Hi Lily. Two problems. You have named the month field "mon" and then refer to it as "month". Second, as the resolution of as.Date is days, it can't produce a valid date without specifying the day. Thus:
df.count.mon<-data.frame(count=sample(1:24,24), year=rep(2014:2015,each=2),mon=rep(1:12,2)) # make each day the first day of the month df.count.mon$time<- as.Date(paste(df.count.mon$year, df.count.mon$mon,1), '%Y %m %d') df.count.mon count year mon time 1 22 2014 1 2014-01-01 2 12 2014 2 2014-02-01 ... You will get values, but I don't think they are the ones you want. Jim On Thu, Feb 23, 2017 at 6:33 AM, lily li <chocol...@gmail.com> wrote: > Hi R users, > > I have a dataframe, with year, month, day, and other variables. I wanted to > calculated monthly values of the variables. For example, there is one > variable called 'count'. I use the code below to convert daily data to > monthly data. > > df.count.mon = aggregate(count ~ year+month, data= df, sum) > > The new dataframe has three columns: year, month, and count. Now I want to > add one more column as 'time', which has the format 'yyyy-mm'. I use the > code below but the new column has all NA values. What is the problem and > how to solve it? > > df.count.mon$time = as.Date(paste(df.count.mon$year, df.count.mon$month), > '%Y %m') > > I had experience to add one more column with the format 'yyyy-mm-dd', which > works, but not with monthly format. Thanks for your help. > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
