Calculating number of elapsed days from starting date
On 27/09/12 17:16, Marcel Curlin wrote:
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.
X <- data
X$date <- with(X,as.Date(date,format="%m/%d/%y"))
X$Days <- unlist(with(X,tapply(date,Person,function(x){x-x[1]})))
And then if you *really* want to keep that shaganappi date format:
X$date <- data$date
X
# Giving:
Person date Days
1 bob 1/1/00 0
2 bob 1/2/00 1
3 bob 1/3/00 2
4 dave 1/7/00 0
5 dave 1/8/00 1
6 dave 1/10/00 3
7 kevin 1/2/00 0
8 kevin 1/3/00 1
9 kevin 1/4/00 2
BTW: You really *shouldn't* call your data "data". See:
fortune("dog")
cheers,
Rolf Turner