created a reprex to troubleshoot kaijagahm/vultureUtils#44

spatsoc_data.table_reprex.R

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+# A reprex for trying to understand how data.table works in the context of spatsoc
+library(spatsoc)
+library(tidyverse)
+
+# A sample dataset
+load("data/repr.Rda")
+
+# convert to a data.table for spatsoc
+data.table::setDT(repr)
+
+# Group times
+spatsoc::group_times(repr, datetime = "timestamp", threshold = "10 minutes")
+
+# group spatial
+spatsoc::group_pts(repr, threshold = 1000, id = "trackId", coords = c("utmE", "utmN"), timegroup = "timegroup")
+
+names(repr) # this works! We have modified `repr` by reference, adding both `timegroup` and `group`
+
+# Okay, so even though it's quite poorly documented in spatsoc, both of these functions have the ability to modify by reference.
+# BUT! If we make changes to the data between doing group_times and group_pts, group_pts won't successfully modify by reference.
+# A sample dataset
+load("data/repr.Rda")
+data.table::setDT(repr)
+spatsoc::group_times(repr, datetime = "timestamp", threshold = "10 minutes")
+repr2 <- repr %>%
+  dplyr::select(-ground_speed)
+spatsoc::group_pts(repr2, threshold = 1000, id = "trackId", coords = c("utmE", "utmN"), timegroup = "timegroup")
+names(repr2)
+# In this case, it doesn't work. The `group` column doesn't get added.
+
+# So, this leads me to ask, what does the dplyr::select() step do to the data that prevents pass-by-reference from working?
+class(repr)
+class(repr2) # the classes are the same!
+
+str(repr)
+str(repr2) # except for the removed ground_speed column (expected), there seems to be no difference!
+# aaaaargh what the heck is going on????
+
+# Just to confirm: if we run setDT again on repr2, and *then* try running group_pts, it should work
+data.table::setDT(repr2)
+spatsoc::group_pts(repr2, threshold = 1000, id = "trackId", coords = c("utmE", "utmN"), timegroup = "timegroup")
+names(repr2)
+# wait WHAT??? Doing that did not fix the issue.
+# So it seems like the dplyr operation has broken this irreversibly and invisibly. I cannot figure out what was changed that is making this not work.