are returning a pointer to a static buffer which is overlaid by the next call to any function which also returns CHAR *.

### Expression Components

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 "-'".
A field is simply a field name in the default database, or is in the form of database->fieldname.

### Sample Program Demonstrating Expression Processing

    /* 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.GetCharResult();
       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;
    }

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