R data.table find lags between current row to previous row

问题

> tempDT <- data.table(colA = c("E","E","A","A","E","A","E")
+                      , lags = c(NA,1,1,2,3,1,2))
> tempDT
   colA lags
1:    E   NA
2:    E    1
3:    A    1
4:    A    2
5:    E    3
6:    A    1
7:    E    2

I have column colA, and need to find lags between current row and the previous row whose colA == "E".

Note: if we could find the row reference for the previous row whose colA == "E", then we could calculate the lags. However, I don't know how to achieve it.

回答1:

1) Define lastEpos which given i returns the position of the last E among the first i rows and apply that to each row number:

lastEpos <- function(i) tail(which(tempDT$colA[1:i] == "E"), 1)
tempDT[, lags := .I - shift(sapply(.I, lastEpos))]

Here are a few variations:

2) i-1 In this variation lastEpos returns the positions of the last E among the first i-1 rows rather than i:

lastEpos <- function(i) tail(c(NA, which(tempDT$colA[seq_len(i-1)] == "E")), 1)
tempDT[, lags := .I - sapply(.I, lastEpos)]

3) Position Similar to (2) but uses Position:

lastEpos <- function(i) Position(c, tempDT$colA[seq_len(i-1)] == "E", right = TRUE)
tempDT[, lags := .I - sapply(.I, lastEpos)]

4) rollapply

library(zoo)
w <- lapply(1:nrow(tempDT), function(i) -rev(seq_len(i-1)))
tempDT[, lags := .I - rollapply(colA == "E", w, Position, f = c, right = TRUE)]

5) sqldf

library(sqldf)

sqldf("select a.colA, a.rowid - b.rowid lags
       from tempDT a left join tempDT b
       on b.rowid < a.rowid and b.colA = 'E'
       group by a.rowid")

来源：https://stackoverflow.com/questions/49142288/r-data-table-find-lags-between-current-row-to-previous-row