Fix NOTE about undefined global variable t.1 in pw_info()

[jdblischak]

This is a follow-up to PR #579. It introduced the following NOTE from R CMD check:

* checking R code for possible problems ... [15s/10s] NOTE
pw_info: no visible binding for global variable ‘t.1’
  (/home/runner/work/gsDesign2/gsDesign2/check/gsDesign2.Rcheck/00_pkg_src/gsDesign2/R/pw_info.R:189)
Undefined global functions or variables:
  t.1

This is caused by the following line, which uses the non-standard evaluation feature of {data.table} to update the table by reference:

gsDesign2/R/pw_info.R

Line 189 in 5103d9c

ans[, t.1 := NULL]

There are various workarounds to fix this:

• Add t.1 <- NULL just above the line. This is sufficient to satisfy R CMD check, but clutters the code with another line related to an unimportant intermediate variable
• Replace ans[, t.1 := NULL] with ans[["t.1"]] <- NULL. I confirmed via data.table::address() that the latter makes a copy of the data table, but for such a small object, this will have a negligible effect on performance

However, I was concerned why t.1 was introduced in the first place. I realized it came from this step, where t is both calculated and used as a grouping variable. Since t is in by, there is by definition only a single value, and thus no need to run min(t).

gsDesign2/R/pw_info.R

Lines 176 to 182 in 5103d9c

	# pool strata together for each time period
	tbl_event <- tbl_event[, .(
	t = min(t),
	event = sum(event),
	info0 = sum(info0),
	info = sum(info)
	), by = .(time, stratum, t, hr)]

After I removed t = min(t), the column t.1 was no longer created because tbl_event only had a single column t. The NOTE is removed, and the tests all pass. I also ran the code examples below and confirmed the results were identical before and after my change:

pw_info()
##   time stratum t  hr  n    event     info    info0
## 1   30     All 0 0.9 12 21.24782 5.300180 5.311956
## 2   30     All 3 0.6 96 37.24314 9.027063 9.310786

pw_info(
  enroll_rate = define_enroll_rate(
    duration = 12,
    rate = 20
  ),
  fail_rate = define_fail_rate(
    duration = c(9, Inf),
    fail_rate = log(2) / c(10, 20),
    hr = c(0.72, 0.72),
    dropout_rate = 0.001
  ),
  total_duration = 50,
  ratio = 1
)
##   time stratum t   hr   n    event     info    info0
## 1   50     All 0 0.72 180 98.70948 24.29858 24.67737
## 2   50     All 9 0.72  60 87.45497 21.86273 21.86374

[LittleBeannie]

Thanks for fixing this, @jdblischak! I appreciate you tackling it.
This PR touches an important low-level function, and it took me a while to fully understand the context. Your fix is very helpful.