将日期时间从字符转换为POSIXct对象

Question

我有一种仪器，以不规则的时间格式导出数据。我需要将date和time载体组合成以下POSIXct格式的新的datetime载体：%Y-%m-%d %H:%M:%S。出于好奇，我试图用三种不同的方式做到这一点，使用as.POSIXct(),strftime()和strptime()。当使用我的示例数据下面，只有as.POSIXct()和strftime()功能工作，但我很好奇为什么strptime()生产NAs？此外，我不能strftime()输出转换成使用as.POSIXct()一个POSIXct对象...

当我真实数据（其中我只给你提供了第一个用于行）尝试这些相同的功能，我遇到完全不同的问题。只有strftime()功能正在工作。出于某种原因，as.POSIXct()功能也产生NAs，这是我真正需要的转换我datetime为POSIXct对象的唯一命令

好像有这些功能之间的细微差别，我想知道如何更有效地使用它们。谢谢！

重复的例子：

## Creating dataframe: 
date <- c("2017-04-14", "2017-04-14","2017-04-14","2017-04-14") 
time <- c("14:24:24.992000","14:24:25.491000","14:24:26.005000","14:24:26.511000") 
value <- c("4.106e-06","4.106e-06","4.106e-06","4.106e-06") 
data <- data.frame(date, time) 
data <- data.frame(data, value) ## I'm sure there is a better way to combine three vectors... 
head(data) 

## Creating 3 different datetime vectors: 

## This works in my example code, but not with my real data... 
data$datetime1 <- as.POSIXct(paste(data$date, data$time), format = "%Y-%m-%d %H:%M:%S",tz="UTC") 
class(data$datetime1) 

## This is producing NAs, and I'm not sure why: 
data$datetime2 <- strptime(paste(data$date, data$time), format = "%Y-%m-%d %H:%M%:%S", tz = "UTC") 
class(data$datetime2) 

## This is working just fine 
data$datetime3 <- strftime(paste(data$date, data$time), format = "%Y-%m-%d %H:%M%:%S", tz = "UTC") 
class(data$datetime3) 
head(data) 

## Since I cannot get the as.POSIXct() function to work with my real data, I tried this workaround. Unfortunately I am running into trouble... 
data$datetime4 <- as.POSIXct(x$datetime3, format = "%Y-%m-%d %H:%M%:%S", tz = "UTC") 

链接到真实数据： here

实施例使用real_data.txt：

## Reading in the file: 
fpath <- "~/real_data.txt" 
x <- read.csv(fpath, skip = 1, header = FALSE, sep = "", stringsAsFactors = FALSE) 
names(x) <- c("date","time","bscat","scat_coef","pressure_mbar","temp_K","CH1","CH2") ## This is data from a Radiance Research Integrating Nephelometer Model M903 for anyone who is interested! 

## If anyone could get this to work that would be awesome! 
x$datetime1 <- as.POSIXct(paste(x$date, x$time), format = "%Y-%m-%d %H:%M%:%S", tz = "UTC") 

## This still doesn't work... 
x$datetime2 <- strptime(paste(x$date, x$time), format = "%Y-%m-%d %H:%M%:%S", tz = "UTC") 

## This works: 
x$datetime3 <- strftime(paste(x$date, x$time), format = "%Y-%m-%d %H:%M%:%S", tz = "UTC") 

## But I cannot convert from strftime character to POSIXct object, so it doesn't help me at all... 
x$datetime4 <- as.POSIXct(x$datetime3, format = "%Y-%m-%d %H:%M%:%S", tz = "UTC")  

head(x) 

解决方案：

我没有提供as.POSIXct()功能以正确的格式字符串。一旦我将%Y-%m-%d %H:%M%:%S更改为%Y-%m-%d %H:%M:%S，data$datetime2,data$datetime4,x$datetime1和x$datetime2正常工作！非常感谢PhilC的调试！

Answer 1

为了您的真实数据问题更换%m%与%m：

## Reading in the file: 
fpath <- "c:/r/data/real_data.txt" 
x <- read.csv(fpath, skip = 1, header = FALSE, sep = "", stringsAsFactors = FALSE) 
names(x) <- c("date","time","bscat","scat_coef","pressure_mbar","temp_K","CH1","CH2") ## This is data from a Radiance Research Integrating Nephelometer Model M903 for anyone who is interested! 

## issue was the %m% - fixed 
x$datetime1 <- as.POSIXct(paste(x$date, x$time), format = "%Y-%m-%d %H:%M:%S", tz = "UTC") 

## Here too - fixed 
x$datetime2 <- strptime(paste(x$date, x$time), format = "%Y-%m-%d %H:%M:%S", tz = "UTC") 
head(x) 

Answer 2

导致NAs的格式字符串错误;试试这个：

## This is no longer producing NAs: 
data$datetime2 <- strptime(paste(data$date, data$time), format = "%Y-%m-%d %H:%M:%S",tz="UTC") 
class(data$datetime2)