[R] Calculating number of elapsed days from starting date

[arun]

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 <cemarcel at u.washington.edu>
To: r-help at 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.




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