Subtle thing going on with NotebookDirectory[]—if the notebook exists at all as a file which was ever saved, then NotebookDirectory[] will return the save location, but it doesn't have anything to say about the "current state" of the notebook vs. its "saved state".

You can get something kind of related via NotebookInformation[]—namely, you can get a "saved" time and a "modified in memory" time. I thought that maybe taking their difference could be instructive, but I think that just running the code itself counts as "modified in memory", so you wind up getting the time in seconds since your last save with this:

Subtract @@ 
 Lookup[Association[NotebookInformation[]], {"MemoryModificationTime",
    "FileModificationTime"}]

As for the greyed out icon, maybe I can tell you another new thing re: macOS—the reason that that icon gets greyed out is that it's a fully-functional "link" to the file in question! When you have unsaved changes, the saved file and the "working file" no longer match up, so it greys out to indicate that the "link" is no longer active. But if you've got an up-to-date file open, you can drag that icon around and it will act like the file itself—that is, you could drag it onto this page to embed it in a post, or drag it over to an email to attach it, and so on.

In any case, I'm glad you're having fun with the code!

EDIT: However, if you're in another notebook, you can point to the Notebook of Interest as follows and this will work:

Subtract @@ 
 Lookup[Association[
   NotebookInformation[
    NotebookOpen[
     "path/to/NotebookOfInterest.nb"]]], {"MemoryModificationTime", "FileModificationTime"}]

Arben, the best instructor ever~ Yeah....

Hi Arben! Christine emailed me the notebook and recording with a passcode. Seems there was glitch with the zoom auto generated(?) email. All good there.

Regarding whether the notebook is saved, I see three saved notebook verifying cues:

1. The red circle button in the upper left corner of the window. It has a black dot in it if not saved. I did not know that Mac OS motif. Thanks!
2. The Mathematica logo is greyed out if the file is not saved. The circle button is more overt.
3. I tried NotebookDirectory[]. It seems to return the same directory regardless whether notebook is saved.

See the google drive link to video.

Having a blast tinkering with the code from your lessons. Thanks.

Thanks Ray! I can't say I'm the most organized notebook creator out there; back when I explicitly had research that I was working on with my notebooks I usually would just iterate live, keep comments, and save a new version of the notebook once I had made "significant enough" changes. I was often working with fairly chunky sets of data (high resolution topographies), so my workflow involved getting to the point where I'd do the relevant calculations, exporting to .mx, then commenting out the code and just reimporting the relevant things back in.

Here at Wolfram U, we do have a git repository where we keep all of our course materials. In some courses there are tables of contents which link to other notebooks, but largely we try and have everything web deployed (which we've mostly automated as well) so those links often just go to cloud notebooks rather than linking to local copies. Relatedly, there's some new functionality in 14.1 that may interest you: https://reference.wolfram.com/language/ref/Diff.html

Hey Soomi! No final exam for this—just a quiz! (Also, I see your emails coming in but I'm way behind on everything that isn't Data Science Boot Camp right now!)