R help - Jan 2003 - dataframe subsetting behaviour

Hi,

I'm trying to understand a behaviour that I have encountered
and can't fathom.


Here's some code I will use to illustrate the behaviour:

# start with some data frame "a" having some named columns
a <- data.frame(a=rep(1,3),c=rep(2,3),d=rep(3,3),e=rep(4,3))

# create a subset of the original data frame, but include a
# name "b" that is not present in my original data frame
b <- a[,c("a","b","c")]


## Up until now no errors are issued, but the following commands
## will give the error shown:

b[1,]     ## "Error in x[[j]] : subscript out of bounds"
b[1,2]    ## "Error in "names<-.default"(*tmp*, value = cols)
:
          ##  names attribute must be the same length as the vector"


Can anyone explain to me the meaning of these error messages in terms
of R is actually doing?  These error messages had me baffled and 
it took me hours to track down that the source of the error was an 
incorrect column name in my data frame subsetting.

Thanks,
Doug Grove
Statistical Research Associate
Fred Hutchinson Cancer Research Center
Seattle, WA

Douglas Grove <dgrove at fhcrc.org> writes:
> Hi,
> 
> I'm trying to understand a behaviour that I have encountered
> and can't fathom.
> 
> 
> Here's some code I will use to illustrate the behaviour:
> 
> # start with some data frame "a" having some named columns
> a <- data.frame(a=rep(1,3),c=rep(2,3),d=rep(3,3),e=rep(4,3))
> 
> # create a subset of the original data frame, but include a
> # name "b" that is not present in my original data frame
> b <- a[,c("a","b","c")]
> 
> 
> ## Up until now no errors are issued, but the following commands
> ## will give the error shown:
> 
> b[1,]     ## "Error in x[[j]] : subscript out of bounds"
> b[1,2]    ## "Error in "names<-.default"(*tmp*, value =
cols) :
>           ##  names attribute must be the same length as the vector"
> 
> 
> Can anyone explain to me the meaning of these error messages in terms
> of R is actually doing?  These error messages had me baffled and 
> it took me hours to track down that the source of the error was an 
> incorrect column name in my data frame subsetting.
Looks like a (semi-)bug. Indexing outside of the data frame creates a
"column" which is really the single value NULL, e.g. 
> dput(a[,4:5])
structure(list(e = c(4, 4, 4), "NA" = NULL), .Names = c("e",
NA), row.names = c("1", "2", "3"), class =
"data.frame")

This will print because the format.data.frame called inside
print.data.frame will recycle the NULL and give you
> a[,4:5]
  e   NA
1 4 NULL
2 4 NULL
3 4 NULL

However, it confuses the h*ck out of "[.data.frame"
> (a[,4:5])[2]
Error in "[.data.frame"((a[, 4:5]), 2) : undefined columns
selected
> (a[,4:5])[,2]
NULL
> (a[,4:5])[,1]
[1] 4 4 4

and also the examples you found. However, the main issue is that you
have managed to construct a corrupt data frame. So indexing outside
the array should probably either give an error or return a column of
NA.

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907