## Identification of new values cumulatively by groups in data.table in r

cumulative sum in r
r cumulative sum by group
r cumulative percentage by group
r cumulative sum by group dplyr
running total by group in r
cumulative sum by factor r
cumulative sum by row in r
r group by count

How to create a new column that identifies new value appearance in `Letter` column cumulatively by groups of unique combs of `Year` + `Month`?

Data sample.

```require(data.table)
dt <- data.table(Letter = c(LETTERS[c(5, 1:2, 1:2, 1:4, 3:6)]),
Year = 2018,
Month = c(rep(5,5), rep(6,4), rep(7,4)))
```

Print.

```    Letter Year Month
1:      E 2018     5
2:      A 2018     5
3:      B 2018     5
4:      A 2018     5
5:      B 2018     5
6:      A 2018     6
7:      B 2018     6
8:      C 2018     6
9:      D 2018     6
10:      C 2018     7
11:      D 2018     7
12:      E 2018     7
13:      F 2018     7
```

Result I'm trying to get:

```    Letter Year Month   New
1:      E 2018     5  TRUE
2:      A 2018     5  TRUE
3:      B 2018     5  TRUE
4:      A 2018     5  TRUE
5:      B 2018     5  TRUE
6:      A 2018     6 FALSE
7:      B 2018     6 FALSE
8:      C 2018     6  TRUE
9:      D 2018     6  TRUE
10:      C 2018     7 FALSE
11:      D 2018     7 FALSE
12:      E 2018     7 FALSE
13:      F 2018     7  TRUE
```

Detailed Question:

1. Group1 ("E", "A", "B", "A", "B") all TRUE by default as nothing to compare with.
2. Which of the letters in group2 ("A", "B", "C", "D") is not duplicated in group1.
3. Then, which of letters in group3 ("C", "D", "E", "F") in not duplicated in both groups 1&2 ("E", "A", "B", "A", "B", "A", "B", "C", "D").

Initialize to FALSE; then join to first Year-Month with each Letter and update to TRUE:

```dt[, v := FALSE]
dt[unique(dt, by="Letter"), on=.(Letter, Year, Month), v := TRUE][]

Letter Year Month     v
1:      E 2018     5  TRUE
2:      A 2018     5  TRUE
3:      B 2018     5  TRUE
4:      A 2018     5  TRUE
5:      B 2018     5  TRUE
6:      A 2018     6 FALSE
7:      B 2018     6 FALSE
8:      C 2018     6  TRUE
9:      D 2018     6  TRUE
10:      C 2018     7 FALSE
11:      D 2018     7 FALSE
12:      E 2018     7 FALSE
13:      F 2018     7  TRUE
```

Calculate cumulative sum within each ID (group), frame to a data.table by reference; Calculate the cumulative sum of value grouped by id and assign it by reference; Print (the last [] there) the  3 Identification of new values cumulatively by groups in data.table in r Sep 20 '18 3 How to effectively determine the maximum difference between the variable value in each row and same variable subsequent row values in data.table in R Jan 19 '19

Simply:

``` # dt[,new := ifelse(Letter %in% dt\$Letter[dt\$Month<Month],F,T), by="Month"][]

#   Letter Year Month   new
#1:      E 2018     5  TRUE
#2:      A 2018     5  TRUE
#3:      B 2018     5  TRUE
#4:      A 2018     5  TRUE
#5:      B 2018     5  TRUE
#6:      A 2018     6 FALSE
#7:      B 2018     6 FALSE
#8:      C 2018     6  TRUE
#9:      D 2018     6  TRUE
#10:      C 2018     7 FALSE
#11:      D 2018     7 FALSE
#12:      E 2018     7 FALSE
#13:      F 2018     7  TRUE
```

With very valid comments of David A., a much faster and less verbose version: (recommended)

```dt[, new := !(Letter %in% dt\$Letter[dt\$Month<Month]), by=Month][]
```

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Another possible approach:

```dupes <- c()
dt[, New := {
x <- !Letter %chin% dupes
dupes <- c(dupes, unique(Letter[x]))
x
}, by=.(Year, Month)]
```

Some timings for reference below:

if Letter is an integer:

```library(microbenchmark)
microbenchmark(mtd0=dt0[, New := !(Letter %in% dt0\$Letter[dt0\$Month<Month]), by=Month],
mtd1={
dt1[, v := FALSE]
dt1[unique(dt1, by="Letter"), on=.(Letter, Year, Month), v := TRUE]
},
mtd2={
dupes <- c()
dt2[, New := {
x <- !Letter %in% dupes
dupes <- c(dupes, unique(Letter[x]))
x
}, by=.(Year, Month)]
},
times=3L)
```

integer timing output:

```Unit: milliseconds
expr       min       lq      mean    median        uq      max neval
mtd0 1293.3100 1318.775 1331.7129 1344.2398 1350.9143 1357.589     3
mtd1  377.1534  391.178  402.4423  405.2026  415.0868  424.971     3
mtd2 2015.2115 2020.926 2023.7209 2026.6400 2027.9756 2029.311     3
```

if Letter is a character:

```microbenchmark(mtd0=dt0[, New := !(Letter %chin% dt0\$Letter[dt0\$Month<Month]), by=Month],
mtd1={
dt1[, v := FALSE]
dt1[unique(dt1, by="Letter"), on=.(Letter, Year, Month), v := TRUE]
},
mtd2={
dupes <- c()
dt2[, New := {
x <- !Letter %chin% dupes
dupes <- c(dupes, unique(Letter[x]))
x
}, by=.(Year, Month)]
},
times=3L)
```

timing output:

```Unit: milliseconds
expr       min        lq      mean    median        uq       max neval
mtd0 1658.5806 1689.8941 1765.9329 1721.2076 1819.6090 1918.0105     3
mtd1  849.2361  851.1807  852.8632  853.1253  854.6768  856.2283     3
mtd2  420.1013  426.0941  433.9202  432.0869  440.8296  449.5723     3
```

check:

```> identical(dt2\$New, dt1\$v)
 TRUE
> identical(dt0\$New, dt1\$v)
 FALSE
```

data:

```set.seed(0L)
nr <- 1e7
dt <- unique(data.table(Letter=sample(nr/1e2, nr, replace=TRUE),
Year=sample(2014:2018, nr, replace=TRUE),
Month=sample(1:12, nr, replace=TRUE)))
setorder(dt, Year, Month)#[, Letter := as.character(Letter)]
dt0 <- copy(dt)
dt1 <- copy(dt)
dt2 <- copy(dt)

#for seed=0L, dt has about 4.8mio rows
```

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• @Andre BTW replacing `%in%` with data.table's native `%chin%` makes it even more efficient.
• Fyi, your dupes object outside of DT[...] doesn't get modified. That's not a problem, but some alternatives that might be of interest: have it exist only inside DT[...] like `dt[, New := { if (.GRP == 1L) dupes <- c(); x <- !Letter %chin% dupes; dupes <- c(dupes, unique(Letter[x])); x }, by=.(Year, Month)]` or use `<<-` to modify the object outside.
• thanks, Frank. yeah if `unique` set is required, then can use `<<-` to store dupes, then no need to call `unique` again