r - 以毫秒为单位计算时间差

Question

我想以毫秒为单位逐行计算时间戳的差异。下面示例中的差异（time_diff）应始终为 0.2，但我也得到高于和低于 0.2 的值。

当将毫秒除以秒和/或向对象添加numeric值时，问题就开始了POSIXct。

我认为这是一些浮点问题，所以我尝试指定digits.sec 和数字的数量、舍入、截断等......但问题仍然存在。

为什么会这样以及如何解决这个问题？

library(dplyr)

options("digits.secs"=10)
options(digits = 10)

id <- 1:12
time <- c("9:34:50" , "9:34:50" , "9:34:51" , "9:34:51" , "9:34:51" , "9:34:51" ,
  "9:34:51" , "9:34:52" , "9:34:52" , "9:34:52" , "9:34:52" , "9:34:52")
ms <- c(600,800,0,200,400,600,800,0,200,400,600,800)

time <- as.POSIXct(time, format="%H:%M:%S", tz="GMT")

# Problem begins here
timeNew <- time + (ms/1000)

timeNewDf <- data.frame(id=id,
                      time=timeNew)

timeNewDf %>% dplyr::mutate(
  time_diff = c(diff(time),0)) %>% 
  dplyr::rowwise()

标签： rfloating-pointdivisionmilliseconds

Answer 1

POSIXct 将比精确时间更接近于其最近的浮点数。添加偏移量以适应浮点近似并显示到毫秒的 1 位数。

library(dplyr) 
options("digits.secs"=1)                                        # showing upto 1 digit

# test data
id <- 1:12
time <- c("9:34:50" , "9:34:50" , "9:34:51" , "9:34:51" , "9:34:51" , 
          "9:34:51" , "9:34:51" , "9:34:52" , "9:34:52" , "9:34:52" , "9:34:52" , "9:34:52")
ms <- c(600,800,0,200,400,600,800,0,200,400,600,800)
time <- as.POSIXct(time, format="%H:%M:%S", tz="GMT")

# offset to cater to floating point approximations
timeNew <- time + (ms/1000) + .0001

timeNewDf <- data.frame(id=id, time=timeNew)
timeNewDf %>% mutate(time_diff = round(c(diff(time),0),1))      # rounding to 1 decimal digit

#1   1 2018-05-30 09:34:50.6  0.2 secs
#2   2 2018-05-30 09:34:50.8  0.2 secs
#3   3 2018-05-30 09:34:51.0  0.2 secs
#4   4 2018-05-30 09:34:51.2  0.2 secs
#5   5 2018-05-30 09:34:51.4  0.2 secs
#6   6 2018-05-30 09:34:51.6  0.2 secs
#7   7 2018-05-30 09:34:51.8  0.2 secs
#8   8 2018-05-30 09:34:52.0  0.2 secs
#9   9 2018-05-30 09:34:52.2  0.2 secs
#10 10 2018-05-30 09:34:52.4  0.2 secs
#11 11 2018-05-30 09:34:52.6  0.2 secs
#12 12 2018-05-30 09:34:52.8  0.0 secs