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Tag Archives: scope
Command Terms
Clause :
An optional or required section of a Clipper language command beginning with a keyword that modifies or enhances the command.
Command :
A statement to be translated by the Clipper preprocessor into source code that will perform a particular operation. All Clipper language commands are defined in the standard header file, STD.CH, located in …INCLUDE. Also, the preprocessor directives that define a command.
See Also: Header File, Statement, STD.CH
Condition :
A logical expression that determines whether an operation will take place. With database commands, a logical expression that determines what records are included in an operation. Conditions are specified as arguments of the FOR or WHILE clause.
Keyword :
A word that has a special meaning to a compiler or other utility program. Commands, directives, or options are often recognized by examining supplied text to see if it contains keywords.
List :
A list of expressions, field names, or filenames, separated by commas specified generally as command, procedure, or function arguments. Code blocks can also execute a list of expressions.
Optional Clause :
A portion of a match pattern that is enclosed in square ([ ]) brackets. An optional clause specifies part of a match pattern that need not be present for source text to match the pattern. An optional clause may contain any of the components legal within a match pattern, including other optional clauses. When a match pattern contains a series of optional clauses that are immediately adjacent to each other, the matching portions of the source text are not required to appear in the same order as the clauses in the match pattern. If an optional clause is matched by more than one part of the source text, the multiple matches may be handled using a repeating clause in the result pattern.
Scope :
In a database command, a clause that specifies a range of database records to be addressed by the command. The scope clause uses the qualifiers ALL, NEXT, RECORD, and REST to define the record scope.
See Also: Condition
Skeleton :
A wildcard mask used to specify a group of filenames or memory variables. The * is used to specify one or more characters and the ? to specify a single character.
Toggle :
As a verb, to choose between an on or off state. As a noun, a value or setting that can be either on or off. A toggle is often represented using a logical value, with true (.T.) representing on, and false (.F.) representing off.
Verb :
The first word of a command that describes the action to perform.
See Also: Command
Database Terms
Alias :
The name of a work area; an alternate name given to a database file. Aliases are often used to give database files descriptive names and are assigned when the database file is opened. If no alias is specified when the database file is USEd, the name of the database file becomes the alias.
An alias can be used to reference both fields and expressions (including user-defined functions). In order to alias an expression, the expression must be enclosed in parentheses.
See also : Work Area
Attribute :
As a formal DBMS term, refers to a column or field in a table or database file.
See Also: Column, Field
Beginning of File :
The top of the database file. In Clipper language there is no beginning of file area or record. Instead, it is indicated by BOF() returning true (.T.) if an attempt is made to move the record pointer above the first record in the database file or the database file is empty.
Cell :
In a table, a cell is the intersection of a Row and a Column.
Column :
A database term used to describe a field in a table or database file.
See Also: Field
Concurrency :
The degree to which data can be accessed by more than one user at the same time.
Condition :
A logical expression that determines whether an operation will take place. With database commands, a logical expression that determines what records are included in an operation. Conditions are specified as arguments of the FOR or WHILE clause.
See Also: Scope
Controlling/Master Index :
The index currently being used to refer to records by key value or sequential record movement commands.
See Also: Index, Natural Order
Database :
An aggregation of related operational data used by an application system. A database can contain one or more data files or tables.
See Also: Field, Record, Tuple, View
DBMS :
An acronym for the term database management system. A DBMS is a software system that mediates access to a database through a data manipulation language.
Delimited File :
A text file that contains variable-length database records with each record separated by a carriage return/linefeed pair (CHR(13) + CHR(10)) and terminated with an end of file mark (CHR(26)). Each field within a delimited file is variable length, not padded with either leading or trailing spaces, and separated by a comma. Character strings are optionally delimited to allow for embedded commas.
End of File :
The bottom of a database file. In Clipper language, this is LASTREC() + 1 and is indicated by EOF() returning true (.T.).
Field :
The basic column unit of a database file. A field has four attributes: name, type, length, and decimals if the type is numeric.
See Also: Database, Record, Tuple, Vector, View
Field Variable :
A variable that refers to data in a database field, as opposed to data in memory.
See Also: Local Variable, Memory Variable, Variable
Index :
An ordered set of key values that provides a logical ordering of the records in an associated database file. Each key in an index is associated with a particular record in the database file. The records can be processed sequentially in key order, and any record can be located by performing a SEEK operation with the associated key value.
See Also: Controlling/Master Index, Key Value, Natural Order
Join :
An operation that takes two tables as operands and produces one table as a result. It is, in fact, a combination of other operations including selection and projection.
See Also: Projection, Selection
Key Expression :
An expression, typically based on one or more database fields, that when evaluated, yields a key value for a database record. Key expressions are most often used to create indexes or for summarization operations.
See Also: Index, Key Value
Key Value :
The value produced by evaluating a key expression. When placed in an index, a key value identifies the logical position of the associated record in its database file.
See Also: Index, Key Expression
Master Index :
The index currently being used to refer to records by key value or sequential record movement commands.
See Also : Controlling/Master Index
Memo Type :
A special database field type consisting of one or more characters in the extended character set. The maximum size of a memo field In Clipper language is 65,534 bytes. A memo field differs only from a character string by the fact it is stored in a separate memo (.DBT file) and the field length is variable-length.
See Also: Character String
Natural Order :
For a database file, the order determined by the sequence in which records were originally entered into the file. Also called unindexed order.
See Also: Index
Normalization :
The process of elimination and consolidation of redundant data elements in a database system.
Projection :
A DBMS term specifying a subset of fields. In Clipper, the analogy is the FIELDS clause.
See Also: Join Selection
Query :
A request for information to be retrieved from a database. Alternately, a data structure in which such a request is encoded.
Record :
The basic row unit of a database file consisting of one or more field elements.
See Also: Database, Field, Table, Tuple
Relation :
A link between database files that allows the record pointer to move in more than one database file based on the value of a common field or expression. This allows information to be accessed from more than one database file at a time.
Relational Database System :
A system that stores data in rows and columns, without system dependencies within the data. In other words, relationships between different databases are not stored in the actual database itself, as is the case in a system that uses record pointers.
Row :
A group of related column or field values that are treated as a single entity. It is the same as a Clipper language record.
See Also: Column, Field, Record
Search Condition :
See : Condition, Scope
Scope :
In a database command, a clause that specifies a range of database records to be addressed by the command. The scope clause uses the qualifiers ALL, NEXT, RECORD, and REST to define the record scope.
See Also: Condition
SDF File :
A text file that contains fixed-length database records with each record separated by a carriage return/linefeed pair (CHR(13) + CHR(10)) and terminated with an end of file mark (CHR(26)). Each field within an SDF file is fixed-length with character strings padded with trailing spaces and numeric values padded with leading spaces. There are no field separators.
See Also: Database, Delimited File, Text File
Selection :
A DBMS term that specifies a subset of records meeting a condition. The selection itself is obtained with a selection operator. In Clipper language, the analogy is the FOR clause.
Separator :
The character or set of characters that differentiate fields or records from one another. In Clipper language, the DELIMITED and SDF file types have separators. The DELIMITED file uses a comma as the field separator and a carriage return/linefeed pair as the record separator. The SDF file type has no field separator, but also uses a carriage return/linefeed pair as the record separator.
See Also: Delimiter
Sort Order :
Describes the various ways database files and arrays are ordered.
. Ascending
Causes the order of data in a sort to be from lowest value to highest value.
. Descending
Causes the order of data in a sort to be from highest value to lowest value.
. Chronological
Causes data in a sort to be ordered based on a date value, from earliest to most recent.
. ASCII
Causes data in a sort to be ordered according to the ASCII Code values of the data to be sorted.
. Dictionary
The data in a sort is ordered in the way it would appear if the items sorted were entries in a dictionary of the English language.
. Collating Sequence
Data in a sort will be placed in sequence following the order of characters in the Extended Character Set.
. Natural
The order in which data was entered into the database.
Table :
A DBMS term defining a collection of column definitions and row values. In Clipper, it is represented and referred to as a database file.
Tuple :
A formal DBMS term that refers to a row in a table or a record in a database file. In DIF files, tuple also refers to the equivalent of a table record.
See Also: Database, Field, Record
Update :
The process of changing the value of fields in one or more records. Database fields are updated by various commands and the assignment operator.
Vector :
In a DIF file, vector refers to the equivalent of a table field.
See Also: Database, Field, Record, Tuple
View :
A DBMS term that defines a virtual table. A virtual table does not actually exist but is derived from existing tables and maintained as a definition. The definition in turn is maintained in a separate file or as an entry in a system dictionary file. In Clipper, views are supported only by DBU.EXE and are maintained in (.vew) files.
See Also: Database, Field, Record
Work Area :
The basic containment area of a database file and its associated indexes. Work areas can be referred to by alias name, number, or a letter designator.
See Also: Alias
#include directive
What is #include directive ?
#include
Include a file into the current source file
Syntax :
#include “<headerFileSpec>”
Arguments
<headerFileSpec> specifies the name of another source file to include in the current source file. As indicated in the syntax, the name must be enclosed in double quotation marks.
<headerFileSpec> may contain an explicit path and file name as well as a file extension. If, however, no path is specified, the preprocessor searches the following places:
. Source file directory
. Directories supplied with the /I option
. Directories specified in the INCLUDE environment variable
#include directives may be nested up to 15 levels deep–that is, a file that has been included may contain #include directives, up to 15 levels.
Description
#include inserts the contents of the specified file in place of the #include directive in the source file. By convention, the file inserted is referred to as a header file. Header files should contain only preprocessor directives and external declarations. By convention header files have a .ch extension.
When deciding where to locate your header files, you have two basic choices. You can place them in the source file directory where they are local to the current system; or, you can make them globally available by placing them in the directory specified in the INCLUDE environment variable. A list of one or more directories can be specified.
Header files overcome the one major drawback of defining constants or inline functions–the #define directive only affects the file in which it is contained. This means that every program which needs access to these statements must have a list of directives at the top. The solution to this problem is to place #define statements in a separate file and use the #include directive to tell the preprocessor to include that file before compiling.
For example, suppose the file “Inkey.ch” contains a list of #define directives assigning key values to constants. Instead of including these directives at the top of each program file (.prg) requiring access to them, you can simply place the following line at the top of each program file:
#include “Inkey.ch”
This causes the preprocessor to look for Inkey.ch and place all the directives contained within it at the top of this program.
Another advantage of using the #include directive is that all the #define statements are contained in one file. If any modifications to these statements are necessary, only the #include file need be altered; the program itself remains untouched.
Note that the scope of definitions within an included header file is the current program file unless the header file is included on the compiler command line with the /U option. In this case, the scope is all the program files compiled in the current invocation of the compiler.
Notes
Supplied header files: The compiler provides a number of header files containing manifest constants for common operations.
Std.ch–the standard header file: Std.ch is the standard header file provided with compiler. Std.ch contains the definitions of all compiler commands and the standard functions specified as pseudofunctions. It is strongly recommended that no changes be made to Std.ch. If changes are desired, it is advisable to copy Std.ch to a new name, make the changes, and compile with /U.
This header file differs somewhat from a header file you might #include in that everything defined in Std.ch, with #define, #translate, or #command, has a scope of the entire compile rather than the current source file.
Examples
This example uses #include to insert Inkey.ch, a file of common keyboard definitions, into a key exception handler called by an interface function:
#include "Inkey.ch"
FUNCTION GetEvent()
LOCAL nKey, nResult
nKey = INKEY(0)
DO CASE
CASE nKey = K_F10
nResult := DoMenu("Browse")
CASE nKey = K_ESC
nResult := DoQuit()
.
. <statements>
.
CASE nKey = K_CTRL_RIGHT
nResult := DoNextRec()
ENDCASE
RETURN nResult
