Read a trace of a dynamic target.

Read a target's dynamic trace from the cache. Best used on its own outside a drake plan.

read_trace(
  trace,
  target,
  cache = drake::drake_cache(path = path),
  path = NULL,
  character_only = FALSE
)

Arguments

trace	Character, name of the trace you want to extract. Such trace names are declared in the .trace argument of map(), cross() or group().
target	Symbol or character, depending on the value of character_only. target is T=the name of a dynamic target with one or more traces defined using the .trace argument of dynamic map(), cross(), or group().
cache	drake cache. See new_cache(). If supplied, path is ignored.
path	Path to a drake cache (usually a hidden .drake/ folder) or NULL.
character_only	Logical, whether name should be treated as a character or a symbol (just like character.only in library()).

Value

The dynamic trace of one target in another: a vector of values from a grouping variable.

Details

In dynamic branching, the trace keeps track of how the sub-targets were generated. It reminds us the values of grouping variables that go with individual sub-targets.

See also

get_trace(), subtargets()

Examples

if (FALSE) {
isolate_example("demonstrate dynamic trace", {
plan <- drake_plan(
  w = LETTERS[seq_len(3)],
  x = letters[seq_len(2)],

  # The first trace lets us see the values of w
  # that go with the sub-targets of y.
  y = target(paste0(w, x), dynamic = cross(w, x, .trace = w)),

  # We can use the trace as a grouping variable for the next
  # group().
  w_tr = read_trace("w", y),

  # Now, we use the trace again to keep track of the
  # values of w corresponding to the sub-targets of z.
  z = target(
    paste0(y, collapse = "-"),
    dynamic = group(y, .by = w_tr, .trace = w_tr)
  )
)
make(plan)

# We can read the trace outside make().
# That way, we know which values of `w` correspond
# to the sub-targets of `y`.
readd(y)
read_trace("w", y)

# And we know which values of `w_tr` (and thus `w`)
# match up with the sub-targets of `y`.
readd(z)
read_trace("w_tr", z)
})
}