On Jan 7, 2013, at 5:33 AM, Paolo Donatelli wrote:
Hi all,
I have a very basic doubt -- but still, I am a newby!
My question is about referring to the previous row: in a sample as the
following...
ID X1 X2
1 A 12
2 A 6
3 A 10
1 B 17
2 B 19
1 C 22
1 D 13
2 D 19
3 D 21
... I would like to create a dummy variable equal to 1 whenever the
value of ID of the current row is lower or equal than the value of ID
of the previous row -- check the new vector X3 I'd like to obtain:
ID X1 X2 X3
1 A 12 0
2 A 6 0
3 A 10 0
1 B 17 1
2 B 19 0
1 C 22 1
1 D 13 1
2 D 19 0
3 D 21 0
Something like (untested):
dfrm$X3 <- c(0, as.numeric( diff(dfrm$ID) <= 0 ) )
That might be faster than this sort of untested strategy:
... <- with(dfrm, c( 0 , as.numeric( ID[2:nrow(dfrm)] <= ID[1:
(nrow(dfrm)-1] ) ) )
In my newbie days I thought a function named `lag` would do it, but
discovered it was only working on ts-class objects.
I have searched a lot without finding a decent and working solution.
I Adding
suppose it is just some basic matter of indexing language, something
like
X3<- as.numeric ( ID[n] <= ID[n-1])
An explicit sequence rather than using mathematical notation is
needed. And if you are using dataframes, you should not be using
`attach`. That X3 would not be constructed in the dataframe.
--
David Winsemius, MD
Alameda, CA, USA
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