Emphasizing cells using emphasize.strong.cells fails - POSIXct related?

Question

I am attempting to display the following table (which contains two columns of class POSIXct) and emphasize cells that contain a certain value ("B").

> test
   bikeid start_station           starttime end_station             endtime
1       1             A 2017-09-25 01:00:00           B 2017-09-25 01:30:00
2       1             B 2017-09-25 07:30:00           C 2017-09-25 08:00:00
3       1             C 2017-09-25 10:00:00           A 2017-09-25 10:30:00
4       1             A 2017-09-25 13:00:00           C 2017-09-25 13:30:00
5       1             C 2017-09-25 15:30:00           B 2017-09-25 16:00:00
6       1             B 2017-09-25 18:00:00           B 2017-09-25 18:30:00
7       1             B 2017-09-25 19:00:00           A 2017-09-25 19:30:00
8       1             А 2017-09-25 20:00:00           C 2017-09-25 20:30:00
9       1             C 2017-09-25 22:00:00           B 2017-09-25 22:30:00
10      1             B 2017-09-25 23:00:00           C 2017-09-25 23:30:00

When I use the following code, the table is successfully displayed:

library(pander)
panderOptions('table.split.table', Inf)
pander(test)

However, when I include emphasize.strong.cells to specify which cells to emphasize, the table fails and I receive an error:

panderOptions('table.split.table', Inf)
emphasize.strong.cells(which(test == "B"))
pander(test)

# Error in as.POSIXlt.character(x, tz, ...) : 
#  character string is not in a standard unambiguous format
#  Calls: <Anonymous> ... as.POSIXct.default -> as.POSIXct -> as.POSIXlt -> 
# as.POSIXlt.character
# Execution halted

Even when the columns containing POSIXct values are removed completely, the code still fails.

Why does this happen and what is the right workaround?

Here is a sample of the dataset:

> dput(test)
structure(list(bikeid = c(1, 1, 1, 1, 1, 1, 1, 1, 1, 1), start_station = c("A", 
"B", "C", "A", "C", "B", "B", "А", "C", "B"), starttime = structure(c(1506315600, 
1506339000, 1506348000, 1506358800, 1506367800, 1506376800, 1506380400, 
1506384000, 1506391200, 1506394800), class = c("POSIXct", "POSIXt"
), tzone = ""), end_station = c("B", "C", "A", "C", "B", "B", 
"A", "C", "B", "C"), endtime = structure(c(1506317400, 1506340800, 
1506349800, 1506360600, 1506369600, 1506378600, 1506382200, 1506385800, 
1506393000, 1506396600), class = c("POSIXct", "POSIXt"), tzone = "")), .Names = c("bikeid", 
"start_station", "starttime", "end_station", "endtime"), row.names = c(NA, 
-10L), class = "data.frame")

Answer 1

Your problem has nothing to do with emphasize.strong.cells, but is caused by the comparison you make in which: test == "B". You are comparing the character "B" with the entire data set "test". In "test", your focal variables are characters, but you also have some non-character variables, i.e. the two POSIXct variables. Thus, you are making a comparison (==) between arguments of different type. Then see ?Comparison:

If the two arguments are atomic vectors of different types, one is coerced to the type of the other.

When you try compare "B" with a POSIXct variable, R attempts to coerce "B" to POSIXct which fails:

as.POSIXct("2017-09-25 01:00:00") == "B"
# Error in as.POSIXlt.character(x, tz, ...) : 
#   character string is not in a standard unambiguous format

Just to show an example of when the coercion of the RHS characterwould have worked:

as.POSIXct("2017-09-25 01:00:00") == "2017-09-25 01:00:00"
# [1] TRUE 

A similar issue would have occured if you had a Date variable in your data frame:

as.Date("2017-09-25") == "B"
# Error in charToDate(x) : 
#   character string is not in a standard unambiguous format

Thus, as I mentioned in my comments, one workaround in your case would be to coerce the POSIXct columns to character before making the comparison.

---

On the other hand, if you had been looking for a certain numeric value to emphasize (e.g. test > 10), the comparison with Date or POSIXct columns wouldn't cause an error (check e.g typeof(as.POSIXct("2017-09-25 01:00:00"))).

---

A small example:

d <- data.frame(time = as.POSIXct("2017-09-25 21:30:20") + 0:1,
                x = c("A", "B"), y = 1:2)
d

emphasize.strong.cells(which(d == "B", arr.ind = TRUE))
# Error in as.POSIXlt.character(x, tz, ...) : 
#   character string is not in a standard unambiguous format

d$time <- as.character(d$time)
emphasize.strong.cells(which(d == "B", arr.ind = TRUE))
pander(d)
# ---------------------------------
#          time             x     y 
# --------------------- ------- ---
#   2017-09-25 21:30:20     A     1 
# 
#   2017-09-25 21:30:21   **B**   2 
# ---------------------------------


emphasize.strong.cells(which(d == 1, arr.ind = TRUE))
pander(d)
# ---------------------------------
#         time           x     y   
# --------------------- --- -------
#  2017-09-25 21:30:20   A   **1** 
#  
#  2017-09-25 21:30:21   B     2   
# ---------------------------------