My BASIC program Macro is a drawing tool. There are 8 drawing commands available, and the coordinates for each are recorded and stored, along with the command number, in a single element of _LM. It's almost done, but it's missing a desperately needed feature: An undo button. I can't just do the opposite of the command that was just performed, because that could change pixels that weren't changed by the instruction being undone. The only way I can see to circumvent this situation would be to remove the last element of the list and call Read to redraw the whole thing, which would be an absolute pain. Can anyone think up a more efficient way to implement an undo feature?

In computer programs, what is usually done is to save all the things that are done and then undo them, but this is theory, I have no idea about the practical way of 'Undo' in Basic.

Before every action, you could store the screen to an OS Pic variable, and then using Undo would recall the pic var onto the screen, effectively "undoing" the last action

But that would only allow one undo. That's not enough. I want the ability to undo ad infinitum.

Then it'd requite saving infinite Pic Variables, but they are finite.

Hence the necessity of another method.

I think you could use the algorithm of a snake.

And what algorithm would that be?

I'd be curious about where the RAM to store all of that information is. TI-BASIC doesn't give you access to a whole lot of memory with which to store data.

However, I made a CAD tool awhile back in BASIC and here's what I did:

Every time the user drew a point, the 3D spatial coordinates of that point would be stored in an array in sequential order. When the user needed to erase something, the program searched searched the list for a point close to the cursor and erased it. All elements after the deleted element were then placed in a temporary list and the lists then augmented together again to remove that element. The location on the screen that corresponded to the coordinates was simultaneously erased pixel by pixel. That way, the point would be erased until the next time the screen needed to be redrawn, at which the point would not show up because it was no longer present in the array.

Yeah one issue with multiple undos is that it would take loads of memory. Even some computer programs lets the user set a limit on undos, because otherwise they run out of RAM quick.

I'm with ztrumpet with this one, store a max of 3 or 4 picture variables and recall them when you want to undo.  Quick, simple, and it works.  Any other way is going to be really complicated, and probably not the fastest