尽管存在值，但 R 显示 NA

Question

我有两列 PosixLT 时间没有 NA 值，但检查时显示 NA 值

> sum(is.na(check$start))
[1] 19 
> sum(is.na(check$end))
[1] 23

数据存在于单元格中，为什么会发生这种情况？我听说 PosixLT 会发生这种情况，但即使我将其转换为 posixCT，也会出现非常奇怪的行为。如何解决这个问题？

> as.POSIXct(check$start, format = "%Y-%m-%d %H:%M:%S", tz = "CST6CDT")
 [1] NA                        "2014-03-09 01:35:01 CST" NA                        "2014-03-09 01:53:30 CST" NA                       
 [6] NA                        NA                        NA                        NA                        "2014-03-09 04:17:11 CDT"
[11] NA                        NA                        "2015-03-08 01:54:43 CST" NA                        NA                       
[16] NA                        NA                        NA                        NA                        NA                       
[21] NA                        NA                        NA  


> dput(check)
structure(list(start = structure(list(sec = c(24, 1, 27, 30, 
8, 21, 40, 9, 43, 11, 31, 43, 43, 55, 39, 54, 41, 19, 2, 35, 
6, 54, 40), min = c(45L, 35L, 14L, 53L, 36L, 37L, 47L, 48L, 54L, 
17L, 57L, 53L, 54L, 3L, 52L, 22L, 34L, 28L, 41L, 42L, 52L, 52L, 
53L), hour = c(2L, 1L, 2L, 1L, 2L, 2L, 2L, 2L, 2L, 4L, 2L, 2L, 
1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L), mday = c(9L, 9L, 
9L, 9L, 9L, 9L, 9L, 9L, 9L, 9L, 9L, 8L, 8L, 8L, 8L, 8L, 8L, 8L, 
8L, 8L, 8L, 8L, 8L), mon = c(2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L), 
    year = c(114L, 114L, 114L, 114L, 114L, 114L, 114L, 114L, 
    114L, 114L, 114L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 
    115L, 115L, 115L, 115L, 115L), wday = c(0L, 0L, 0L, 0L, 0L, 
    0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 
    0L, 0L, 0L), yday = c(67L, 67L, 67L, 67L, 67L, 67L, 67L, 
    67L, 67L, 67L, 67L, 66L, 66L, 66L, 66L, 66L, 66L, 66L, 66L, 
    66L, 66L, 66L, 66L), isdst = c(-1L, 0L, -1L, 0L, -1L, -1L, 
    -1L, -1L, -1L, 1L, -1L, -1L, 0L, -1L, -1L, -1L, -1L, -1L, 
    -1L, -1L, -1L, -1L, -1L), zone = c("", "CST", "", "CST", 
    "", "", "", "", "", "CDT", "", "", "CST", "", "", "", "", 
    "", "", "", "", "", ""), gmtoff = c(NA_integer_, NA_integer_, 
    NA_integer_, NA_integer_, NA_integer_, NA_integer_, NA_integer_, 
    NA_integer_, NA_integer_, NA_integer_, NA_integer_, NA_integer_, 
    NA_integer_, NA_integer_, NA_integer_, NA_integer_, NA_integer_, 
    NA_integer_, NA_integer_, NA_integer_, NA_integer_, NA_integer_, 
    NA_integer_)), .Names = c("sec", "min", "hour", "mday", "mon", 
"year", "wday", "yday", "isdst", "zone", "gmtoff"), class = c("POSIXlt", 
"POSIXt"), tzone = c("CST6CDT", "CST", "CDT")), end = structure(list(
    sec = c(7, 59, 38, 45, 29, 46, 39, 14, 52, 29, 37, 5, 23, 
    41, 10, 43, 46, 46, 53, 24, 57, 13, 51), min = c(55L, 47L, 
    30L, 2L, 43L, 51L, 53L, 56L, 54L, 54L, 57L, 56L, 6L, 3L, 
    13L, 29L, 37L, 32L, 48L, 47L, 55L, 55L, 55L), hour = c(2L, 
    2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
    2L, 2L, 2L, 2L, 2L, 2L, 2L), mday = c(9L, 9L, 9L, 9L, 9L, 
    9L, 9L, 9L, 9L, 9L, 9L, 8L, 8L, 8L, 8L, 8L, 8L, 8L, 8L, 8L, 
    8L, 8L, 8L), mon = c(2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
    2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L), 
    year = c(114L, 114L, 114L, 114L, 114L, 114L, 114L, 114L, 
    114L, 114L, 114L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 
    115L, 115L, 115L, 115L, 115L), wday = c(0L, 0L, 0L, 0L, 0L, 
    0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 
    0L, 0L, 0L), yday = c(67L, 67L, 67L, 67L, 67L, 67L, 67L, 
    67L, 67L, 67L, 67L, 66L, 66L, 66L, 66L, 66L, 66L, 66L, 66L, 
    66L, 66L, 66L, 66L), isdst = c(-1L, -1L, -1L, -1L, -1L, -1L, 
    -1L, -1L, -1L, -1L, -1L, -1L, -1L, -1L, -1L, -1L, -1L, -1L, 
    -1L, -1L, -1L, -1L, -1L), zone = c("", "", "", "", "", "", 
    "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", 
    "", ""), gmtoff = c(NA_integer_, NA_integer_, NA_integer_, 
    NA_integer_, NA_integer_, NA_integer_, NA_integer_, NA_integer_, 
    NA_integer_, NA_integer_, NA_integer_, NA_integer_, NA_integer_, 
    NA_integer_, NA_integer_, NA_integer_, NA_integer_, NA_integer_, 
    NA_integer_, NA_integer_, NA_integer_, NA_integer_, NA_integer_
    )), .Names = c("sec", "min", "hour", "mday", "mon", "year", 
"wday", "yday", "isdst", "zone", "gmtoff"), class = c("POSIXlt", 
"POSIXt"), tzone = c("CST6CDT", "CST", "CDT"))), .Names = c("start", 
"end"), row.names = c(1559963L, 1560092L, 1560157L, 1560220L, 
1560240L, 1560247L, 1560252L, 1560253L, 1560255L, 1560258L, 1560260L, 
2004432L, 2004583L, 2004591L, 2004594L, 2004596L, 2004598L, 2004599L, 
2004600L, 2004603L, 2004609L, 2004610L, 2004611L), class = "data.frame")

Answer 1

工作原理 is.na在这种情况下？

> is.na.POSIXlt
function (x) 
is.na(as.POSIXct(x))
<bytecode: 0x0000000014232980>

怎么样 as.POSIXct在这里表现？

> as.POSIXct(check$start)
 [1] NA                        "2014-03-09 01:35:01 CST" NA                        "2014-03-09 01:53:30 CST"
 [5] NA                        NA                        NA                        NA                       
 [9] NA                        "2014-03-09 04:17:11 CDT" NA                        NA                       
[13] "2015-03-08 01:54:43 CST" NA                        NA                        NA                       
[17] NA                        NA                        NA                        NA                       
[21] NA                        NA                        NA                       

好的，但是为什么？？？？

让我们检查 as.POSIXct 的文档:

Any conversion that needs to go between the two date-time classes requires a time zone: conversion from "POSIXlt" to "POSIXct" will validate times in the selected time zone. One issue is what happens at transitions to and from DST, for example in the UK

让我们来看看:

> check$start$zone
 [1] ""    "CST" ""    "CST" ""    ""    ""    ""    ""    "CDT" ""    ""    "CST" ""    ""    ""    ""    ""    ""    ""   
[21] ""    ""    ""   

这是龙，除了 4 个条目之外没有时区，所以 as.POSIXct无法判断日期是否有效(在 DST 更改范围内？)，如您所见:

> check$start$isdst
 [1] -1  0 -1  0 -1 -1 -1 -1 -1  1 -1 -1  0 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1

因此 POSIXlt(您的数据帧)和 POSIXct 之间的转换无法猜测日期是否有效，并返回 NA。

一种修复方法可能是对所有记录强制执行时区:

> check$start <- as.POSIXlt(strftime(check$start,tz="CST"),tz="CST6CDT")
> is.na(check$start)
 [1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE