Merge nearest date, and related variables from a another dataframe by group

Question

I have two dataframes each with multiple rows per ID. I need to return the closest date and related data from the second dataframe based on the ID and date of the first data

Answer 1

Using data.table, simple and elegant solution:

library(data.table)

setDT(df1)
setDT(df2)

setkey(df2, ID, dateTarget)[, dateMatch:=dateTarget]
df2[df1, roll='nearest']

    ID dateTarget ValueMatch  dateMatch Value
 1:  3 2015-11-14         48 2015-07-06    47
 2:  3 2015-02-22         94 2015-03-09    52
 3:  1 2014-12-29         88 2014-12-06    18
 4:  3 2015-12-08         48 2015-07-06    98
 5:  2 2013-01-14         77 2013-04-08    52
 6:  2 2015-07-29         68 2015-08-01    97
 7:  3 2013-05-30         85 2013-04-01    91
 8:  1 2013-11-04         35 2014-02-21    70
 9:  2 2015-06-15         68 2015-08-01    98
10:  3 2014-11-17         95 2014-12-15    68