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A powerful expression parser/analyzer provides for complex conditional expressions that can be defined and evaluated at runtime.
Filter expression syntax closely follows the C language syntax for expressions, including order of precedence. An expression interpreted by the expression parser should compile without errors with a standard C compiler. As in C, you cannot compare strings directly like LastName > 'S'. However, the expression parser has a number of built-in functions that allow the comparison of strings. Example:
strcmp( LastName, "S" ) > 0
The expression handling assures proper alignment considerations are handled, and ensures buffer size of any record being evaluated is big enough.
Routines that evaluate conditional expressions maintain fixed data record lengths and total data record lengths. This permits correct alignment adjustments and detects if insufficient data is available. The latter condition results in a CVAL_ERR (598) error. The easiest way to produce a CVAL_ERR (598) is to read only the fixed-length portion of a data record, and have an expression that relies on fields in the variable-length portion of the record.
For additional control, a Conditional Expression Callback Function is available. This allows advanced control through an external user created function.
See also
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Use parentheses exactly like they are used in C expressions. There are no limits on the number of parentheses you may use in an expression, as long as each open parenthesis has a closing parenthesis. Parentheses are also used to enclose the arguments of built-in functions.
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The FairCom DB conditional expression parser has numerous built-in functions for advanced conditional filtering possibilities. They are described in the Predefined Functions section of the FairCom DB Developer's Guide.
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The filter expression parser allows you to use explicit type casts in expressions. This is very useful if you are comparing fields of different types and want to control the result of an expression.
For example, suppose "Salary" is a CT_MONEY field and "Average" is a CT_DFLOAT field; type casts can be used as illustrated in the following expression: (Salary - (int)Average) > 500
The following type casts may be used in conditional expressions:
You cannot type cast a string expression.
