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<!DOCTYPE HTML PUBLIC>
<HTML>
<TITLE>Xbase DBMS Chapter 3</TITLE>
<BODY BGCOLOR=#FFFFFF>
<H1><p align="center">Fields and Strings</p></H1>
<p align="center">Chapter Updated 11/21/22</p><hr>
<br><br>
The main objective of this chapter is to provide basic information regarding
various field types supported by the library.<br><br>
Field names can be up to ten bytes in length and can contain characters, numbers
or special characters in the name. The field methods are used to manipulate
the data in a record of a data file. There are several types of fields.<br><br>
<TABLE BORDER>
<CAPTION ALIGN="TOP"><h3>Field Types</H3></CAPTION>
<TR VALIGN="BASELINE">
<TR><TH ALIGN="LEFT">Type<TD>Size<TD>Allowable Values<TD>Schema Value
<TR><TH ALIGN="LEFT">Numeric<TD>0 - 17(include sign and decimal point<TD>+ - . 0 through 9<TD>XB_NUMERIC_FLD
<TR><TH ALIGN="LEFT">Character<TD>0 - 254<TD>Anything<TD>XB_CHAR_FLD
<TR><TH ALIGN="LEFT">Date<TD>8<TD>CCYYMMDD<TD>XB_DATE_FLD
<TR><TH ALIGN="LEFT">Floating Point<TD>0 - 17 (includes sign and decimal point<TD>+ - . 0 through 9<TD>XB_FLOAT_FLD
<TR><TH ALIGN="LEFT">Logical<TD>1<TD>? Y y N n T t F f (? - uninitialized)<TD>XB_LOGICAL_FLD
<TR><TH ALIGN="LEFT">Memo<TD>Fixed length portion - 10<br>Variable length 0 - 32760
<TD>Type III - Text<br>Type IV - Anything<TD>XB_MEMO_FLD
</TABLE>
<br><br>
Field names, types and lengths are defined when a data file is created.
After the file is created, the field characteristics can not be changed. To
change field characteristics, a new database table must be defined with the new
field requirements.<br><br>
<h2>Memo Fields</h2>
Memo fields are variable length data fields which are stored in two parts.
This first part is a ten byte field which is stored
in the fixed length record of the .DBF file. The variable data is stored in
a seperate .DBT file in 512 byte blocks. The ten byte field in the fixed
length portion of the record points to a .DBT block number.<br><br>
There are two versions of memo data files type III and type IV. Type IV
is more advanced in that released space can be reused and it also
supports BLOB data. The type III file is older technology, does not
support dynamic space reclamation and only supports string data.
See method xbDbf::SetVersion for controlling which version type you are
using.
<br><br>
To utilize memo fields, the application program must allocate a buffer
which is large enough to handle the memo data.<br><br>
<h2>Fields and Field Numbers</h2>
The Xbase routines can access field data via using field names or field
numbers. Field numbers are numbered 0-n where the first field in a datafile
is field 0 going through the last field n. Accessing fields by number is
slightly more efficient than accessing by name.<br><br>
<h2>Strings</h2>
Xbase64 includes support for a string class <em>xbString</em>.
The xbString class interface was originally derived from the
<em>Draft Standard C++ Library by P.J. Plauger</em> and modified.
If you are familiar with other string classes, this one should be similar.
Strings can be used to manage strings of character data.
<br><br>
<hr>
<p><img src="xbase.jpg"><hr>
</BODY>
</HTML>
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