Dear Peter, Thomas, et al.,
In the spirit of programming exercises, here's a recursive solution:
insert.values <- function(vector, after, values){
where <- which(vector == after)[1]
if (is.na(where)) vector
else c(vector[1:where], values,
Recall(vector[-(1:where)], after, values))
}
insert.values(a, "3", "7")
Regards,
John
At 08:11 PM 2/17/2003 +0100, Peter Dalgaard BSA wrote:
Thomas Lumley <[EMAIL PROTECTED]> writes:
> N<-length(a)
> threes<- a==3
> offset<- c(0,cumsum(threes)[-N])
> a[offset+(1:N)]<-a
> a[which(threes)+offset[threes]+1]<-7
>
> for a more vectorised version. Equally ugly, but understanding
> these two solutions is probably educational.
>
> Adding elements in the middle is something vectors are not good at, in
> contrast to (pair-based or linked) lists.
Extra exercises for the over-achievers:
1) Show that
offset <- cumsum(threes) - threes
also works.
2) Show that the index in the last line is the same as
which(threes)+seq(length=sum(threes))
-----------------------------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: [EMAIL PROTECTED]
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox
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