find last day of month in a sequence of dates

Question

I have a sequence of dates like this

ds <- seq(as.Date("2011-02-01"), length=100, by="day")

I want to find the indices of the last days of each month

I can do it like this

last_day <- seq(as.Date("2011-02-01"), length=10, by="1 month") - 1

which(ds %in% last_day)

my problem is that my sequence of dates is not complete, some dates are missing and sometimes this can be the last day.

For example, I removed the last day of February

ds[ds == as.Date('2011-02-28')] <- NA

The new last day should now be '2011-02-27'.

How can I find the last of for each month based on the dates in my vector? The dates span over several years.

Answer 1

尝试：

which(ave(as.numeric(ds),format(ds,"%Y%m"),FUN=function(x) x==max(x))==1)

Answer 2

We can group_by month and select the max date from each month

library(zoo)
library(dplyr)

data.frame(ds) %>%
  group_by(month = as.yearmon(ds)) %>%
  slice(which.max(ds))


#        ds         month        
#     <date>     <S3: yearmon>
#1 2011-02-27     Feb 2011     
#2 2011-03-31     Mar 2011     
#3 2011-04-30     Apr 2011     
#4 2011-05-11     May 2011  

---

If we want the indices, we can do

library(zoo) 
which(ds %in% unique(ave(ds, as.yearmon(ds), FUN = max)))
#[1] 27 58 88 99

Answer 3

Function nth_day in package datetimeutils (which I maintain) allows you to get the last day of a month. It won't handle NA values, though.

library("datetimeutils")
ds <- seq(as.Date("2011-02-01"), length = 100, by = "day")

nth_day(ds, n = "last")
## [1] "2011-02-28" "2011-03-31" "2011-04-30" "2011-05-11"

nth_day(ds, n = "last", index = TRUE)
## [1]  28  59  89 100

Answer 4

Using endpoints from xts package:

ds <- seq(as.Date("2011-02-01"), length=100, by="day")
ds[ds == as.Date('2011-02-28')] <- NA


library(xts)
#need to remove NA's. xts can handle dates that are not there, but doesn't like NA's
ep <- endpoints(xts(ds[!is.na(ds)], order.by = ds[!is.na(ds)]), on = "months")
ds[ep]
[1] "2011-02-27" "2011-03-30" "2011-04-29" "2011-05-10"