Date column coerced to numeric when indexing dataframe with [[ and a vector

问题

I am creating a data.frame with a column of type Date. When indexing the data frame with [[ and a numeric vector, the Date becomes a number. This is causing a problem when using purrr::pmap. Can anyone explain why this is happening and is there a work around?

Example:

x <- data.frame(d1 = lubridate::ymd(c("2018-01-01","2018-02-01")))

class(x$d1)
# [1] "Date"

x[[1]]
# [1] "2018-01-01" "2018-02-01"

x[[c(1, 1)]]
# [1] 17532

回答1:

Overview

After reading why does unlist() kill dates in R and the documentation of unlist(), you've got to manually prevent purrr::map() from coercing the Date objects to integer by way of using the base::c() function.

Load mikmart's PR version of purrr::pmap()

After reading pmap strips Date, it looks like someone very awesome submitted a pull request to resolve this issue within a refactored version of the indexing that happens under the hood in purrr::pmap().

Using devtools::dev_mode(), you can install mikmart/purrr's "pmap" branch version of purrr to retain Date objects while using pmap().

# ******pmap() example ****
# load necessary packages -----
library(devtools)
library(lubridate)

# enter dev mode so you don't have to uninstall the cran version of purrr ----
dev_mode(on = TRUE)

# install mikmart's PR to fix the coercing of Dates to integer ----
install_github(repo = "mikmart/purrr", ref = "pmap")

# load mikmart's PR version of purrr ----
library(purrr)

# load necessary data
x <- data.frame(d1 = lubridate::ymd(c("2018-01-01","2018-02-01")))

# for the first column in x ----
# give me each element
# note: no need for c()
list.of.dates <-
  x %>%
  pmap(.f = ~ .x)

# view results -----
list.of.dates
# [[1]]
# [1] "2018-01-01"
# 
# [[2]]
# [1] "2018-02-01"

# view the class of each list -----
map_chr(list.of.dates, class) # [1] "Date" "Date"
#
# 
# turn off dev mode ---
dev_mode(on = FALSE)
#
# restart R -----
# Manually hit Shift+Cmd+F10 or point in click under the "Session" tab
#
# clear global environment ----
rm(list = ls())
#
# ******map() example********
# load necessary packages -----
library(tidyverse)
library(lubridate)

# load necessary data ----
x <- data.frame(d1 = lubridate::ymd(c("2018-01-01","2018-02-01")))

# from the first column ------
# give me each element
# and ensure the dates don't get coerced to integers
list.of.dates <-
  x$d1 %>%
  map(.f = ~ .x %>% c()) 

# view results -----
list.of.dates
# [[1]]
# [1] "2018-01-01"
# 
# [[2]]
# [1] "2018-02-01"

# # view the class of each list -----
map_chr(list.of.dates, class) # [1] "Date" "Date"

# end of script #

来源：https://stackoverflow.com/questions/52579980/date-column-coerced-to-numeric-when-indexing-dataframe-with-and-a-vector