The ABC’s of AutoLISP by George Omura

Writing ASCII Files to Disk

AutoLISP lets you create ASCII files. You can use this capability to store different types of information ranging from the current drawing status to general notes that appear in a drawing. The program in figure 7.13 demonstrates this capability.

;Program to export text from AutoCAD -- Exprt.lsp (Defun C:EXPRT (/ fname txt selset count nme oldtx)

(setq fname (getstring "\nEnter name of file to be saved: "))

Figure 7.13: A text export program

Using a Text Export Program

Exit AutoCAD then create a file called Exprt.lsp containing the program shown in figure 7.16. Return to AutoCAD, load the file and proceed with the following:

1.Erase the text you imported previously.

2.Use the dtext command and starting the text at the coordinate 2,8, write the following lines:

For want of a nail, the shoe was lost; For want of a shoe, the horse was lost; For want of a horse, the rider was lost;

3.Enter exprt to start the text export program.

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The ABC’s of AutoLISP by George Omura

4. At the prompt:

Enter name of file to save:

Enter test.txt.

5. At the prompt:

Select objects:

Pick the lines of text you just entered picking each one individually from top to bottom. Press return when you are done.

The file test.txt is created containing the text you had just entered using AutoCAD's Dtext command. To make sure the file exists, enter the following at the command prompt:

type test.txt

The AutoCAD type command is the same as the DOS type command. It displays the contents of a text file. AutoCAD will switch to text mode and the contents of test.txt will be displayed on the screen.

The Exprt program starts by prompting the user to enter a name for the file to be saved to:

(Defun c:exprt (/ ts n dir)

(setq fname (getstring "\nEnter name of file to save: "))

Then, just as with the imprt program, the open function opens the file:

(setq txt (open n "w"))

In this case, the "w" code is used with the open functions since this file is to be written to. The next line obtains a group of objects for editing using the ssget function:

(setq selset (ssget))

This selection set is saved as the variable selset. Next, a variable count is give the value 0 in preparation for the while expression that follows:

(setq count 0)

The next if expression checks to see that the user has indeed picks objects for editing.

(if (/= selset nil)

If the variable selset does not return nil, then the while expression is evaluated:

(while (< count (sslength selset))

(setq entnme (ssname selset count))

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The ABC’s of AutoLISP by George Omura

(setq oldtx (cdr (assoc 1 (entget entnme))))

(write-line oldtx txt)

(setq count (1+ count))

)

This while expression uses the count variable to determine the number of times it must evaluate its set of expressions:

(while (< count (sslength selset))

The sslength function returns the number of objects contained in a selection set. In this case, it returns the number of objects recorded in the variable selset. This value is compared with the variable count to determine whether or not to evaluate the expressions found under the while expression.

The next two expressions extract the text string value from the first of the objects selected:

(setq entnme (ssname selset count))

(setq oldtx (cdr (assoc 1 (entget entnme))))

This extraction process involves several new functions which are discussed in chapter . For now, just accept that the end result is the assignment of the text value of the selected object to the variable oldtx.

Now the actual writing to the file occurs:

(write-line oldtx txt)

Here the write-line functions reads the string held by oldtx and writes it to the file represented by the variable txt.

Write-line's syntax is:

(write-line [string][file descriptor])

The first argument is the string to be written to file while the second argument is a variable assigned to the open file.

The next line increases the value of count by one:

(setq count (1+ count))

This expression counts the number of times a string of text, and therefore an object, has been processed. Since the while test expression checks to see if the value of count is less than the number of objects selected, once count reaches a value equivalent to the number of objects selected and processed, the while expression stops processing.

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The ABC’s of AutoLISP by George Omura

Finally, the all important close expression appears:

)

(close txt)

)

Just as with the C:IMPRT program, close must be used to properly close the file under DOS, otherwise its contents may be in-accessible.

Read-line and write-line are two of several file read and write functions available in AutoLISP. Table shows several other functions along with a brief description.

write-line string file_descriptor)Writesfile. a string to the screen prompt or if a file descriptor is provided, to an open

The three functions prin1, princ, and print are nearly identical with some slight differences. All three use the same syntax as shown in the following:

(princ [string or string variable][optional file descriptor])

The file descriptor is a symbol that has been assigned an open file.

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The ABC’s of AutoLISP by George Omura

The main difference between these three functions is in what they produce as values. The following shows an expression using prin1 followed by the resulting value:

(prin1 "\nFor want of a nail...")

"For want of a nail...""\nFor want of a nail..."

Notice that the string argument to prin1 is printed twice to the prompt line. This is because both prin1 and AutoLISP will print something to the prompt. Prin1 prints its a literal version of its string argument. AutoLISPs read-evaluate- print loop also prints the value of the last object evaluated. The net result it the appearance of the string twice on the same line.

Princ differs from prin1 in that instead of printing a literal version of its string argument, it will act on any control characters included in the string:

(princ "\nFor want of a nail...")

For want of a nail..."\nFor want of a nail..."

In the prin1 example, the \n control character is printed without acting on it. In the princ example above, the \n character causes the AutoCAD prompt to advance one line. Also, the string is printed without the quotation marks. Again, the AutoLISP interpreter prints the value of the string directly to the prompt line after princ does it's work. Table Shows the other control characters and what they do.

The print function differs from the prin1 function in that it advances the prompt one line before printing the string then it adds a space at the end of the string:

(print "\nFor want of a nail...")

"\nFor want of a nail..." "\nFor want of a nail..."

Just as with prin1, print prints a literal version of its string argument.

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The ABC’s of AutoLISP by George Omura

Conclusion

While a good deal of your effort using AutoLISP will concentrate on graphics and numeric computation, the ability to manipulate string data will be an important part of your work. You have been introduced to those functions that enable you to work with strings.

You have also seen how these functions work together to perform some simple tasks like reading and writing text files. But you don't have to limit yourself to using these functions for text editing. Since any kind of information can be stored as a string, you may find ways to further enhance your use of AutoCAD through the manipulation of string data. One of the more obvious uses that comes to mind is the importation of numeric data for graphing, charting or other types of data analysis. As long as data is stored in an ASCII format, AutoLISP can read it. And with AutoLISP's data conversion functions, numeric data can be easily translated from ASCII files.

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