Elegant way to identify runs inside a data.table

Question

I've run into this problem twice in the last two weeks alone, so I figured it's worth a post. I'm trying to identify "runs" inside a data.table, but I can't figure out an elegant way to do it.

Example

set.seed(2016)
dt <- data.table(ID = 1:50, Char = sample(LETTERS, 50, replace=TRUE))
dt <- dt[order(Char, ID)]

    ID Char
 1:  9    A
 2: 10    B
 3: 20    C
 4: 42    C
 5:  2    D
 6:  4    D
 7:  6    D
 8: 18    D
 ...

Here, I'd like to identify and group rows where the ID is within 2 of the row above/below. Here's my currently ugly solution

# Runs of 2 or more IDs within 2 of each other
dt[, `:=`(InRun = FALSE, InRunStart = FALSE)]
dt[abs(ID - shift(ID, type="lag")) <= 2 | abs(shift(ID, type="lead") - ID) <= 2, InRun := TRUE]
dt[InRun == TRUE & abs(ID - shift(ID, type="lag")) > 2 | is.na(shift(ID, type="lag")), InRunStart := TRUE]
dt[InRun == TRUE, RunID := cumsum(InRunStart)]
dt[, c("InRun", "InRunStart") := NULL]
dt
    ID Char RunID
 1:  9    A     1
 2: 10    B     1
 3: 20    C    NA
 4: 42    C    NA
 5:  2    D     2
 6:  4    D     2
 7:  6    D     2
 8: 18    D    NA
 ...

Is there a better way to do this?

---

EDIT: It seems there's been some confusion over how I'm defining a "run". To put it more explicitly, row_i and row_i+1 should have the same RunID if and only if their IDs are within a distance of 2.

Answer 1

I would stop after making this run ID:

dt[, run_id0 := 1L + cumsum(abs(ID - shift(ID, fill=ID[1L])) > 2)]

But to get the OP's run ID (which ignores length-one runs), here are a couple ways to go:

dt[duplicated(run_id0) | duplicated(run_id0, fromLast=TRUE), run_id1 := .GRP, by=run_id0 ]
# or
dt[, run_len := .N, by=run_id0 ][ run_len > 1L, run_id2 := .GRP, by=run_id0 ]

Answer 2

Don't know if this is elegant or not, but how about:

dt <- data.table(ID = c(9, 10, 15, 18, 21, 22, 25))
run_ids <- abs(dt[1:(.N-1), ID] - dt[2:.N, ID]) <= 2
run_ids <- c(run_ids[1], run_ids)
foo <- with(rle(run_ids), rep(cumsum(values) * values, lengths))
foo[foo == 0] = foo[which(foo == 0) + 1]
dt[, RunID := foo]
dt[RunID == 0, RunID := NA]
#    ID RunID
# 1:  9     1
# 2: 10     1
# 3: 15    NA
# 4: 18    NA
# 5: 21     2
# 6: 22     2
# 7: 25    NA