Find the minimum distance between two data frames, for each element in the second data frame

Question

I have two data frames ev1 and ev2, describing timestamps of two types of events collected over many tests. So, each data frame has columns \"test_id\", and \"timestamp\". W

Answer 1

May be this helps:

library(data.table)
setkey(setDT(ev1), test_id)
DT <- ev1[ev2, allow.cartesian=TRUE][,distance:=time-i.time]
DT[DT[,abs(distance)==min(abs(distance)), by=list(test_id, i.time)]$V1]
#    test_id time i.time distance
#1:       0    3      6        3
#2:       0    1      1        0
#3:       0    3      8        5
#4:       1    4      4        0
#5:       1    4      5        1
#6:       1    4     11        7

Or

 ev1[ev2, allow.cartesian=TRUE][,distance:= time-i.time][,
      .SD[abs(distance)==min(abs(distance))], by=list(test_id, i.time)]

Update

Using the new grouping

setkey(setDT(ev1), test_id, group_id)
setkey(setDT(ev2), test_id, group_id)
DT <- ev1[ev2, allow.cartesian=TRUE][,distance:=i.time-time]
DT[DT[,abs(distance)==min(abs(distance)), by=list(test_id, 
                                group_id,i.time)]$V1]$distance
#[1]  2  3  4 -1  0  4

Based on the code you provided

min_data$distance
#[1]  2  3  4 -1  0  4