Creating variable in R data frame depending on another data frame

Question

I am seeking help after having wasted almost a day. I have a big data frame (bdf) and a small data frame (sdf). I want to add variable z to bdf depending on the value of sdf

Answer 1

This seems now absolutely unnecessary, but in base R

bdf$z <- numeric(nrow(bdf))
for(i in seq_along(bdf$z)){
  ind <- which.min(abs(bdf$tb[i] - sdf$ts))
  bdf$z[i] <- sdf$y[ind]
}

While being little clumsy, it has an advantage in clarity, which accomodates easy adaptation to dplyr

library(dplyr)
bdf %>% rowwise() %>% 
  mutate(z= sdf$y[which.min(abs(as.numeric(tb)-as.numeric(sdf$ts)))])

#Source: local data frame [10 x 2]
#Groups: 

#                    tb    z
#1  2013-05-19 17:11:22  0.2
#2  2013-05-21 06:40:58  0.2
#3  2013-05-22 20:10:34  0.2
#4  2013-05-24 09:40:10 -0.1
#5  2013-05-25 23:09:46 -0.1
#6  2013-05-27 12:39:22  0.3
#7  2013-05-29 02:08:58  0.3
#8  2013-05-30 15:38:34  0.3
#9  2013-06-01 05:08:10  0.3
#10 2013-06-02 18:37:46  0.3