从30分钟的数据中提取R中某一天的最后时间戳

Question

我有一个以30分钟频率不定期采样的数据集，如下所示。我需要提取每天最后一个时间戳的索引。数据集如下:

datetime <-c("8/19/2011 16:00",
"8/19/2011 17:30",
"8/19/2011 18:30",
"8/19/2011 19:30",
"8/22/2011 4:00",
"8/22/2011 6:00",
"8/22/2011 7:00",
"8/22/2011 19:00",
"8/22/2011 19:30",
"8/23/2011 4:00",
"8/24/2011 5:30",
"8/24/2011 7:00",
"10/25/2011 7:30")

我已将其转换为POSIXlt对象，如下所示。

datetime <- strptime(datetime, format="%m/%d/%Y  %H:%M")
datetime <- as.POSIXlt(datetime)

但是，我无法提取每天的最后一个索引。我想要一个输出，作为每天最后一个时间戳的索引，也就是说，我的输出将是

list of (4, 9, 10,12,13) corresponding to datetime values of 
"8/19/2011 19:30"
"8/22/2011 19:30"
"8/23/2011 4:00"
"8/24/2011 7:00"
"10/25/2011 7:30"

任何帮助将不胜感激。谢谢!

Answer 1

将这些字符值转换为日期时间，然后按日期(日-月)转换为split。在每个日期内，使用which.max:

选择最后一个值

dt <- as.POSIXct(datetime, format="%m/%d/%Y %H:%M") 
lapply( split( dt, format(dt,"%m-%d") ), function(d) as.POSIXct(d[which.max(d)] ) )
$`08-19`
[1] "2011-08-19 19:30:00 PDT"
$`08-22`
[1] "2011-08-22 19:30:00 PDT"
$`08-23`
[1] "2011-08-23 07:30:00 PDT"
$`08-24`
[1] "2011-08-24 07:00:00 PDT"

Answer 2

尝试:

数据:

 datetime <- c("8/19/2011 16:00", "8/19/2011 17:30", "8/19/2011 18:30", "8/19/2011 19:30", "8/22/2011 4:00", "8/22/2011 6:00", "8/22/2011 7:00", "8/22/2011 19:00", "8/22/2011 19:30", "8/23/2011 4:00", "8/24/2011 5:30", "8/24/2011 7:00", "8/23/2011 7:30", "12/23/2012 19:23", "11/24/2015 7:13")

代码:

splitter <- strftime(strptime(datetime,"%m/%d/%Y %H:%M"), "%m/%d/%Y")    
lapply(split(datetime, splitter), function(x) {
        match(x[length(x)], datetime)
    })

如果您还想返回索引处的值，您可以像下面这样调整代码:

lapply(split(datetime, splitter), function(x) {
    val_index <- match(x[length(x)], datetime)
    c(val_index,datetime[val_index])
})

Output1:

$`08/19/2011`
[1] 4
$`08/22/2011`
[1] 9
$`08/23/2011`
[1] 13
$`08/24/2011`
[1] 12
$`11/24/2015`
[1] 15
$`12/23/2012`
[1] 14

Output2:

$`08/19/2011`
[1] "4"               "8/19/2011 19:30"
$`08/22/2011`
[1] "9"               "8/22/2011 19:30"
$`08/23/2011`
[1] "13"             "8/23/2011 7:30"
$`08/24/2011`
[1] "12"             "8/24/2011 7:00"
$`11/24/2015`
[1] "15"              "11/24/2015 7:13"
$`12/23/2012`
[1] "14"               "12/23/2012 19:23"