Sorry about that, it was sent by accident.
I have a data frame that looks something like this:
id day k
56 -1 566
63 -1 680
73 -1 773
56 2 298
63 2 273
Of course, it is a very simplified version of the
real data frame I am working on. I need to add another
column that would represent a percent change in k from
day -1, by id. I put only two ids at day 2 to
emphasize the fact that after day -1 some subjects
won't be on the data frame any more.
I tried something like this:
pck<-by(dat,dat[,c("id","day")], function(x) {
pc<-((x$k-x$k[x$day==-1])/x$k[x$day==-1])*100
})
but it didn't work.
Then I tried:
for(i in dat$id) {
for(s in dat$day) {
pc<-((dat$k[dat$id==i &
dat$day==s]-dat$k[dat$id==i &
dat$day==-1])/dat$k[dat$id==i & dat$day==-1])*100
}
}
without success.
I am sure it is very simple to do, but I would
appreciate any hints.
Thank you,
Judith
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