[R] Bizarre - R crashes on merge

[Philipp Pagel]

>   I have a large data.frame, 1530 observation with 6 columns. I want to
> merge a 7th column, a transformation of the response variable (hospital
> admissions), namely 
> trans<-sqrt(copd$admissions+0.25)
> trans<-data.frame(trans)
> And now when I do 
> copd2<-merge(copd,trans)
> (copd being my original data.frame), R either crashes or is taking an
> extremely long time to do the computation. I had expected the
> computation to be done almost instantly as I have done similar things in
> R recently, however my system becomes very slow to the point of being
> unusable.

If I understand correctly, all you want to do is add another column to
your data.frame. Unless I have overlooked something, a simple
assignement should do:

copd$trans <- copd$admissions+0.25

The reason your computation takes long/crashes is that you are merging
two data frames which have no common columns to merge on. So merge() is
generating all possible combinations for you.

> a = data.frame(1:3)
> b = data.frame(4:6)
> merge(a,b)
  X1.3 X4.6
1    1    4
2    2    4
3    3    4
4    1    5
5    2    5
6    3    5
7    1    6
8    2    6
9    3    6

This is most likely not what you intended. I guess you were looking for
cbind() rather than merge()

cu
	Philipp


-- 
Dr. Philipp Pagel
Lehrstuhl für Genomorientierte Bioinformatik
Technische Universität München
Wissenschaftszentrum Weihenstephan
85350 Freising, Germany
http://mips.gsf.de/staff/pagel