Site Map

Index
   AutoCAD LT
      Tips & Tricks
      DIESEL
         Defined
         Listing of Functions
         Storing Variables
         Evaluation Sequence
         Getting User Input
         Errors
         Examples
       System Variables
       Custom Menus
   Visual Basic
   Resources on the Net
   Contact BigByte

DIESEL was introduced to AutoCAD users in AutoCAD R12 for DOS. DIESEL can return or pass string values to AutoCAD commands and return the value of any AutoCAD variable. DIESEL expressions can also return string values to the menu itself, altering the appearance and content of a menu label. In the absence of AutoLisp, DIESEL can be used to write simple macros for AutoCAD LT.

Listing of Functions

Storing Variables

Storing Integers
Five AutoCAD system variables that have been included in the LT subset of system variables are the variables USERI1, USERI2.......USERI5. These variables are not mentioned in the AutoCAD LT Users Guide nor the Online Help. They are not used directly by the program and as such can be used to store any whole number between -32768 and 32767. The value is saved in the drawing and will persist between sessions.

Storing Real Numbers
Another five AutoCAD system variables that made it into LT are the variables USERR1, USERR2.......USERR5. Again they are not documented and can safely be used to store a number between 9.9E-99 and 9.9e+99. The value is saved in the drawing and will persist between sessions. 

Storing Strings
Unfortunately five variables that didn't make the transition to LT are the AutoCAD system variables USERS1-5 which can be used to store string variables. This was either an amazing lack of foresight on the behalf of AutoDesk or a deliberate (successful) attempt to limit the potential of LT thereby protecting AutoCAD's market.

What ever the case, we need to find another way to store string values. Fortunatly LT provides the command SETENV variablename myvalue which will create the string key variablename in the System Registry and assign it the value of myvalue. The command GETENV variablename can then be used to retrieve the value of variablename.

Evaluation Sequence

If you start to write your own DIESEL macros sooner or later you will come across an extremely frustrating idiosyncrasy of the DIESEL macro language. That is the order in which DIESEL expressions are evaluated.

As an example lets look at how DIESEL evaluates the following macro

^C^C_id;\_setenv;pt1;$m=$(getvar,lastpoint);_line;$(getenv,pt1);\;

Looking at the above example one would expect it to be evaluated like this:

• The _id command is issued with a backspace pausing the macro to allow the user to pick a point on the screen.
• The _setenv command is then issued to save the value of the system variable lastpoint to a variable named pt1.
• The _line command is the issued and the value of pt1 retrieved as the first point and another backspace pausing for the User to pick the second point.

What actually happens can be seen by setting the system variable macrotrace to 1 (on). This displays all DIESEL expressions and there results on the command line. This is what would be seen if the above macro was run with macrotrace on.

Command: _id
Point: X = 5.8769 Y = 6.1587 Z = 0.0000
Command: _setenv
Variable name: pt1
Value <6.22905364,5.59305622,0>: Eval: $(GETVAR, lastpoint)
===> 5.87689209,6.1586937,0
Eval: $(GETENV, pt1)
===> 6.22905364,5.59305622,0
5.87689209,6.1586937,0
Command: _line
From point: 6.22905364,5.59305622,0
To point:

What is all this telling us. Compare the co-ordinates returned by the _id command with the co-ordinates used as the from point in the _line command. It is a different point so what is going wrong.

Nothing! This is how DIESEL macros are evaluated by AutoCAD. Let me explain

• The macro is copied directly to the command processor until the charactors '$m=' are encountered which indicates that a DIESEL expression follows. The remainer of the macro is passed to the DIESEL interpreter and evaluated and the result passed to the command processor
• The end result is that value of pt1 as used by the _line command is returned BEFORE pt1 has been set to the value of lastpoint. In actual fact the first time you run a example macro it will fail altogather because pt1 has not been initialised and will return null

So if this is how DIESEL expressions are evaluated but it is not what we want what are we going to do about it. We enclose the second DIESEL function with quotes (") like so:

^C^C_id;\_setenv;pt1;$m=$(getvar,lastpoint);_line;"$m=$(getenv,pt1)";\;

the output with macrotrace on would now be

Command: _id
Point: X = 6.2291 Y = 6.1740 Z = 0.0000
Command: _setenv
Variable name: pt1
Value <11.69521383,5.71535621,0>: Eval: $(GETVAR, lastpoint)
===> 6.22905364,6.1739812,0
6.22905364,6.1739812,0
Command: _line
From point: Eval: $(GETENV, pt1)
===> 6.22905364,6.1739812,0
6.22905364,6.1739812,0
To point:

The addition of the pair of quotes prevents the DIESEL interpreter from evaluating the second expression. After evaluating the first expression the second expression is passed back to the command processor minus the pair of quotes. The command processor then continues with the _line command before the second '$m=' charactor sequence is encountered and the remainder of the macro is again passed the the DIESEL interpreter this time returning the new value of pt1

As a pair of quotes are discarded each time the macro is passed to the DIESEL interpreter each level of nesting requires an additional pair of quotes. Also you must preceed all nested DIESEL expressions with $m= to call the DIESEL interpreter unlike in the first example where the second DIESEL function opened with $(

Note: I do not pretend to be an expert on this subject as I still find it very difficult to get complex macros to be evaluated in the sequence desired. The evaluation sequence I have explained here is merely my observations and will gladly stand corrected if I anyone has a better explanation.

Getting User Input

Error Messages

If the DIESEL intepreter encounters an error whilst evaluating a macro it will embed an error message in the output string

Copyright © 1998 by BigByte Drafting.
All rights reserved. Revised: 30 Dec 1999 20:30.