Rolling Join Grouped by a Second Variable in Data.Table

Rolling join grouped by a second variable in data.table

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You can join on multiple columns simultaneously. So, in addition to "Date", you can include "Field" in the on clause. But please note the description of the roll argument in ?data.table:

Rolling joins apply to the last join column

Thus, for "Date" to be used for the rolling join, specify it as the last variable in on:

library(data.table)
d1[d2, roll = "nearest", on = .(Field, Date)]

For better verification, the result can be ordered

d1[d2, roll = "nearest", on = .(Field, Date)][order(Field, Date)]

    Field       Date  NlbsAcre      TotN
 1:   12S 2016-05-24        NA 208.62194
 2:   12S 2016-05-27        NA 172.57658
 3:   12S 2016-07-31        NA 318.97092
 4:   12S 2016-08-18        NA 428.54011
 5:   12S 2016-08-29        NA 393.81545
 6:   12S 2017-03-13 44728.184 145.15091
 7:   12S 2017-03-16 44728.184 128.14334
 8:   12S 2017-08-01 12621.083 132.72365
 9:   12S 2017-08-04 12621.083 422.63032
10:   12S 2017-08-14 12621.083 337.91388
11:   12S 2017-10-04 22162.203 692.15276
12:  19-1 2016-05-01 12630.923 476.17492
13:  19-1 2016-08-15 12630.923 110.70600
14:  19-1 2016-09-10 12630.923 215.88105
15:  19-1 2016-09-19 12630.923 224.68906
16:  19-1 2016-12-16 12630.923 338.59349
17:  19-1 2017-01-13 12630.923 305.35394
18:  19-1 2017-03-27 12630.923 435.04925
19:  19-1 2017-05-30 12630.923 818.80997
20:     6 2016-05-05        NA 102.53240
21:     6 2016-06-14        NA 149.06045
22:     6 2016-06-29        NA 125.82803
23:     6 2016-06-29        NA 125.82803
24:     6 2016-07-11        NA  79.24480
25:     6 2016-07-25        NA  62.24449
26:     6 2016-08-25        NA  75.77014
27:     6 2017-01-03  2014.772  47.49660
28:     6 2017-01-12  2014.772  45.53730
29:     6 2017-01-17  2014.772  43.92222
30:     6 2017-02-11  3082.318  21.96791
31:     6 2017-03-19  2477.083  21.39367
32:     6 2017-04-17  2427.536  79.03807
33:     6 2017-07-12        NA 103.52417
34:     6 2017-07-17        NA  65.53112
35:     6 2017-09-06        NA  47.40618
36:     7 2016-06-02        NA 147.49353
37:     7 2016-07-11        NA  59.26973
38:     7 2016-08-04        NA  72.62146
39:     7 2016-08-30        NA  58.27003
40:     7 2016-08-30        NA  58.27003
41:     7 2016-10-30        NA  73.88811
42:     7 2017-02-11  2279.609  21.07551
43:     7 2017-02-22  2279.609  19.92023
44:     7 2017-03-19 15842.916  31.71433
45:     7 2017-05-17        NA  44.96872
46:     7 2017-07-17        NA  58.53364
47:   W62 2016-05-05 16764.975  96.72854
48:   W62 2016-05-31 16764.975  72.96954
49:   W62 2016-08-31 16764.975  86.33588
50:   W62 2016-12-05 16764.975  94.19370
51:   W62 2017-01-02 18874.656 119.39040
52:   W62 2017-02-22 18874.656  75.46591
    Field       Date  NlbsAcre      TotN

Rolling join in Data.Table by two variables without creating duplicates

First of all, you could use unique instead of distinct
(the latter presumably from dplyr; you don't specify)
to avoid coercing the data table to a data frame.

You were pretty close,
but you need to switch the tables in the join,
i.e. something like df2[df1],
so that the rows from df1 are used as search keys,
and then you can use mult to remove duplicates.

Here's one way to do what you want with a non-equi join:

setkey(df1, departure)
setkey(df2, departure)

df1[, max_departure := departure + as.difftime(1, units = "hours")
    ][, observed_departure := df2[df1,
                                  x.departure,
                                  on = .(stop_id, departure >= departure, departure <= max_departure),
                                  mult = "first"]
      ][, max_departure := NULL]

We order by departure (via setkey) so that mult = "first" returns the closest match in the future within what's allowed.
The intermediate column max_departure has to be assigned and subsequently removed because non-equi joins can only use existing columns.
Also note that the syntax used takes from this answer
(the version with .SD instead of df1 doesn't work in this case,
I don't know why).

EDIT: based on the comments,
it occurs to me that when you say "duplicated",
you might be referring to something different.
Say you have planned departures at 10 and 10:30,
but the one at 10 never takes place,
and an observed departure is 10:31.
Perhaps you mean that 10:31 is the observed departure for the one scheduled at 10:30,
and cannot be used for the one at 10?
If that's the case,
perhaps this will work:

setkey(df1, departure)
setkey(df2, departure)

max_dep <- function(departure) {
  max_departure <- departure + as.difftime(1, units = "hours")

  next_departure <- shift(departure,
                          fill = max_departure[length(max_departure)] + as.difftime(1, units = "secs"),
                          type = "lead")

  invalid_max <- max_departure >= next_departure

  max_departure[invalid_max] <- next_departure[invalid_max] - as.difftime(1, units = "secs")
  max_departure
}

df1[, max_departure := max_dep(departure), by = "stop_id"
    ][, observed_departure := df2[df1,
                                  x.departure,
                                  on = .(stop_id, departure >= departure, departure <= max_departure),
                                  mult = "first"]
      ][, max_departure := NULL]

The max_dep helper checks,
for each stop and scheduled departure,
what would be the next scheduled departure,
and sets max_departure as "next minus 1 second" if the next departure is within one hour.

The other solution wouldn't work for this because,
as long as an observed departure falls within one hour of the scheduled one,
it is a valid option.
In my example that means 10:31 would be valid for both 10:30 and 10.

Rolling join two data.tables with date in R

We may use non-equi join

dt1[dt2, date_2 := date2, on = .(group, date1 > date2), mult = "first"]

R rolling join two data.tables with error margin on join

A data.table answer has been given here by user Uwe:

https://stackoverflow.com/a/62321710/12079387

Merging two sets of data by data.table roll='nearest' function

Here is a step-by-step example based on the sample data you give:

# Sample data
library(data.table)
setDT(set_A)
setDT(set_B)    

# Create time column by which to do a rolling join
set_A[, time := time_a]
set_B[, time := time_b]
setkey(set_A, time)
setkey(set_B, time)

# Rolling join by nearest time
set_merged <- set_B[set_A, roll = "nearest"]

unique(set_merged[order(ID_b)], by = "time")
#    ID_b   b1     b2     source   time_b     time ID_a    a1     a2       a3
# 1:    2 34.2 15.114 set1.csv.1 20.35750 20.01000    8 85640 5274.1 301.6041
# 2:    7 67.2 16.114 set1.csv.2 21.35778 21.00972    7 85697 5345.2 301.6043
# 3:   12 12.2 33.114 set1.csv.3 22.35806 22.00972    4 65694 9375.2 301.6049
# 4:   17 73.2 67.114 set2.csv.1 23.35833 23.00972    3 85694 9278.9 301.6051
# 5:   23 88.2 42.114 set2.csv.2 19.35861 19.00972    5 85653 4375.5 301.6047
# 6:   28 90.2 52.114 set3.csv.1  0.35889  0.00944    2 35694 5245.2 301.6053
#      time_a
# 1: 20.01000
# 2: 21.00972
# 3: 22.00972
# 4: 23.00972
# 5: 19.00972
# 6:  0.00944

Two comments:

We create a new time column to avoid losing one of the original time columns from set_A and set_B. You can always remove the time column after the join if required.
We use unique to remove duplicated time rows by order of ID_b. You mention in your post that "it doesn't really matter which row will be merged" but in case that you do want to retain specific rows, you might need to adjust this line of code.

Update (thanks to @Henrik)

As @Henrik pointed out, what you're after is actually a rolling join of set_A with respect to set_B, in which case you don't need to deal with the duplicate rows.

That translates to

library(data.table)
setDT(set_A)
setDT(set_B)    

# Create time column by which to do a rolling join
set_A[, time := time_a]
set_B[, time := time_b]

set_A[set_B, on = "time", roll = "nearest"][order(ID_a)]
#   ID_a    a1     a2       a3   time_a     time ID_b   b1     b2     source
#1:    2 35694 5245.2 301.6053  0.00944  0.35889   28 90.2 52.114 set3.csv.1
#2:    3 85694 9278.9 301.6051 23.00972 23.35833   17 73.2 67.114 set2.csv.1
#3:    5 85653 4375.5 301.6047 19.00972 19.35861   23 88.2 42.114 set2.csv.2
#4:    6 12694 5236.3 301.6045 22.00972 22.35806   12 12.2 33.114 set1.csv.3
#5:    7 85697 5345.2 301.6043 21.00972 21.35778    7 67.2 16.114 set1.csv.2
#6:    9 30694 5279.0 301.6039 20.01000 20.35750    2 34.2 15.114 set1.csv.1
#  time_b
#1:  0.35889
#2: 23.35833
#3: 19.35861
#4: 22.35806
#5: 21.35778
#6: 20.35750

Sample data

set_A <- read.table(text =
    "ID_a    a1  a2  a3  time_a
2   35694   5245.2  301.6053    00.00944
3   85694   9278.9  301.6051    23.00972
4   65694   9375.2  301.6049    22.00972
5   85653   4375.5  301.6047    19.00972
6   12694   5236.3  301.6045    22.00972
7   85697   5345.2  301.6043    21.00972
8   85640   5274.1  301.6041    20.01000
9   30694   5279.0  301.6039    20.01000", header = T)

set_B <- read.table(text =
    "ID_b    b1  b2  source  time_b
2   34.20   15.114  set1.csv.1  20.35750
7   67.20   16.114  set1.csv.2  21.35778
12  12.20   33.114  set1.csv.3  22.35806
17  73.20   67.114  set2.csv.1  23.35833
23  88.20   42.114  set2.csv.2  19.35861
28  90.20   52.114  set3.csv.1  00.35889", header = T)

R data.table rolling join mult not working as expected

When running dt2[dt1, roll = "nearest"] you are basically saying "return the rows from dt2 according to the nearest join to each row in dt1 using the key. So

dates2 in row one in dt2 is the nearest to dates1 in row one in dt1
dates2 in row one in dt2 is the nearest to dates1 in row two in dt1
dates2 in row two in dt2 is the nearest to dates1 in row three in dt1

Hence,

dt2[dt1, roll = "nearest"]
#                 dates2 values2 values1
# 1: 2015-10-26 12:00:00       A       a
# 2: 2015-10-26 13:00:00       A       b
# 3: 2015-10-26 14:00:00       C       c

Which are all the rows from dt1 with the joined values2 from dt2.

Instead, we want to join the other way around, namely "extract values2 from dt2 according to the nearest join by each row in dt2 using the key and update the matched rows in dt1", namely

dt1[dt2, roll = "nearest", values2 := i.values2] 
dt1
#                 dates1 values1 values2
# 1: 2015-10-26 12:00:00       a       A
# 2: 2015-10-26 13:00:00       b      NA
# 3: 2015-10-26 14:00:00       c       C

Some additional notes

You don't need to wrap first to data.frame and then to data.table, you can just do dt1 <- data.table(dates1, values1) and etc.
While you at it, you can already set the key on the fly using key parameter data.table, namely dt1 <- data.table(dates1, values1, key = "dates1") and etc.
Or you can skip setting keys all together and use on instead (V 1.9.6+), namely dt1[dt2, roll = "nearest", values2 := i.values2, on = c(dates1 = "dates2")]
Finally, please refrain from making unnecessary copies, e.g., instead of <- and data.table(df) use := and setDT(df), see here for more information

Nearest n rolling join in R data table

Here is something very raw (we go row by row):

n <- 2L
sen <- 1L:n
for (i in 1:nrow(dt1)) {
  set(dt1, i, j = "nearest", list(which(frank(abs(dt1$x[i] - dt2$x)) %in% sen)))
}
dt1[, .(id1, nearest = unlist(nearest)), by = x
    ][, id2 := dt2$id2[nearest]
      ][, roll := paste0("nr", frank(abs(dt2$x[nearest] - x))), by = x][]

#      x id1 nearest id2 roll
# 1:  15   x       1   a  nr1
# 2:  15   x       2   b  nr2
# 3: 101   y       2   b  nr2
# 4: 101   y       3   c  nr1

Slightly cleaner:

dt1[, 
    {
      nrank <- frank(abs(x - dt2$x), ties.method="first")
      nearest <- which(nrank %in% sen)
      .(x = x, id2 = dt2$id2[nearest], roll = paste0("nr", nrank[nearest]))
    }, 
    by = id1] # assumes unique ids.

Data:

dt1 <- data.table(x = c(15, 101), id1 = c("x", "y"))
dt2 <- data.table(x = c(10, 50, 100, 200), id2 = c("a", "b", "c", "d"))

EDIT (as suggested/written by OP)
Joining with multiple keys:

dt1[, 
    {
      g <- group
      dt_tmp <- dt2[dt2$group == g]
      nrank <- frank(abs(x - dt_tmp$x), ties.method="first")
      nearest <- which(nrank %in% sen)
      .(x = x, id2 = dt_tmp$id2[nearest], roll = paste0("nr", nrank[nearest]))
    }, 
    by = id1]

Unable to perform rolling join on time and another variable in data.table

This seemed to solve the problem

test = df2[df1, on=.(bike_id, time = time), roll="nearest", 
            .(bike_id, time, st_x, st_y)]

rolling joins data.table in R

That quote from the documentation appears to be from FAQ 1.12 What is the difference between X[Y] and merge(X,Y). Did you find the following in ?data.table and does it help?

roll Applies to the last join column, generally a date but can be any
ordered variable, irregular and including gaps. If roll=TRUE and i's
row matches to all but the last x join column, and its value in the
last i join column falls in a gap (including after the last
observation in x for that group), then the prevailing value in x is
rolled forward. This operation is particularly fast using a modified
binary search. The operation is also known as last observation carried
forward (LOCF). Usually, there should be no duplicates in x's key, the
last key column is a date (or time, or datetime) and all the columns
of x's key are joined to. A common idiom is to select a
contemporaneous regular time series (dts) across a set of identifiers
(ids): DT[CJ(ids,dts),roll=TRUE] where DT has a 2-column key (id,date)
and CJ stands for cross join.

rolltolast Like roll but the data is not rolled forward past the last
observation within each group defined by the join columns. The value
of i must fall in a gap in x but not after the end of the data, for
that group defined by all but the last join column. roll and
rolltolast may not both be TRUE.

In terms of left/right analogies to SQL joins, I prefer to think about that in the context of FAQ 2.14 Can you explain further why data.table is inspired by A[B] syntax
in base. That's quite a long answer so I won't paste it here.

Rolling Join Grouped by a Second Variable in Data.Table