问题
I have a data.table as follows -
dt = data.table(
date = seq(as.Date("2015-12-01"), as.Date("2015-12-10"), by="days"),
v1 = c(seq(1, 9), 20),
v2 = c(5, rep(NA, 9))
)
dt
date v1 v2
1: 2015-12-01 1 5
2: 2015-12-02 2 NA
3: 2015-12-03 3 NA
4: 2015-12-04 4 NA
5: 2015-12-05 5 NA
6: 2015-12-06 6 NA
7: 2015-12-07 7 NA
8: 2015-12-08 8 NA
9: 2015-12-09 9 NA
10: 2015-12-10 20 NA
Question 1: I want to add the current row value of v1 with the previous row value of v2 so the output looks like the following.
date v1 v2
1: 2015-12-01 1 5
2: 2015-12-02 2 7
3: 2015-12-03 3 10
4: 2015-12-04 4 14
5: 2015-12-05 5 19
6: 2015-12-06 6 25
7: 2015-12-07 7 32
8: 2015-12-08 8 40
9: 2015-12-09 9 49
10: 2015-12-10 20 69
I have tried to use rollapplyr function to achieve this but failed.
Question 2: Instead of adding (as in Question 1), I want to roll apply the function qma to the current row value of v1 with the previous row value of v2
qma <- function(x, y){(x+y+7)/2}
I am sure there must be a simple way to do this in one line using data.table.
Thanks
回答1:
You can add first
v2
value to cumulative sum of v1
.
library(data.table)
dt[, v2:= first(v2) + c(0, cumsum(v1[-1]))]
dt
# date v1 v2
# 1: 2015-12-01 1 5
# 2: 2015-12-02 2 7
# 3: 2015-12-03 3 10
# 4: 2015-12-04 4 14
# 5: 2015-12-05 5 19
# 6: 2015-12-06 6 25
# 7: 2015-12-07 7 32
# 8: 2015-12-08 8 40
# 9: 2015-12-09 9 49
#10: 2015-12-10 10 59
来源:https://stackoverflow.com/questions/65746251/r-rolling-sum-of-two-columns-in-data-table