Hi, You can also use either of these: data$date <- as.Date(data$date,format="%m/%d/%y") data$Days<-as.vector(sapply(lapply(split(data,data$Person),`[`,2),function(x) difftime(x[,1],x[1,],units="days")))
#or data$Days<-as.vector(sapply(lapply(split(data,data$Person),`[`,2),function(x) x[,1]-x[1,])) #or data$Days<-unlist(lapply(lapply(split(data,data$Person),`[`,2),function(x) difftime(x[,1],x[1,],units="days"))) data data # Person date Days #1 bob 2000-01-01 0 #2 bob 2000-01-02 1 #3 bob 2000-01-03 2 #4 dave 2000-01-07 0 #5 dave 2000-01-08 1 #6 dave 2000-01-10 3 #7 kevin 2000-01-02 0 #8 kevin 2000-01-03 1 #9 kevin 2000-01-04 2 A.K. ----- Original Message ----- From: Marcel Curlin <cemar...@u.washington.edu> To: r-help@r-project.org Cc: Sent: Thursday, September 27, 2012 1:16 AM Subject: [R] Calculating number of elapsed days from starting date Hi I have data for events in rows, with columns for person and date. Each person may have more than one event; tC <- textConnection(" Person date bob 1/1/00 bob 1/2/00 bob 1/3/00 dave 1/7/00 dave 1/8/00 dave 1/10/00 kevin 1/2/00 kevin 1/3/00 kevin 1/4/00 ") data <- read.table(header=TRUE, tC) close.connection(tC) rm(tC) I would like to add a new column to my dataframe containing the calculated number of elapsed days from the starting date for each person. So the new dataframe would read Person date Days bob 1/1/00 0 bob 1/2/00 1 bob 1/3/00 2 dave 1/7/00 0 dave 1/8/00 1 dave 1/10/00 3 kevin 1/2/00 0 kevin 1/3/00 1 kevin 1/4/00 2 Not sure how to do this, tried looking through the forum but didn't find anything that seemed to apply. Suggestions appreciated. -- View this message in context: http://r.789695.n4.nabble.com/Calculating-number-of-elapsed-days-from-starting-date-tp4644333.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ R-help@r-project.org mailing list 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 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.