[R] dataframes with only one variable

[Prof Brian Ripley]

On Wed, 11 Jan 2006, Erich Neuwirth wrote:

> Subsetting from a dataframe with only one variable
> returns a vector, not a dataframe.
> This seems somewhat inconsistent.

Not at all.  It is entirely consistent with matrix-like indexing (the form 
you used).

> Wouldn't it be better if subsetting would respect
> the structure completely?

It depends how you do it.  [sel1,]  parallels a matrix, and drops 
dimensions unless drop == FALSE is supplied.  [sel1] returns a 
one-column df, and [[sel1]] returns a vector.

It is just a question of choosing the appropriate tool.  And any changes 
to this sort of thing (from the White book) would break a lot of careful 
code.

>
>
> v1<-1:4
> v2<-4:1
> df1<-data.frame(v1)
> df2<-data.frame(v1,v2)
> sel1<-c(TRUE,TRUE,TRUE,TRUE)
>
>> df1[sel1,]
> [1] 1 2 3 4
>> df2[sel1,]
>  v1 v2
> 1  1  4
> 2  2  3
> 3  3  2
> 4  4  1
>
> -- 
> Erich Neuwirth
> Institute for Scientific Computing and
> Didactic Center for Computer Science
> University of Vienna
> phone: +43-1-4277-39464  fax: +43-1-4277-39459

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595