Matching timestamped data to closest time in anoth

Question

I have a timestamp in one data frame that I am trying to match to the closest timestamp in a second dataframe, for the purpose of extracting data from the second dataframe. See below for a generic example of my approach:

library(lubridate)

data <- data.frame(datetime=ymd_hms(c('2015-04-01 12:23:00 UTC', '2015-04-01 13:49:00 UTC', '2015-04-01 14:06:00 UTC' ,'2015-04-01 14:49:00 UTC')),
                   value=c(1,2,3,4))
reference <- data.frame(datetime=ymd_hms(c('2015-04-01 12:00:00 UTC', '2015-04-01 13:00:00 UTC', '2015-04-01 14:00:00 UTC' ,'2015-04-01 15:00:00 UTC', '2015-04-01 16:00:00 UTC')),
                        refvalue=c(5,6,7,8,9))

data$refvalue <- apply(data, 1, function (x){
  differences <- abs(as.numeric(difftime(ymd_hms(x['datetime']), reference$datetime)))
  mindiff <- min(differences)
  return(reference$refvalue[differences == mindiff])
})

data
#              datetime value refvalue
# 1 2015-04-01 12:23:00     1        5
# 2 2015-04-01 13:49:00     2        7
# 3 2015-04-01 14:06:00     3        7
# 4 2015-04-01 14:49:00     4        8

This works fine, except it is very slow, because the reference dataframe is quite large in my real-world application. Is this code properly vectorized? Is there a faster, more elegant way of performing this operation?

Answer 1

I wondered if this would be able to match a data.table solution for speed, but it's a base-R vectorized solution which should outperform your apply version. And since it doesn't actually ever calculate a distance, it might actually be faster than the data.table-nearest approach. This adds the length of the midpoints of the intervals to either the lowest possible value or the starting point of the the intervals to create a set of "mid-breaks" and then uses the findInterval function to process the times. That creates a suitable index into the rows of the reference dataset and the "refvalue" can then be "transferred" to the data-object.

 data$reefvalue <- reference$refvalue[
                      findInterval( data$datetime, 
                                     c(-Inf, head(reference$datetime,-1))+
                                     c(0, diff(as.numeric(reference$datetime))/2 )) ]
 # values are [1] 5 7 7 8

Answer 2

You can try data.tables rolling join using the "nearest" option

library(data.table) # v1.9.6+
setDT(reference)[data, refvalue, roll = "nearest", on = "datetime"]
# [1] 5 7 7 8