`by()` changed POSIXct to numeric value in R

Question

In R, how to prevent the by() function from changing POSIXct to numeric values automatically?

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In the following example, the time object is class "POSIXct" "POSIXt". When using by() function, the output is numeric. But when doing what by() does manually, e.g. min(time[index=='a']), the output is still "POSIXct" "POSIXt", which is desired. Is there a way to prevent the by() function from changing POSIXct to numeric values automatically?

time = c("0001-03-04 05:36:00 UTC", "0001-03-04 05:50:00 UTC", "0001-03-04 06:05:00 UTC")
time = as.POSIXct(time, tz = "UTC")
index = c("a", "a", "b")

by(time, index, min) # results are numeric values.
min(time[index=='a']) # "0001-03-04 05:36:00 UTC"

Answer 1

1) Return a list and then concatenate the elements of the result.

do.call("c", by(time, index, function(x) list(min(x))))
##                         a                         b 
## "0001-03-04 05:36:00 UTC" "0001-03-04 06:05:00 UTC" 

2) split/lapply would also work:

do.call("c", lapply(split(time, index), min))
##                         a                         b 
## "0001-03-04 05:36:00 UTC" "0001-03-04 06:05:00 UTC" 

3) as would tapply with simplify = FALSE:

do.call("c", tapply(time, index, min, simplify = FALSE))
##                         a                         b 
## "0001-03-04 05:36:00 UTC" "0001-03-04 06:05:00 UTC" 

4) aggregate can be used.

with(aggregate(time ~., data.frame(time, index), min), setNames(time, index))
##                         a                         b 
## "0001-03-04 05:36:00 UTC" "0001-03-04 06:05:00 UTC" 

or if names are not needed:

aggregate(time, list(index), min)$x
## [1] "0001-03-04 05:36:00 UTC" "0001-03-04 06:05:00 UTC"

or to get a data frame result:

aggregate(time ~., data.frame(time, index), min)
##   index                time
## 1     a 0001-03-04 05:36:00
## 2     b 0001-03-04 06:05:00