Mission: To understand Undo and all its variations

31 May, 2000 By: Bill Fane

These are the voyages of the CADship LearnCurve. Its mission: To seek out new commands, to explore AutoCAD functions, to boldly go where the manual has never gone before. Captain’s log (rtos (/ (getvar "cdate") 10000))= 2000.03311841. CADfleet Command reports a strange anomaly in the timespace continuum in the AutoCAD sector of the universe.

Mr. Sulu, lay in a Line From point: 2,3 To point: 4.3,6.7 To point: 1.8,2.9 To point: . . .
Excuse me, Captain, I made a mistake typing in the last coordinates. Can we go back to the previous point?

From within a particular run of the Line and Pline commands, you can always enter the single letter U (upper- or lowercase) instead of specifying the next point. This undoes the last segment and reattaches the elastic line to the previous point. From here you can select a new point and carry on, press <Enter> or <Esc> to terminate the command, or enter another U to step back one more segment.

You can repeat this sequence and choices as needed. You can go two steps forward, one back, four forward, five back, and so on. In fact, you can enter a U often enough to back out to the beginning of this particular run of Line or Pline.

At the Command prompt, typing the single letter U reverses the action of the last command. Once again, you can invoke it repeatedly to back up all the way to the start of the editing session.

You can invoke a U in several different ways, and they differ a bit among releases. Typing it in at the Command prompt always works, and an Undo arrow appears on the standard toolbars. In Release 14, clicking the Undo arrow button from within Line and Pline backs you up one segment, but in AutoCAD 2000 it terminates the command and undoes all segments created within it.

In AutoCAD 2000 you can right-click from within the Line and Pline commands. The Undo pick from the pop-up menu undoes only one segment.

Wait—that’s too far
Captain, we’ve wandered too far in the wrong direction. It’s imperative that we return to our starting point immediately! Oh, no! Our scanners canna’ detect the Undo command.

It has disappeared from the menus and toolbars, but it still exists—just type it in at the Command prompt. Unlike the single U, Undo has a number of options. When you invoke it, it responds with: Auto/Back/Control/End/ Group/Mark/<number>.

The default, indicated by the <angle brackets>, asks you to type in a number and press <Enter>. This reverses time, and AutoCAD runs backwards as it undoes the specified number of commands. The name of each command is displayed as its universe unwinds.

This is very fast and very powerful, but it can be a little unpredictable. The problem is that AutoCAD’s definition of what constitutes each step may be different from your expectations. For example, consider the Line command. Although it draws separate, independent line sections, Undo and U from the Command prompt consider each run the Line command to be a single command. All sections that you create in one run are deleted in one hit.

Captain, the Klingons have turned on the invisibility cloaking device!

Any transparent commands invoked within a regular command are considered part of the regular command. They are undone in one step along with their host command. On the other hand, pans and zooms that you perform with the scroll bars or a wheel mouse undo differently.

The semi-good news is that the AutoCAD Two-Step is available. If you U or Undo too far, the Redo command takes us one step forward again.

The bad news is that it only works once—you can U or Undo as many steps as you want, but you can only Redo the last U or Undo, and you must Redo immediately after the Undo to redo what you undid. There is no Doo-Doo command, although AutoCAD users are often overheard muttering an equivalent word.

The Erase command has its own undo capability. From the Command prompt, enter Oops at any time, even after other commands, to unerase the last set of erased objects. Undo has disappeared from the menus and toolbars, but it still exists—just type it in at the Command prompt.

Leave your mark
Captain, we are heading into uncharted territory. I suggest that we leave a beacon so we can find our way back.

Very well, Mr. Spock. Drop the beacon on my mark.

One option of the Undo command lets you leave a mark at the current condition of a drawing. You can then explore to your heart’s content—experimenting, testing, and trying alternatives. If things get away from you, simply Undo Back to return to the mark in one step.

Everything between the Undo Back and the mark is undone, including the mark placement. If you want to go exploring again, be sure to leave another Undo mark first.

Like Hansel and Gretel with their trail of bread crumbs, you can leave a series of Undo marks at any place to which you might want to return. Repeated Undo Back operations step you back one mark at a time, removing each mark as you go.

The Undo mark option outranks Undo <number>. If you leave a mark, proceed ahead five steps, then issue the command Undo 999, the operation stops at the mark.

All engines—stop
Captain, we seem to have encountered a strange force field that won’t let us go back any further.

The Undo Group sequence is very similar to the Undo mark sequence except for one significant difference. Undo mark is open-ended, but you can close off Undo group with Undo end.

When U and Undo encounter Undo end, the next step back undoes everything back to the initial Undo group as a single command. The sequence of commands between Undo group and Undo end behaves like a single block of commands in the same way that a block of entities behaves like a single object.

The problem arises when AutoCAD automatically invokes an Undo group without telling you. This happens when Undo auto is on (the default). AutoCAD then automatically inserts Undo group at the start of any menu pick and Undo end when it is complete, so that all commands within a menu macro are a single group.

The main problem arises when you cancel out of the middle of a menu macro. AutoCAD opens the group but doesn’t end it, so it’s left hanging. Any attempt to Undo back past it produces the message: Start of Group encountered; enter Undo End to go back further.

The Undo command appears to be stuck. If you invoke it again, it won’t go back any further—it just repeats the message. If all else fails, follow the prompt’s advice and enter Undo End to close off the group (it now contains zero commands) and carry on. This is the photon torpedo that lets us blast through.

Empty the hold
AutoCAD tracks everything you do in a scratch file that it opens so it can retrace your steps as you Undo. Writes and reads over a network are relatively slow. You should use the Preferences command (or File|Options from the menu) to change the temporary drawing file to a local drive.

Captain! The temporary drive is overloading— we’re going to crash.

The bad news is that if the drive you specify for temporary files fills up, AutoCAD self-destructs with little warning. I suspect this is the real source of many unexplained AutoCAD crashes.

Luckily, there is a manually operated safety valve. Enter Undo Control, and AutoCAD gives you three choices:
  All <Default>
  (Enables all features of Undo.)
  (Disables Undo completely.)
  (Limits Undo to a single U or Undo operation—you can back up one step, but that’s all.)

If you invoke Undo none or Undo one within an editing session, AutoCAD dumps any Undo information saved earlier and frees up affected disk space. This can prevent a catastrophic crash when you heavily edit a large drawing. If desired, you can turn Undo control (all) back on for more editing.

And now for something completely different
When traveling by air, do not tightly close the covers on SCUBA diving cameras and flash units. They are designed to withstand high pressures from outside, but not from within. As the aircraft ascends and the pressure decreases in the cargo hold, the excess pressure within the case finds ways to leak out.

When the aircraft descends and the outside air pressure increases back to normal, the covers reseal, creating a partial vacuum within the cases. This can make them almost impossible to open at your destination.

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