[R] why does data frame subset return vector

[Arne.Muller@aventis.com]

Hello,

I've a weired problem with a data frame. Basically it should be just one
column with
specific names coming from a data file (the file contains 2 rows, one should
be
the for the rownames of the data frame the other contains numeric values).

> df.rr <- read.table("RR_anova.txt", header=T, comment.char="", row.names=1)
> df.rr[c(1,2,3),]
[1] 1.11e-16 1.11e-16 1.11e-16

Why are the rownames not displayed?

The data file itself look slike this:
> df.rr <- read.table("RR_anova.txt", header=T, comment.char="")
> df.rr[c(1,2,3),]
            QUAL   PVALUE
1    AJ224120_at 1.11e-16
2 rc_AA893000_at 1.11e-16
3 rc_AA946368_at 1.11e-16

and assigning the rownames explicitely works as I'd expect:   
> rownames(df.rr) <- df.rr$'QUAL'
> df.rr[c(1,2,3),]
                         QUAL   PVALUE
AJ224120_at       AJ224120_at 1.11e-16
rc_AA893000_at rc_AA893000_at 1.11e-16
rc_AA946368_at rc_AA946368_at 1.11e-16

Ok, now they are displayed, but it's a duplication to keep the "QUAL" colum.

below I create the a new data frame to skip the "QUAL" column, since it is
already
a rowname.
> df.rr2 <- data.frame(PVALUE=df.rr, row.names=1)
> df.rr2[1:4,]
[1] 1.11e-16 1.11e-16 1.11e-16 1.11e-16

However, the rowname is still there ..., you just cannot see it:
> df.rr2["AJ224120_at",]
[1] 1.11e-16

The code below shows that "sub-setting" the df.rr data frame in deed creates
a
vector rather than a data frame whereas sub-setting the 2 column data frame
returns
a new data frame (as I'd expect).
 
> df.rr[1:4,]
[1] 1.11e-16 1.11e-16 1.11e-16 1.11e-16
> is.vector(df.rr[1:4,])
[1] TRUE
> is.data.frame(df.rr[1:4,])
[1] FALSE
> df.rr <- read.table("CLO_RR_anova.txt", header=T, comment.char="")
> is.data.frame(df.rr[1:4,])
[1] TRUE

Any explanation is appreciated. There must be a good reason for this I guess
... . On
the other hand is there a way to fore the subset of the 1 colum data frame to
be
dataframe itself? I'd just like to see the rownames displayed, that's it ...

	thanks alot for your help,

	Arne