[R] merge function

[peter dalgaard]

On Jul 1, 2011, at 06:48 , Jeff Newmiller wrote:

> You haven't provided a reproducible example.
> 
> I do notice you are using T and F which are variables that can be redefined (which is why TRUE and FALSE are preferred.

Also, if x and y really are "vectors" (I bet they're not, though), you'll get the cartesian product whatever all.x and all.y are, unless you specify by.x="x" and by.y="y". I.e.,

> merge(1:3,2:4,all.y=F,all.x=T)
  x y
1 1 2
2 2 2
3 3 2
4 1 3
5 2 3
6 3 3
7 1 4
8 2 4
9 3 4

> merge(1:3,2:4,by.x="x",by.y="y")
  x
1 2
2 3

> merge(1:3,2:4,by.x="x",by.y="y", all.x=T)
  x
1 1
2 2
3 3

All just to point out the importance of actual examples. Mind reading is sort of fun and some correspondents on mailing lists get rather good at it, but it is more expedient to have a well-defined problem from the outset. 

-pd

> 
> "Downey, Patrick" <PDowney at urban.org> wrote:
> 
> Hello,
> 
> I'm clearly confused about the merge function. In the following
> 
> r <- merge(x,y,all.x=T,all.y=F)
> 
> my y vector has only unique values (no duplicates). So I don't understand
> how this can ever generate an r which is of greater length than x. 

-- 
Peter Dalgaard
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com