Separate day of year into month and day of month columns in R

Question

For simplicity, I have data that has two columns. One column is the year (year) and the other is the number of days (yday). So year with a value of 1980 and yday with a value of 1 is January 1, 1980. Year with a value of 1980 and yday with a value of 365 is December 31, 1980. How do I separate the single yday column into two columns; a month column and the day of the month column? For example, 365 would be 12 for the month and 31 for the day. Thanks in advance.

Answer 1

Create a Date from the yday + year columns, then extract the day of month, and month separately:

dat <- data.frame(year=1980, yday=c(1,365))

#  year yday
#1 1980    1
#2 1980  365

dat[c("month","day")] <- lapply(c("%m","%d"), \(x) {
  d <- as.Date(paste(dat$year, dat$yday), format="%Y %j")
  as.integer(format(d, x)) 
}) 
#  year yday month day
#1 1980    1     1   1
#2 1980  365    12  30