[R] inserting elements in a list
j+rhelp at howard.fm
Tue Feb 18 07:27:03 CET 2003
On 17 Feb 2003 20:11:11 +0100, "Peter Dalgaard BSA"
<p.dalgaard at biostat.ku.dk> said:
> Thomas Lumley <tlumley at u.washington.edu> 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.
Indeed they're not - and the code above fails to do this insertion:
 1 2 3 7 6 3 NA 7
The problem is that it is placing the '7's in the correct place, but is
not shifting the right-hand side of the list across to make room.
> 2) Show that the index in the last line is the same as
That's a neat way to find the right spots to place the '7's - wherever
the '3's were before, plus one spot for each '3' that we've seen so
far... I'm not sure how to insert into the vector rather than replace
though - a quick browse through the list archives didn't turn up any
quick solutions (other than the types of loops suggested in earlier
answers in this thread).
jhoward at fastmail.fm
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