Ask anyone how much time has elapsed since September last year and they’ll probably start counting on their fingers: “October, November…” and tell you “just over 9 months.”
So, when faced as I was today with a data frame (named dates) like this:
pmid1 year1 month1 pmid2 year2 month2 21355427 2010 Dec 21542215 2011 Mar 21323727 2011 Feb 21521365 2011 Jun 21297532 2011 Feb 21336080 2011 Mar 21291296 2011 Apr 21591868 2011 Jun ...
How to add a 7th column, with the number of months between “year1/month1” and “year2/month2”?
R has lots of methods for date/time calculations, many of which seem notoriously difficult to get your head around. One answer for my problem came, as so often, from the R-help mailing list. I liked the title of the question too: “difference between 2 dates: IN MONTHS the way Mothers compute it“. It uses the zoo library and goes like this:
library(zoo) dates$months <- 12 * as.numeric(as.yearmon(paste(dates$year2, dates$month2, sep = "-"), "%Y-%b") - as.yearmon(paste(dates$year1, dates$month1, sep = "-"), "%Y-%b"))
Rather ungainly and difficult to read, but it works. The trick is to paste year and month together to generate, e.g. “2011-Jun”, then tell as.yearmon() the date format: %Y = YYYY (e.g. 2011), %b = abbreviated month (e.g. Jun). Result:
pmid1 year1 month1 pmid2 year2 month2 months 1 21355427 2010 Dec 21542215 2011 Mar 3 2 21323727 2011 Feb 21521365 2011 Jun 4 3 21297532 2011 Feb 21336080 2011 Mar 1 4 21291296 2011 Apr 21591868 2011 Jun 2
Note to self: improve understanding of R dates and times.
4 thoughts on “R: calculations involving months”
libidate + reshape ?
What about them? :-)
This uses the same basic idea but improves on the code so its not so ungainly:
to.ym <- function(y, m) as.yearmon(paste(y, m), "%Y %b")
12 * with(dates, to.ym(year2, month2) – to.ym(year1, month1))
Also its almost never a good idea to represent dates using multiple columns in data frames and that is the reason its ungainly — not because of yearmon: Suppose we had represented them as DF below. Then we could have reduced it to just this:
DF <- data.frame(d1 = "2010-01", d2 = "2011-02")
12 * with(DF, as.yearmon(d2) – as.yearmon(d1))
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