Pascal Newsletter #19 - Free ezine for Delphi (and Kylix) programmers

Pascal Newsletter #19
The full source code examples of this issue are available for download.
Pascal Newsletter #19 - 25-MAR-2001

INDEX

1. A FEW WORDS FROM THE EDITOR
2. DELPHI PROGRAMMING - EXPRESSION EVALUATION THE XBASE WAY
   - INTRODUCTION
   - EVALUATING EXPRESSIONS GIVEN AS STRINGS OF CHARACTERS
   - TO BE OR NOT TO BE, THAT IS THE QUESTION
   - HOW EXPRESSIONS GET EVALUATED?
   - AND THE VARIABLES?
   - WE CANNOT USE CASE...OF
   - BINARY SEARCH
   - IMPLEMENTATION OF SEARCHES BY "HASHING"
   - WHERE SQL HAS NOT GONE BEFORE
   - NOTES ABOUT THE EXAMPLE
   - GOING BACK TO THE PACKAGE
   - WHAT'S NEXT
3. CAPTURING APPLICATION MESSAGES
4. DYNAMIC ARRAYS
   - MULTIDIMENSIONAL DYNAMIC ARRAYS
5. DELPHI ON THE NET
   ARTICLES IN THE BORLAND COMMUNITY
   - Delphi articles
     · Component Building for the Professional - by Ray Konopka
     · Migrating from Visual Basic to Delphi - by Borland Staff
     · Image feathering - Lab report by Earl F. Glynn II
     · Effectively Using Action Lists - by Ray Konopka
     · Delphi math resources
   - Kylix articles
     · Are you ready for Kylix? - by David Intersimone
     · Adventures in Kylix - by James R. Knowles
     · FreeCLX source code available on SourceForge - by John Ray Thomas
   - Interbase articles
     · 30 Days from Paradox to InterBase - by Skip Rowland
     · InterBase V6 - Performance Enhancements - by Dave Schnepper
     · The ABCs of Migrating from InterBase 5 to InterBase 6 - by Mark
     · InterBase Install and Licensing APIs - by Inprise Staff

________________________________________________________________________


1. A FEW WORDS FROM THE EDITOR


I'm pleased to announce that the winner of the license of the JfControls
Library (http://www.jfactivesoft.com) is Santiago Cubero, a young
programmer from Barcelona (Spain) who had the number 62.

In this issue I'm pleased to introduce another great article from Alirio
Gavidia that this time will show us how to evaluate expressions at
runtime in our Delphi applications.

Best regards,

Ernesto De Spirito
eds2004 @ latiumsoftware.com

________________________________________________________________________

JfControls Library. Multi-language. Multi-appearance. Skins. Privileges.
More than  40  integrated  and customizable components.  Impressive GUI.
Centralized  resources  administration.  Multiple  programming  problems
solved. For Delphi 3-7 and C++ Builder 3-6. http://www.jfactivesoft.com/
________________________________________________________________________


2. DELPHI PROGRAMMING - EXPRESSION EVALUATION THE XBASE WAY

   By Alirio A. Gavidia B.


INTRODUCTION
------------

A couple of years ago I was asked to solve a common "Compatibility"
problem. This was to be able to evaluate xBase expressions from Delphi
(by xBase we understand dBase and its derivatives like Clipper and
FoxPro).

All this happens because the xBase languages based all in some form
of interpretation support what is known as "Macros" (don't mistake them
with the storing of a series of commands and actions for future
execution).


EVALUATING EXPRESSIONS GIVEN AS STRINGS OF CHARACTERS
-----------------------------------------------------

The application created in that opportunity asked expressions to the user
as search filters for queries and reports, showing each time a button
that poped up a dialog form with the elements to build an expression.
The expression is of xBase type, so if the user knows this kind of
applications he/she can provide the expression directly.


TO BE OR NOT TO BE, THAT IS THE QUESTION
----------------------------------------

Basically there were two alternatives:

1.- Creating a control to evaluate xBase expressions.
2.- Using an existent control.

The third, teaching SQL to the users, was discarded due to the principle
of compatibility with the previous application.

There are interesting components in this area, matter of fact I found
that there are two sets of components for this: oriented to arithmetic
and oriented to databases. The first ones are practical, nice, cheap
(many are freeware) but they are usually oriented to the resolution of
graphs and mathematical problems. The second ones consider the matter
of the fields. I got interested in the components of the second kind. I
ended up using a BDE replacement with support for Clipper files that
accepts xBase expressions as filters.

However, at the same time I developed some functions to evaluate
expressions (they are in my web page and they are freeware, only
documented in Spanish). As a part of this article I introduce the class
TExpression. On the other hand I developed a components set (these are
shareware in English and with documentation in English/Spanish) that
allow to evaluate expressions like:

  (CUSTOMER->DOB > CTOD('01/01/1900')) .AND. CREDIT

when the majority of the components that evaluate formulas don't go
beyond constants expressions like:

  3-2/3.14


HOW EXPRESSIONS GET EVALUATED?
------------------------------

In general there are two steps: first the analysis (search) of patterns
and second the combination of the values obtained in the previous step
to get a result.

The patterns search (lexicography) allows to recognize in a series of
characters the type and functionality of certain patterns. For example
to recognize if a word starts with a digit it indicates that possibly
it's a numeric literal (or an error), or otherwise the word is possibly
and identifier that defines some value or operator that alters a value.

Once discovered the patterns, these are stored in some data structure
(normally a stack, also I've seen it with two stacks) and they get
reduced combining the values according to the identifiers. In the end
what remains is the result of the expression on the stack.


AND THE VARIABLES?
------------------

Variables and constants and in general anything associated to an
identifier has to be provided by the programmer thru an event or a list.
The components I'm introducing consider both ways. They also consider
arrays and functions (many predefined, mainly similar to the ones of
Clipper).

For example:

               HalSimpleExpr.Eval('Variable*Pi()');

In this case there are two identifiers "Variable" and "Pi". Since the
second one is followed by a pair of parenthesis it's a function. The
component fires the event OnRequestIdentifier (to evaluate Variable)
and OnRequestFunction (to evaluate Pi).

The evaluation of Pi would be handled this way:

  procedure TForm1.HalSimpleExpr1RequestFunction(Sender: TObject;
    ident: String; var value: Variant; var cancel: Boolean);
  Var
    col, row : integer;
  begin
    if CompareText(ident,'Pi')=0 then
      value := Pi
    else
      cancel := true
  end;

Note: I must point out that there is a great absent among the parameters
of the event; this is an array with the arguments of the function. In
the next version the argument of the evaluated function will be part of
the parameters, but for now it's just a public field of the class.

You'll see that if the function isn't Pi, the control raises an
exception (this is what the parameter "cancel" defines). The solution
to evaluate "Variable" is analogous but with another event. All this
works pretty well until we have some 50 functions.


WE CANNOT USE CASE...OF
-----------------------

It's not possible to use the "case" sentence since it works with
"integral" types (integer, char, boolean, etc.) and the alternative to
implement one "if" after the other is efficient for the first identifier
but terrible for the latests. The next step is using a sorted list.


BINARY SEARCH
-------------

The binary search on sorted lists looks like a good alternative, if we
talk of 1000 elements we can find any value in 10 steps 50% of the times
(the other 50% in less). In most of the cases this is good, but there's
an alternative worthy of study.


IMPLEMENTATION OF SEARCHES BY "HASHING"
---------------------------------------

Better than revising a list is having a function that beforehand signals
the position of the identifier in the list. For example a function that
adds the first character, the last minus 97 and the length of the
string. This way, if we would be looking for the word "program" we would
apply something like IndexOf("program") that would return Ord('p') +
Ord('m') - 97 + Length('program'), that is 112 + 109 + 7 – 97 -> 131.
This would mean the position 131 of the table.

As you can see, the searching with "Hash" tables improve the search
efforts dramatically. However, they waste memory. It's up to the
programmer to define which path to follow, nevertheless the components
I referred as shareware present lists of this kind as part of their
intrinsic working.


WHERE SQL HAS NOT GONE BEFORE
-----------------------------

It happens that SQL solves very well the problems regarding filters and
searching. But what happens if I wanted to make something less voluminous
like a small spreadsheet with formulas definable by the user? For this
purpose I'm attaching an example that uses a "StringGrid" and the class
TExpression. I believe that in general there are small needs for which
it it might result convenient the use of a formulas evaluator rather
than intending to imitate the xBase macros.


NOTES ABOUT THE EXAMPLE
-----------------------

The example is a "TStringGrid" control that makes use of the properties
"Cells" and "Objects". The first shows the result of a formula stored in
"Objects". The "strings" have to be specified with single quotes. The
files that save/load are simple text. Each defined formula requires one
to press the 'ok' key to be stored. There's a defined event to handle
the function "Cell(c,r)" where 'c' is the column and 'r' is the row.


GOING BACK TO THE PACKAGE
-------------------------

The use is simple, one method: eval. The expression is given as a string
parameter. However, there are different methods according to the type to
handle.

The components consider different types of syntax: xBase, Basic, Pascal
and Java. Fundamentally altering the use of boolean operators between
dots like being done in xBase expressions.

Three components with different scopes: ThalSimpleExpr, ThalMacroExpr
and ThalArithmeticExpr.

The first one doesn't have functions and identifiers lists management,
the second implements about 75 functions, lists management and datasets
references. The third one doesn't manage "aliases" nor datasets, but
nevertheless it supports many of the predefined functions.

The details are documented with the package and there are many examples
included. I hope you enjoy it.

You can download the shareware demo directly from my web page:

  http://www.gavidia.org/he-shareware.zip

You can also ask for it to the following email address:

  delphi@gavidia.org.

For the example attached to this article

   http://www.latiumsoftware.com/download/p0019.zip

In http://www.gavidia.org/pod there is freeware material related with
the subject.


WHAT'S NEXT
-----------

I'll take the time to give more information about the "hash" lists that
I mentioned. That will be the subject of my next article.


------------------------------

Copyright © 2001 by Alirio A. Gavidia B. All rights reserved.

The publication of this material is allowed by any means from anyone
as long as this material is not modified and the original source is
mentioned.

________________________________________________________________________


3. CAPTURING APPLICATION MESSAGES


Sometimes we need to capture Windows messages at an application level.
One way is by a hook, and another way is by using the Message event of
the Application object which is triggered every time the application
gets a Windows message. We are going to follow the second path. There
are two ways to set an event handler for this event: one is using an
ApplicationEvents object that comes in Delphi 5 (simply double click its
OnMessage combo box in the Object Inspector) and the other is doing it
"by hand":

  1) In the private section of your main form add the following
     declaration:

     procedure ApplicationMessage(var Msg: TMsg; var Handled: Boolean);

  2) Assign the OnMessage property in the Create event of the form
     for example:

     procedure TForm1.FormCreate(Sender: TObject);
     begin
       Application.OnMessage := ApplicationMessage;
     end;

  3) When you want to stop capturing messages all you have to do is set
     OnMessage to nil:

     procedure TForm1.FormDestroy(Sender: TObject);
     begin
       Application.OnMessage := nil;
     end;

Finally you should implement the procedure. For example, we are going to
trap the keyboard messages WM_KEYUP and WM_KEYDOWN to convert the
decimal point of the numeric keypad to a comma (this is useful in Spanish
applications).

  procedure TForm1.ApplicationMessage(var Msg: TMsg;
    var Handled: Boolean);
  begin
    case Msg.Message of
    WM_KEYDOWN, WM_KEYUP:
      case Msg.wParam of
      // Replace the period in the numeric keypad (key code = 110)
      // with a comma (key code = 188).
      110: Msg.wParam := 188;
      end;
    end;
  end;

Now you can drop an Edit control on your form and test it.

________________________________________________________________________


4. DYNAMIC ARRAYS


Dynamic arrays are arrays that can grow or shrink at runtime to
accommodate more or less elements. The declaration of a dynamic array is
much like the declaration of a static array, except that the index range
is omitted. For example:

  var
    a: array of integer;

A dynamic array initially has no elements. You should use the SetLength
procedure in your code to assign the number of elements needed. You can
call SetLength many times as needed. If the new length is greater than
the current number of elements, space for new elements is added, and if
it's smaller then the last elements of the array are disposed.

For example SetLength(a, 10) assigns space for 10 elements. Elements in
a dynamic array are indexed starting from 0 to one less the Length of
the array, so for example the following code can be used to initialize
the array elements to zero:

    for i := 0 to Length(a)-1 do
      a[i] := 0;

In the example attached to this article you'll see the use of a
dynamic array to store the names of the files in a folder. Naturally, a
string list would be better for the task, but a dynamic array was used
for a pedagogic purpose.


MULTIDIMENSIONAL DYNAMIC ARRAYS
-----------------------------

A two-dimension dynamic array can be declared this way:

  var
    a: array of array of integer;

This way you can assign both dimensions at runtime. The SetLength
procedure admits as many NewLength parameters as dimensions. For
example SetLength(a, 10, 10) would set the size of the array to an
square of 10 x 10.

One interesting thing is that instead of seeing your array as a matrix
(i.e. rectangular) you can see it as a vector of vectors that don't
necessarily have to have the same number of elements. For example the
following code creates a "triangular" array and initialize its elements
with consecutive numbers:

    k := 0;
    SetLength(a, 4);              // 4 rows
    for i := 0 to 3 do begin      // For each row
      SetLength(a[i], i + 1);     //   Set the number of columns
      for j := 0 to i do begin    //   Initialize the row
        a[i,j] := k;
        inc(k);
      end;
    end;

          0   1   2   3
        +---+
      0 | 0 |
        +---+---+
      1 | 1 | 2 |
        +---+---+---+
      2 | 3 | 4 | 5 |
        +---+---+---+---+
      3 | 6 | 7 | 8 | 9 |
        +---+---+---+---+

________________________________________________________________________


5. DELPHI ON THE NET


ARTICLES IN THE BORLAND COMMUNITY

Some interesting things can be found in the Borland Community this
month...


Delphi articles
===============

* Component Building for the Professional - by Ray Konopka
  This paper describes the critical extra steps that must be taken in
  order to develop professional, commercial-quality components.
  Specific topics will include creating online component help,
  effectively designing packages for component distribution, supporting
  multiple versions of Delphi, and how supporting automatic one-step
  installation.
  http://community.borland.com/article/0,1410,27057,00.html

* Migrating from Visual Basic to Delphi - by Borland Staff
  http://community.borland.com/article/images/26225/vbtodelphi.pdf

* Image feathering - Lab report by Earl F. Glynn II
  This article demonstrates how to merge a foreground and background
  image by fading from one image to the other, but only along a
  transition edge drawn with a simple drawing tool.
  http://www.efg2.com/Lab/ImageProcessing/Feathering.htm

* Effectively Using Action Lists - by Ray Konopka
  This paper describes, in detail, what actions and action lists can do
  for an application.
  http://community.borland.com/article/0,1410,27058,00.html

* Delphi math resources
  Here's an updated compendium of Delphi math resources.
  http://www.efg2.com/Lab/Library/Delphi/MathInfo/Resources.htm


Kylix articles
==============

* Are you ready for Kylix? - by David Intersimone
  Are you ready for Kylix? Of course you are. Is your system ready for
  Kylix? Here is a way to find out.
  http://community.borland.com/article/0,1410,26998,00.html

* Adventures in Kylix - by James R. Knowles
  Description of a first experience with Kylix
  http://www.ifm-services.com/people/jamesk/kylix/

* FreeCLX source code available on SourceForge - by John Ray Thomas
  FreeCLX, the Open Source project for Borland's CLX Component Library
  for Linux, is now available for collaboration.
  http://community.borland.com/article/0,1410,27100,00.html


Interbase articles
==================

* 30 Days from Paradox to InterBase - by Skip Rowland
  This article reviews the problems, the band-aids, the solutions, and
  the successes encountered during a large database migration from
  Paradox to InterBase
  http://community.borland.com/article/0,1410,27006,00.html

* InterBase V6 - Performance Enhancements - by Dave Schnepper
  This article will cover changes in InterBase 6 that directly or
  indirectly enhance throughput and general performance of queries.
  http://community.borland.com/article/0,1410,27005,00.html

* The ABCs of Migrating from InterBase 5 to InterBase 6 - by Mark
  Duquette
  This technical article is to help you learn about the ins and outs of
  migrating your applications from InterBase 5 to InterBase 6.
  http://community.borland.com/article/0,1410,27004,00.html

* InterBase Install and Licensing APIs - by Inprise Staff
  InterBase 6 has a new install API that automates many chores involved
  in embedding the database into an application. This session describes
  the API and how to use it.
  http://community.borland.com/article/0,1410,26415,00.html

________________________________________________________________________


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________________________________________________________________________

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________________________________________________________________________

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     Copyright (c) 2001 by Ernesto De Spirito. All rights reserved.
________________________________________________________________________
The full source code examples of this issue are available for download.
Errors? Omissions? Comments? Please contact us!