Merge dataframes on matching A, B and *closest* C?

Question

I have two dataframes like so:

set.seed(1)
df <- cbind(expand.grid(x=1:3, y=1:5), time=round(runif(15)*30))
to.merge <- data.frame(x=c(2, 2, 2, 3, 2),
         


        

Answer 1

Use data.table and roll='nearest' or to limit to 1, roll = 1, rollends = c(TRUE,TRUE)

eg

library(data.table)
# create data.tables with the same key columns (x, y, time)
DT <- data.table(df, key = names(df))
tm <- data.table(to.merge, key = key(DT))

# use join syntax with roll = 'nearest'


tm[DT, roll='nearest']

#     x y time val
#  1: 1 1    8  NA
#  2: 1 2   27  NA
#  3: 1 3   28  NA
#  4: 1 4    2  NA
#  5: 1 5   21  NA
#  6: 2 1   11   c
#  7: 2 2    6  NA
#  8: 2 3   20  NA
#  9: 2 4    6   e
# 10: 2 5   12  NA
# 11: 3 1   17  NA
# 12: 3 2   27  NA
# 13: 3 3   19  NA
# 14: 3 4    5  NA
# 15: 3 5   23   d

You can limit your self to looking forward and back (1) by setting roll=-1 and rollends = c(TRUE,TRUE)

new <- tm[DT, roll=-1, rollends  =c(TRUE,TRUE)]
new
    x y time val
 1: 1 1    8  NA
 2: 1 2   27  NA
 3: 1 3   28  NA
 4: 1 4    2  NA
 5: 1 5   21  NA
 6: 2 1   11   c
 7: 2 2    6  NA
 8: 2 3   20  NA
 9: 2 4    6  NA
10: 2 5   12  NA
11: 3 1   17  NA
12: 3 2   27  NA
13: 3 3   19  NA
14: 3 4    5  NA
15: 3 5   23   d

Or you can roll=1 first, then roll=-1, then combine the results (tidying up the val.1 column from the second rolling join)

new <- tm[DT, roll = 1][tm[DT,roll=-1]][is.na(val), val := ifelse(is.na(val.1),val,val.1)][,val.1 := NULL]
new
    x y time val
 1: 1 1    8  NA
 2: 1 2   27  NA
 3: 1 3   28  NA
 4: 1 4    2  NA
 5: 1 5   21  NA
 6: 2 1   11   c
 7: 2 2    6  NA
 8: 2 3   20  NA
 9: 2 4    6  NA
10: 2 5   12  NA
11: 3 1   17  NA
12: 3 2   27  NA
13: 3 3   19  NA
14: 3 4    5  NA
15: 3 5   23   d