如何在ff或ffbase中将因子向量转换为POSIXct

Question

在使用read.csv.ffdf读取大型数据集之后，其中一列是time。例如2014-10-18 00:01:02，用于该列中100万行。这一栏是一个因素。如何将其转换为ff支持的POSIXct？只需使用as.POSIXct()即可将值转换为NA

或者，当我在开始读取数据集时，是否可以将该列指定为POSIXct

我的目标是获得月份和天数(甚至小时)。因此，我对其他解决方案持开放态度，而不是转换到POSIXct。

例如，我们有9 x 2的桌子，

test <- read.csv.ffdf(file="test.csv", header=T, first.rows=-1)

两列是ID (数字类)和时间(因子类)

下面是dput

structure(list(virtual = structure(list(VirtualVmode = c("integer", 
"integer"), AsIs = c(FALSE, FALSE), VirtualIsMatrix = c(FALSE, 
FALSE), PhysicalIsMatrix = c(FALSE, FALSE), PhysicalElementNo = 1:2, 
    PhysicalFirstCol = c(1L, 1L), PhysicalLastCol = c(1L, 1L)), .Names = c("VirtualVmode", 
"AsIs", "VirtualIsMatrix", "PhysicalIsMatrix", "PhysicalElementNo", 
"PhysicalFirstCol", "PhysicalLastCol"), row.names = c("ID", "time"
), class = "data.frame", Dim = c(9L, 2L), Dimorder = 1:2), physical = structure(list(
    ID = structure(list(), physical = <pointer: 0x000000000821ab20>, virtual = structure(list(), Length = 9L, Symmetric = FALSE), class = c("ff_vector", 
    "ff")), time = structure(list(), physical = <pointer: 0x000000000821abb0>, virtual = structure(list(), Length = 9L, Symmetric = FALSE, Levels = c("10/17/2003 0:01", 
    "12/5/1999 0:02", "2/1/2000 0:01", "3/23/1998 0:01", "3/24/2013 0:00", 
    "5/29/2004 0:00", "5/9/1985 0:01", "6/14/2010 0:01", "6/25/2008 0:02"
    ), ramclass = "factor"), class = c("ff_vector", "ff"))), .Names = c("ID", 
"time")), row.names = NULL), .Names = c("virtual", "physical", 
"row.names"), class = "ffdf")

Answer 1

您可以使用from package ffbase，如下所示的玩具示例。最好的。

require(ff)
x <- data.frame(id = 1:100000, timepoint = seq(from = Sys.time(), by = "sec", length.out = 100000))
x$timepoint <- as.factor(x$timepoint)

xff <- as.ffdf(x)
class(xff)
require(ffbase)
xff$time <- with(xff, as.POSIXct(as.character(timepoint)), by = 10000)
ramclass(xff$time)
[1] "POSIXct" "POSIXt" 
str(xff[1:10, ])
'data.frame':   10 obs. of  3 variables:
 $ id       : int  1 2 3 4 5 6 7 8 9 10
 $ timepoint: Factor w/ 100000 levels "2014-10-20 09:14:10",..: 1 2 3 4 5 6 7 8 9 10
 $ time     : POSIXct, format: "2014-10-20 09:14:10" "2014-10-20 09:14:11" "2014-10-20 09:14:12" "2014-10-20 09:14:13" ...

Answer 2

在读取数据时使用colClasses。例如，以两列为例：ID (数字类)和time (因子类)：

test <- read.csv.ffdf(file="test.csv", header=T, first.rows=-1,colClasses = c("integer","POSIXct"))