使用[[和向量对数据框进行索引时，日期列将被强制转换为数字

Question

我正在创建一个类型为Date的列的data.frame。当使用[[和数字向量索引数据帧时，日期变为数字。使用purrr::pmap时会导致问题。谁能解释为什么会这样，有没有解决的办法？

示例：

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

Answer 1

概述

在阅读why does unlist() kill dates in R和unlist()的文档之后，您必须使用purrr::map()函数来手动阻止base::c()将Date对象强制转换为整数。

加载mikmart的purrr::pmap()的PR版本

在阅读pmap strips Date之后，看来submitted a pull request的人非常厉害，可以在purrr::pmap()的幕后重构版本的索引中解决此问题。

使用devtools::dev_mode()，您可以安装purrr的{​​{3}}版本，以在使用pmap()时保留Date对象。

# ******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 #