r-有效地生成两个日期之间的时间和日期的随机样本

Question

我写了一个（相当天真的）函数，可以在两个指定的日期/时间之间随机选择

# set start and end dates to sample between
day.start <- "2012/01/01"
day.end <- "2012/12/31"
# define a random date/time selection function
rand.day.time <- function(day.start,day.end,size) {
  dayseq <- seq.Date(as.Date(day.start),as.Date(day.end),by="day")
  dayselect <- sample(dayseq,size,replace=TRUE)
  hourselect <- sample(1:24,size,replace=TRUE)
  minselect <- sample(0:59,size,replace=TRUE)
  as.POSIXlt(paste(dayselect, hourselect,":",minselect,sep="") )
}

结果是：

> rand.day.time(day.start,day.end,size=3)
[1] "2012-02-07 21:42:00" "2012-09-02 07:27:00" "2012-06-15 01:13:00"

但随着样本量的增加，这种情况似乎正在显著放缓。

# some benchmarking
> system.time(rand.day.time(day.start,day.end,size=100000))
   user  system elapsed 
   4.68    0.03    4.70 
> system.time(rand.day.time(day.start,day.end,size=200000))
   user  system elapsed 
   9.42    0.06    9.49 

有人能建议如何以更有效的方式做这样的事情吗？

Answer 1

Ahh，我们可以将另一个日期/时间问题简化为在浮点中工作：）

尝试此功能

R> latemail <- function(N, st="2012/01/01", et="2012/12/31") {
+     st <- as.POSIXct(as.Date(st))
+     et <- as.POSIXct(as.Date(et))
+     dt <- as.numeric(difftime(et,st,unit="sec"))
+     ev <- sort(runif(N, 0, dt))
+     rt <- st + ev
+ }
R>

我们以秒为单位计算difftime，然后"仅仅"在它上面画上制服，对结果进行排序。把它加到开头，你就完成了：

R> set.seed(42); print(latemail(5))     ## round to date, or hour, or ...
[1] "2012-04-14 05:34:56.369022 CDT" "2012-08-22 00:41:26.683809 CDT" 
[3] "2012-10-29 21:43:16.335659 CDT" "2012-11-29 15:42:03.387701 CST"
[5] "2012-12-07 18:46:50.233761 CST"
R> system.time(latemail(100000))
   user  system elapsed 
  0.024   0.000   0.021 
R> system.time(latemail(200000))
   user  system elapsed 
  0.044   0.000   0.045 
R> system.time(latemail(10000000))   ## a few more than in your example :)
   user  system elapsed 
  3.240   0.172   3.428 
R> 

Answer 2

类似的东西也会起作用。很抱歉出现了随机数据帧，我只是把它放进去，这样你就可以看到一个情节了。

data=as.data.frame(list(ID=1:10,
                   variable=rnorm(10,50,10)))
#This function will generate a uniform sample of dates from 
#within a designated start and end date:
rand.date=function(start.day,end.day,data){   
  size=dim(data)[1]    
  days=seq.Date(as.Date(start.day),as.Date(end.day),by="day")  
  pick.day=runif(size,1,length(days))  
  date=days[pick.day]  
}
#This will create a new column within your data frame called date:
data$date=rand.date("2014-01-01","2014-02-28",data)
#and this will order your data frame by date:
data=data[order(data$date),]
#Finally, you can see how the data looks
plot(data$date,data$variable,type="b")