相关疑难解决方法(0)

我写了一个(相当幼稚的)函数来随机选择两个指定日期之间的日期/时间

# 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="") )
}

结果如下:

> 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,这似乎在大幅放缓.

# 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 

有人能够以更有效的方式建议如何做这样的事情吗？

假设我有两个数据框,例如:

set.seed(123)
df1<-data.frame(bmi=rnorm(20, 25, 5),
                date1=sample(seq.Date(as.Date("2014-01-01"),
                             as.Date("2014-02-28"),by="day"), 20))

df2<-data.frame(epi=1:5, 
                date2=as.Date(c("2014-1-8", "2014-1-15", "2014-1-28", 
                                "2014-2-05", "2014-2-24")))

我的问题是如何匹配bmi与epi其中DATE1最接近之前或在date2？像这样的结果:

  epi      date2   bmi      date1
1   1 2014-01-08 33.58 2014-01-08
2   2 2014-01-15 22.64 2014-01-15
3   3 2014-01-28 22.22 2014-01-26
4   4 2014-02-05 15.17 2014-02-01
5   5 2014-02-24 27.49 2014-02-15