ifelse and POSIXct: coercion and default class of NA

Question

Here is my example:

library(dplyr)
my_df <- data.frame(col_1 = c(1,2,4), col_2 = c('2017-12-1', '2015-11-2', '2011-2-5'))
my_df$col_2 <- as.POSIXct(my_df$col_2)
out <- ifelse(my_df$col_1 ==2, my_df$col_2+ as.difftime(3, units = 'days'), NA)
print(out)

It produces:

NA 1446703200         NA

So coercion happens because of different data type. What NA datatype should I use to prevent it: NA_date_ or NA_POSIX_ or ...?

akrun · Accepted Answer · 2018-03-06T17:27:10.873

It may be better to do this stepwise with assignment instead of ifelse.

1) Create an index

i1 <- my_df$col_1 == 2

2) Assign values based on the index

my_df$col_2[i1] <- my_df$col_2[i1] + as.difftime(3, units = 'days')

3) Change other elements to NA

my_df$col_2[!i1] <- NA
my_df
#  col_1      col_2
#1     1       <NA>
#2     2 2015-11-05
#3     4       <NA>

The reason is that POSIXct storage mode is numeric, within ifelse it gets coerced to the numeric form

If we are using tidyverse, then

library(tidyverse)
my_df %>%
    mutate(col_2 =  col_2 + days(3), 
           col_2 = replace(col_2, col_1 != 2, NA))

The solution with index is much more universal and conceptual! Thank you! — user1700890, Mar 06 '18 at 17:52

score 1 · Answer 2 · answered Mar 06 '18 at 16:56

data.table makes it easy to modify specific rows

require(data.table)
my_df <- data.frame(col_1 = c(1,2,4), col_2 = c('2017-12-1', '2015-11-2', '2011-2-5'))
setDT(my_df)
my_df[, col_2 := as.POSIXct(col_2)
      ][col_1 == 2, col_2 := col_2 + as.difftime(3, units = 'days')
      ][col_1 != 2, col_2 := NA]

score 1 · Answer 3 · answered Mar 06 '18 at 20:00

Because you (for some reason ;)) have loaded dplyr, you may use if_else and wrap the false NA in as.POSIXct*:

if_else(my_df$col_1 == 2, my_df$col_2 + as.difftime(3, units = 'days'), as.POSIXct(NA))
# [1] NA               "2015-11-05 CET" NA

*See ?as.POSIXct:

Logical NAs can be converted to either of the classes [POSIXlt and POSIXct]

ifelse and POSIXct: coercion and default class of NA

3 Answers3