The ABC’s of AutoLISP by George Omura

Chapter 9: Using Lists to store data

Introduction

Using an Element of a List as a Marker

Getting Data from a List

Finding the Properties of AutoCAD Objects

Using Simple Lists for Data Storage

Using Selection Sets and Object Names

Evaluating Data from an entire list at once

Understanding the Structure of Property Lists

Using Complex Lists for Data Storage

Changing the Properties of AutoCAD Objects

Using Lists for Comparisons

Getting Object Names and Coordinates Together

Location Elements in a List

Conclusion

Searching Through a List

Introduction

We mentioned that there are actually two classes of lists, those meant to be evaluated, which are called expressions or forms, and lists that are repositories for data such as a coordinate list. No matter what the type of list you are dealing with, you can manipulate lists to suite the needs of your program. In this section we will look at the general subject of lists and review some of the functions that allow you to manipulate them.

There are a several functions provided to access and manipulate lists in a variety of ways. You have already seen car and cdr. Table 9.1 shows a list of other functions with a brief description of their uses.

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The ABC’s of AutoLISP by George Omura

So far, we have concentrated on the use of lists as a means of structuring and building your programs. But lists can also be use as repositories for data. You have already seen how coordinate list are used to store the x and y coordinate values of a point. Lists used for storing data can be much larger than the coordinate example. Consider the mdist program you saw in chapter 5. This program uses the append function to constantly add values to a list. This list is then evaluated to obtain the sum of its contents (see figure 9.1).

(Defun C:MDIST (/ dstlst dst) (setq dstlst '(+ 0))

(while (setq dst (getdist "\nPick point or Return to exit: ")) (Setq dstlst (append dstlst (list dst)))

(princ (Eval dstlst))

)

)

Figure 9.1: The Mdist program

We have an example here of a list being both a repository of data and a form or a list that can be evaluated. This is accomplished by starting the list with a functions, in this case, the plus function. Each time a value is appended to

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The ABC’s of AutoLISP by George Omura

the list, it is evaluated to get the sum of the numeric elements of that list.

Suppose your have a list that does not contain a function, but you want to apply some function to it. The following sections discusses ways you can use the functions listed in table 9.1 perform computations on lists.

Getting Data from a List

In the mdist program, a function was applied to a list to get the total of all the numbers in that list. Functions like plus, minus, multiply and divide accept multiple numeric values for arguments. But what if you want to apply a list to a function that will only take single atoms for arguments.

Using Simple Lists for Data Storage

Mapcar is used where you want to use a list as a queue for arguments to a function. It allows you to perform a recursive function on a list of items. For example, suppose you want the sequential numbering program from chapter 5 to place the numbers at points you manually select rather than in a straight line. Figure 9.2 shows a program that does this:

;Program to write sequential numbers -- Seqrand.lsp

Figure 9.2: Program to place sequential number in random locations

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The ABC’s of AutoLISP by George Omura

In the C:SEQRAND program, the following while expression is used to allow the user to pick random point locations for the numbered sequence:

(while (not (not rand))

(setq rand (getpoint "\nSelect points in sequence: " ))

(setq ptlst (append ptlst (list rand) ))

)

This while expression creates the list ptlst comprised of points entered by the user. The user see the prompt:

Select points in sequence:

each time he or she selects a point. Once the user is done, the mapcar expression reads the list of points and applies them to a function that enters the sequence of numbers at those points:

(mapcar '(lambda (rand) (if (not (not rand)) (progn

(command "text" rand "" "" currnt) (setq currnt (1+ currnt))

)

)

)

ptlst

)

In this set of expressions, mapcar applies the elements of the list ptlst to a lambda expression. You may recall that a lambda expression is like a function created using defun. The difference being that lambda expressions have no name. The lambda expression above uses the single argument rand and, using the AutoCAD text command, writes the variable currnt to the drawing using rand as a coordinate to place the text. The lambda expression also adds 1 to the currnt variable increasing the number being added to the drawing by 1.

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