adding days to a date in R - r

Question

My data looks like this:
date rmean
1/2/2004 6
1/5/2004 30
1/6/2004 27
1/7/2004 20
1/8/2004 10
1/9/2004 22
1/12/2004 21
1/13/2004 18
1/14/2004 19
1/15/2004 7
1/16/2004 9
1/19/2004 11
1/20/2004 18
1/21/2004 26
1/26/2004 8
1/27/2004 16
1/28/2004 19
1/29/2004 4
1/30/2004 1
2/3/2004 11
2/4/2004 9
2/5/2004 26
2/6/2004 16
2/9/2004 25
2/10/2004 2
2/11/2004 6
2/12/2004 2
2/13/2004 25
2/16/2004 17
2/17/2004 21
2/18/2004 26
2/19/2004 6
2/20/2004 14
2/23/2004 4
2/24/2004 7
2/25/2004 19
2/26/2004 10
2/27/2004 23
I want to find the rmean of (20 days + 15th of each month).
Note: if there isn't a value for rmean of that date in my data (some days are skipped), i want it to find the rmean of closest day of the
something like this but ( 20 + 15th of each month) instead of 15 :
dt <- Dataframe[, list(day15=abs(mday(date)-15) == min(abs(mday(date)-15)),
date, rmean), by=list(year(date), month(date))]
dt[day15==TRUE]
Finale = dt[day15==TRUE , .SD[1,] ,by=list(month, year)]
The expected output for my example above:
date rmean
2/4/2004 9

Answer 1

Here's one way to do it with base R.
First, some dummy data:
d <- data.frame(date=as.Date('1/1/2004', '%d/%m/%Y') + sort(sample(364, 200)),
x=runif(200))
head(d)
# date x
# 1 2004-01-02 0.29818227
# 2 2004-01-03 0.12543617
# 3 2004-01-04 0.78145310
# 4 2004-01-05 0.30456904
# 5 2004-01-06 0.45228066
# 6 2004-01-07 0.07511554
Calculate arrival dates within the date range of the data:
arrival <-
seq(as.Date(sprintf('15/%s', format(min(d$date), '%m/%Y')), '%d/%m/%Y'),
as.Date(sprintf('15/%s', format(max(d$date), '%m/%Y')), '%d/%m/%Y'),
by='month') + 20
arrival
# [1] "2004-02-04" "2004-03-06" "2004-04-04" "2004-05-05" "2004-06-04" "2004-07-05"
# [7] "2004-08-04" "2004-09-04" "2004-10-05" "2004-11-04" "2004-12-05" "2005-01-04"
Find the closest date to each of the arrival dates (taking that with max x value if there are two closest dates), and return a data.frame with the "arrival" dates, the closest dates to each of these arrival dates, and the corresponding values of x.
cbind(arrival, do.call(rbind, lapply(arrival, function(x) {
closest <- which(abs(d$date - x) == min(abs(d$date - x)))
d[closest[which.max(d$x[closest])], ]
})))
# arrival date x
# 25 2004-02-04 2004-02-03 0.78836413
# 45 2004-03-06 2004-03-06 0.61214949
# 63 2004-04-04 2004-04-04 0.49171847
# 79 2004-05-05 2004-05-05 0.02989788
# 93 2004-06-04 2004-06-04 0.25923715
# 109 2004-07-05 2004-07-05 0.90330331
# 120 2004-08-04 2004-08-04 0.48133237
# 139 2004-09-04 2004-09-03 0.12280267
# 151 2004-10-05 2004-10-03 0.46888891
# 169 2004-11-04 2004-11-04 0.40397949
# 186 2004-12-05 2004-12-04 0.18685615
# 200 2005-01-04 2004-12-30 0.97462347