No of monthly days between two dates - r

Question

diff(seq(as.Date("2016-12-21"), as.Date("2017-04-05"), by="month"))
Time differences in days
[1] 31 31 28
The above code generates no of days in the month Dec, Jan and Feb.
However, my requirement is as follows
#Results that I need
#monthly days from date 2016-12-21 to 2017-04-05
11, 31, 28, 31, 5
#i.e 11 days of Dec, 31 of Jan, 28 of Feb, 31 of Mar and 5 days of Apr.
I even tried days_in_month from lubridate but not able to achieve the result
library(lubridate)
days_in_month(c(as.Date("2016-12-21"), as.Date("2017-04-05")))
Dec Apr
31 30

Answer 1

Try this:
x = rle(format(seq(as.Date("2016-12-21"), as.Date("2017-04-05"), by=1), '%b'))
> setNames(x$lengths, x$values)
# Dec Jan Feb Mar Apr
# 11 31 28 31 5

Answer 2

Although we have seen a clever replacement of table by rle and a pure table solution, I want to add two approaches using grouping. All approaches have in common that they create a sequence of days between the two given dates and aggregate by month but in different ways.
aggregate()
This one uses base R:
# create sequence of days
days <- seq(as.Date("2016-12-21"), as.Date("2017-04-05"), by = 1)
# aggregate by month
aggregate(days, list(month = format(days, "%b")), length)
# month x
#1 Apr 5
#2 Dez 11
#3 Feb 28
#4 Jan 31
#5 Mrz 31
Unfortunately, the months are ordered alphabetically as it happened with the simple table() approach. In these situations, I do prefer the ISO8601 way of unambiguously naming the months:
aggregate(days, list(month = format(days, "%Y-%m")), length)
# month x
#1 2016-12 11
#2 2017-01 31
#3 2017-02 28
#4 2017-03 31
#5 2017-04 5
data.table
Now that I've got used to the data.table syntax, this is my preferred approach:
library(data.table)
data.table(days)[, .N, .(month = format(days, "%b"))]
# month N
#1: Dez 11
#2: Jan 31
#3: Feb 28
#4: Mrz 31
#5: Apr 5
The order of months is kept as they have appeared in the input vector.