R help - Jun 2010 - problem with rbind on data.frames that contain data.frames

It took me some time to find this bug in my code. Is this a feature of  
R? Am I doing something wrong?

 > a=data.frame(x=1:10,y=1:10)
 > b=data.frame(x=11:20,y=11:20)
 > z=data.frame(1:10,11:20)

 > a$z=z
 > b$z=z

 > rbind(a,b)

Error in `row.names<-.data.frame`(`*tmp*`, value = c("1",
"2", "3",
"4",  :
   duplicate 'row.names' are not allowed
In addition: Warning message:
non-unique values when setting 'row.names': ?1?, ?10?, ?2?, ?3?, ?4?,  
?5?, ?6?, ?7?, ?8?, ?9?

adding rownames to a and b doesn't help:
 > rownames(a)=1:10
 > rownames(b)=11:20
 > rbind(a,b)
Error in `row.names<-.data.frame`(`*tmp*`, value = c("1",
"2", "3",
"4",  :
   duplicate 'row.names' are not allowed
In addition: Warning message:
non-unique values when setting 'row.names': ?1?, ?10?, ?2?, ?3?, ?4?,  
?5?, ?6?, ?7?, ?8?, ?9?

the problem is with the rownames of a$z and b$z...
 > rownames(a$z)=1:10
 > rownames(b$z)=11:20
 > rbind(a,b)
     x  y z.X1.10 z.X11.20
1   1  1       1       11
2   2  2       2       12
3   3  3       3       13
4   4  4       4       14
5   5  5       5       15
6   6  6       6       16
7   7  7       7       17
8   8  8       8       18
9   9  9       9       19
10 10 10      10       20
11 11 11       1       11
12 12 12       2       12
13 13 13       3       13
14 14 14       4       14
15 15 15       5       15
16 16 16       6       16
17 17 17       7       17
18 18 18       8       18
19 19 19       9       19
20 20 20      10       20

I was creating data.frames with data.frame members when I was doing  
computations on the data.frames.
Something like:
 > a=data.frame(x=1:10,y=1:10)
 > b=data.frame(x=11:20,y=11:20)
 > a$z=a*2
 > b$z=b*2
 > rbind(a,b)
Error in `row.names<-.data.frame`(`*tmp*`, value = c("1",
"2", "3",
"4",  :
   duplicate 'row.names' are not allowed
In addition: Warning message:
non-unique values when setting 'row.names': ?1?, ?10?, ?2?, ?3?, ?4?,  
?5?, ?6?, ?7?, ?8?, ?9?


Thanks for listening,
Michael

On 30/06/10 20:46, Michael Lachmann wrote:
> It took me some time to find this bug in my code. Is this a feature of 
> R? Am I doing something wrong?
>
> > a=data.frame(x=1:10,y=1:10)
> > b=data.frame(x=11:20,y=11:20)
> > z=data.frame(1:10,11:20)
>
> > a$z=z
You are (kind of) assigning *two* columns from the data frame "z" to
the
name 'z' in "a" which is probably not going to work as you
expect.  R
tries to be clever which may or may not be a Good Thing.  Try

a$z1 <- z[,1]
a$z2 <- z[,2]

or equivalent to keep the names straight.

As you have it, a$z is a data.frame, not a column, so you'd need a$z[,1] 
to get the 1:10 back from the original assignment of z.

The default printing of a does not help: always check using str:

 > str(a)
'data.frame':    10 obs. of  3 variables:
  $ x: int  1 2 3 4 5 6 7 8 9 10
  $ y: int  1 2 3 4 5 6 7 8 9 10
  $ z:'data.frame':    10 obs. of  2 variables:
   ..$ X1.10 : int  1 2 3 4 5 6 7 8 9 10
   ..$ X11.20: int  11 12 13 14 15 16 17 18 19 20


Hope this helps a little.

Allan