[acs-r] List of subgeography ids contained in a geo.set?

[Peter Schmiedeskamp]

If I were looking to get a list of tract / blockgroup / block IDs back
from a geo.set, is there a straightforward way of doing so?

For example, it might be nice to get a list of tract IDs from the
example geo.set in the acs docs:

    > psrc <- geo.make(state="WA", county=c(33,35,53,61), tract="*")
    > tract_ids(psrc) # made up tract_ids function
    [1] 24300 24400 (...)

I see that, if I fetch an acs object, the @geography slot contains the
tract attribute. However, I'm wondering if there is a way of getting
the tracts independent of fetching a full ACS object. I'm guessing
that the tract numbers are only fetched as the result of submitting
the API request to the census, and perhaps there is no way to return
an empty acs object with a NULL table number?

Cheers,
Peter