The ABC’s of AutoLISP by George Omura

Once the new selection set containing the filtered objects is complete, the last expression returns either the selection set of objects, or if there is only one object in the selection set, the object name.

(if (= 1 (sslength newent))(ssname newent 0) newent)

);end defun

The function sslength is used with the = predicate to see if newent contains only one element. If it does, then ssname is used to extract the object name of the element from the selection set newent. Otherwise, the entire selection set newent is returned. This last step is added to allow the user to use lfilter where only one item is accepted for input such as the offset or fillet commands.

In this sample program, layers are used to filter objects, but you can use any object property as a filter. You can filter objects by linetype, color, or any property available from the property list. Consult the list of group codes in Appendix C for a list of group codes and their associated properties.

Selecting Objects Based on Properties

Another method for filtering can be found built into the ssget function. Ssget allows you to select objects based on a filter list. This filter list is an association list much like a property list. The Getlayer function shown in figure 10.10 simply selects the entire contents of a layer that the user specifies.

;function to select all entities on a layer-----------------------------------

(defun GETLAYER (/ lay) (setq lay (list (cons 8

(strcase (getstring "\nEnter layer name: "))))) (ssget "X" lay)

)

Figure 10.10: The Getlayer function.

Let's see what it does:

1.Save and exit the chapt10 file.

2.Open a file called Getlayer.lsp and copy figure 10.10 into the file. Save and exit the file.

3.Return to the chapt10 drawing file then load Getlayer.lsp.

4.Issue the Erase command. At the Select object prompt, Enter:

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The ABC’s of AutoLISP by George Omura

(getlayer)

5. At the prompt

Enter layer name:

Enter center. All the objects on layer Center will ghost.

6. Press return. All the objects on layer Center are erased.

Getlayer does its work by using the "X" argument to ssget. This argument allows ssget to create a selection set based on an association list of properties. In the case of Getlayer, the list is one element long.

First, Getlayer prompts the user for a layer:

(defun GETLAYER (/ lay)

(setq lay (list (cons 8

(strcase (getstring "\nEnter layer name: ")))))

The layer name is used to construct an object property dotted pair much like the one in the flayer function. This dotted pair is further included in a list using the list function. The result is a list containing a single dotted pair:

((8 . "CENTER"))

This list is assigned to the variable lay which is in turn applied to ssget to create the selection set:

(ssget "X" lay)

)

Here we see the "X" argument used with ssget to tell ssget that a filter list is to be used to create the selection set. Ssget then searches the drawing database to find all the objects that have properties that matches the filter list.

Getlayer could have been simplified to one expression thereby eliminating the need for the lay variable:

(ssget "X" (list (cons 8

(strcase (getstring "\nEnter layer name: ")))))

We include the lay variable to help explain how this function works.

A filter list can have more than one property sublist element much like an objects property list. But ssget will only accept certain group codes in the filter list. Table shows those group codes and their associated properties:

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The ABC’s of AutoLISP by George Omura

Group code Meaning

6Linetype name

7Text style name

8Layer name

38Elevation

39Thickness

Accessing AutoCAD's System Tables

The tblnext and tblsearch functions are provided to help you gather information about layers, linetypes, views, text styles, blocks, UCSs and viewports. Each one of these AutoCAD tools is represented in a table that contains the tools status. Tblnext and tblsearch return this table information in the form of association lists similar to object property lists. But unlike object property lists, you cannot modify lists returned from Tblnext and tblsearch. However, you can modify the settings associated with a tblnext or tblsearch listing using the standard AutoCAD commands.

To use tblnext, enter the following at the command prompt:

(tblnext "layer")

You will get an association list similar to the following:

((0 . "LAYER") (2 . "0") (70 . 0)(62 . 7) (6 . "CONTINUOUS"))

The individual dotted pairs can be extracted from this list using Assoc just as with any other association list or property list. Enter the tblnext expression above again and you will get an association list of the next layer. Each time tblnext is used, it advances to the next table setting until it reaches the last item in the particular table you are searching. Once it reaches the end, tblnext returns nil. To reset tblnext to read from the beginning of the table again, you include a second argument that evaluates to non-nil as in the following:

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The ABC’s of AutoLISP by George Omura

(tblnext "layer" T)

If this expression is entered, you will get the same list as the one you got the first time you used tblnext. We used T as the second argument but it could be any expression that evaluates to non-nil. Once you get a list, you can manipulate in the same way as any other association list.

Tblsearch works slightly differently. Instead of stepping through each table item, tblsearch will go to a specific table item which you name. Enter the following:

(tblsearch "layer" "hidden")

You will get the association list pertaining to the layer "hidden".

((0 . "LAYER") (2 . "HIDDEN") (70 . 0)(62 . 7) (6 . "HIDDEN"))

If you include a non-nil third argument to tblsearch, then the next time tblnext is used, it will start from the next item after the one obtained from tblsearch.

Though we used layer settings as an example for tblnext and tblsearch, any of the table settings mentioned at the beginning of this section can be used.

Figure 10.11 shows a program that uses tblnext to store layer settings in an external file. This program can be useful if you use a single multi-layered drawing for several types of output. For example, an architect might have a drawing that serves as both an electrical layout plan and a mechanical floor plan with different layers turned on or off depending on which type of plan you want to edit or print. You can store your different layer settings for the electrical and mechanical plans then restore one or the other group of settings depending on which plan you indent to work on.

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The ABC’s of AutoLISP by George Omura

;Program to save layer settings in a file -- Lrecord.lsp ;-----------------------------------------------------------------------------

(defun c:lrecord (/ fname lafile record)

(setq fname (getstring "\nEnter name of layer file: "));get name of file

Figure 10.11: A program to store layer settings

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The ABC’s of AutoLISP by George Omura

The Lrecord program show at the top of the figure 10.11 simply creates a file then copies each layer association list into the file. Lrecord uses the prin1 function to perform the copying because prin1 does not affect the list in any way. If princ were used, the strings data types within the list would be striped of their quotation marks. Also, Writeline is not used since it expects a string argument. Both princ and prin1 will write any data type to a file.

The Lrestore program simply reads each line back from a file created by Lrecord. Since the Read-line function returns a string, the read function is used to strip the outermost level of quotation marks from the string to return the association list.

(setq flayer (read (read-line lafile)))

The Layer data is then extracted from this list and applied to the layer command which sets the layer back to the saved settings. The if conditional test is used to see if the layer to be restored is the current layer. A different command expression is evaluated depending on whether the layer in question is current or not. This is done since the layer command will issue an extra prompt if the current layer is to be turned off or frozen.

(if (and (< (cdr oldcset) 0) (equal lname clayer)) (command "layer" "C" (cdr oldcset) lname "Y" "") (command "layer" "C" (cdr oldcset) lname "")

)

Conclusion

You have seen a variety of ways to select, edit, and manipulate AutoCAD objects. Selection sets and object filters can provide a powerful means to automating AutoCAD. Tasks that would normally take several minutes to perform manually can be reduced to a few seconds with the proper application of selection sets and recursive expressions.

You have also seen how changes in the way you write your program can affect your programs speed. Though the speed of your programs may not be an issue to you now, as your experience with AutoLISP expands, your need for speed will also expand.

In the next chapter, we will continue the discussion of object access by looking at how polylines and attributes can be edited with AutoLISP.

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