R - Assign column value based on closest match in second data frame

Question

I have two data frames, logger and df (times are numeric):

logger <- data.frame(
time = c(1280248354:1280248413),
temp = runif(60,min=18,max=24.5)
)

df &         


        

Answer 1

I'd use data.table for this. It makes it super easy and super fast joining on keys. There is even a really helpful roll = "nearest" argument for exactly the behaviour you are looking for (except in your example data it is not necessary because all times from df appear in logger). In the following example I renamed df$time to df$time1 to make it clear which column belongs to which table...

#  Load package
require( data.table )

#  Make data.frames into data.tables with a key column
ldt <- data.table( logger , key = "time" )
dt <- data.table( df , key = "time1" )

#  Join based on the key column of the two tables (time & time1)
#  roll = "nearest" gives the desired behaviour
#  list( obs , time1 , temp ) gives the columns you want to return from dt
ldt[ dt , list( obs , time1 , temp ) , roll = "nearest" ]
#          time obs      time1     temp
# 1: 1280248361   8 1280248361 18.07644
# 2: 1280248366   4 1280248366 21.88957
# 3: 1280248370   3 1280248370 19.09015
# 4: 1280248376   5 1280248376 22.39770
# 5: 1280248381   6 1280248381 24.12758
# 6: 1280248383  10 1280248383 22.70919
# 7: 1280248385   1 1280248385 18.78183
# 8: 1280248389   2 1280248389 18.17874
# 9: 1280248393   9 1280248393 18.03098
#10: 1280248403   7 1280248403 22.74372