Relación entre la memoria auditiva inmediata y los procesos lectores

Appendix D lists the types of files dBASE IV creates and uses. You can use aliases to access and relate the information from several different database files.

Filenames

A filename may be the actual name of a file as it is written on disk, or an

indirect reference to the filename. Indirect file references are discussed later in this section. If you enter the name of a file, you may also use the drive specifier and full path to indicate where the file can be found.

dBASE IV accepts any valid DOS filename. When you write a file to disk, dBASE IV assigns an extension to the file that indicates the type of informa-tion it contains. For example, the CREATE command will write a file with a

extension, which indicates to other dBASE commands that the file con-tains data records. Appendix D provides a complete listing of the dBASE IV file extensions.

REFERENCE

I

NOTE

ESSENTIALS

Do not use a DOS device name as a Check your DOS ual for DOS device

An indirect reference is a character expression that evaluates to a filename.

You must use an operator in the expression so that dBASE IV knows the character string is an expression, not the literal filename. For example, an indirect reference may be used in the CREATE command in place of a file-name. If the variable contains the character string CREATE

and CREATE RTRIM(Mfile) will create a database file named Terms.

These commands are the same as CREATE Terms. CREATE Mfile will create a database file named

An indirect reference is similar to using the macro substitution character, as CREATE but operates much faster.

In the following example, the first USE command will call in the compiler at runtime, while the second USE command will not:

The CREATE/MODIFY QUERY/VIEW, CREATE/MODIFY LABEL, CREATE/MODIFY REPORT, CREATE VIEW FROM ENVIRONMENT, and SET CATALOG commands save the names of one or more currently open files, so that these files later can be opened automatically.

CREATE/MODIFY QUERY/VIEW creates a query file, which can extract records matching specified conditions, or an update query file, which can modify the records of a database file. When you build reports or labels with CREATE/MODIFY REPORT or CREATE/MODIFY LABEL, you can save the settings of system memory variables in a print form file.

The name of the file is saved in the corresponding report design

file or label design file. CREATE VIEW FROM ENVIRONMENT builds a view file, which may contain the names of database files, index files, and format files. SET CATALOG creates a catalog file, which contains the names of database and view files, and their associated index, format, label, and report files.

Sometimes the filenames saved in the or files are written with their drive and path names, and sometimes they are not.

This depends on whether the filename can be found on the default drive and in the current directory, and whether you provided an explicit drive and path as part of the filename when building the or

files.

You can provide an explicit drive and path in either of two ways:

You can enter the drive and path as part of the command line.

For example:

C:

gives the drive, as well as the path, \DBASE\MYDATA, where the file can be found.

2. You can use the query clause, which is a question mark, navigate to the location of the file on disk, and then choose the file. Once the file is opened, it is as if you opened it with an explicit drive and path as part of the filename.

dBASE IV determines whether to save the filename with the full drive and path names according to the following conditions:

If you open a file on the default drive and in the current directory, these commands listed above store filenames without the drive and directory information, even if you provide a drive and path as part of the filename.

For example, suppose the default drive is and the current directory is

\DBASE\MYDATA. If a catalog is open with SET CATALOG, and you USE C:\DBASE\MYDATA\Myfile, the file is saved in the catalog as

Of course, the filename will also be saved as Myfile.dbf if you typed USE Myfile.

2. To open a file that is not on the default drive and in the current direc-tory, you may specify the drive and directory as part of the filename, or set up a search path with SET PATH.

a. If you specify the drive and path as part of the filename, the drive and path information is saved as part of the filename. So, if the current drive is still and the current directory is still \DBASE\DATA, and you type USE the file is saved in the catalog as

b. If you SET PATH, the filename is saved just as you enter it. The follow-ing commands:

SET PATH TO

save the filename in the catalog as The following commands:

SET TO USE

save the filename in the catalog as

So, the and files never contain filenames with drive and path names, if the file can be found on the default drive and in the current directory. These and files contain file-names with drive and path file-names, if the file is on another drive or in another directory and if you provide the explicit drive and path names.

Aliases

dBASE IV allows up to ten database files to be open simultaneously by allot-ting each open file its own unique work area.

LANGUAGE REFERENCE

I

dBASE IV keeps track of the open database files by their aliases.

An alias can be an alias name (which may be the same as its filename), a work area letter, or a work area number.

If you provide an alias name with the ALIAS option of the USE command, you may use this alias name as an abbreviation in place of the database file-name when referencing the work area. If you assign an alias name with the ALIAS option of the USE command, dBASE IV uses the filename as the default alias name.

Work area letters are the letters from A to J or a to j. Work area A is the first work area; J is the tenth work area.

Work area numbers are from 1 to 10. In earlier dBASE versions, you could use work area numbers only in the SELECT command. In dBASE IV, you can use a work area number in any command that accepts an alias.

Some valid filenames, such as cannot be used as aliases because they contain characters that dBASE reserves for other uses. If a filename contains characters that prohibit it from being used as the default alias name, and if you do not provide another alias name with the ALIAS option of the USE command, dBASE IV a letter alias, from A to J, as the default.

You may use an indirect reference to an alias, just as you may use an indi-rect reference to a filename. If you include an operator in the alias character string, dBASE IV knows that the string is an expression, not the literal alias.

dBASE IV also evaluates strings that contain an alias symbol or square brackets because these symbols indicate a reference to a field, memory variable, or array element, rather than to a literal alias. In these cases, dBASE IV evaluates the expression for a work area alias name, work area letter, or work area number. For example,

3 GO 5 (Expwa)

positions the record pointer to the fifth record in work area 3.

Indirect alias references are similar to using the macro substitution charac-ter, such as SKIP 2 IN but operate much faster.