[R] Getting subsets of a data frame

[Prof Brian Ripley]

On Sat, 16 Apr 2005, Prof Brian Ripley wrote:

> Perhaps Fernando will also note that is documented in ?"[.data.frame",
> a slightly more appropriate reference than Bill's.
>
> It would be a good idea to read a good account of R's indexing: Bill Venables 
> and I know of a couple you will find in the R FAQ.

BTW,

sw <- swiss
sw[1,,drop=TRUE] *is* a list (not as claimed, but as documented)
sw[1, ]          is a data frame
sw[, 1]          is a numeric vector.

I should have pointed out that "[.data.frame" is in the See Also of Bill's 
reference.

BTW to Andy: a list is a vector, and Kurt and I recently have been trying 
to correct documentation that means `atomic vector' when it says `vector'.
(Long ago lists in R were pairlists and not vectors.)

> is.vector(list(a=1))
[1] TRUE


> On Sat, 16 Apr 2005, Liaw, Andy wrote:
>
>> Because a data frame can hold different data types (even matrices) in
>> different variables, one row of it can not be converted to a vector in
>> general (where all elements need to be of the same type).
>> 
>> Andy
>> 
>>> From: Fernando Saldanha
>>> 
>>> Thanks, it's interesting reading.
>>> 
>>> I also noticed that
>>> 
>>> sw[, 1, drop = TRUE] is a vector (coerces to the lowest dimension)
>>> 
>>> but
>>> 
>>> sw[1, , drop = TRUE] is a one-row data frame (does not convert it into
>>> a list or vector)
>>> 
>>> FS
>>> 
>>> 
>>> On 4/16/05, Bill.Venables at csiro.au <Bill.Venables at csiro.au> wrote:
>>>> You should look at
>>>> 
>>>>> ?"["
>>>> 
>>>> and look very carefully at the "drop" argument.  For your example
>>>> 
>>>>> sw[, 1]
>>>> 
>>>> is the first component of the data frame, but
>>>> 
>>>>> sw[, 1, drop = FALSE]
>>>> 
>>>> is a data frame consisting of just the first component, as
>>>> mathematically fastidious people would expect.
>>>> 
>>>> This is a convention, and like most arbitrary conventions
>>> it can be very
>>>> useful most of the time, but some of the time it can be a very nasty
>>>> trap.  Caveat emptor.
>>>> 
>>>> Bill Venables.
>>>> 
>>>> -----Original Message-----
>>>> From: r-help-bounces at stat.math.ethz.ch
>>>> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of
>>> Fernando Saldanha
>>>> Sent: Saturday, 16 April 2005 1:07 PM
>>>> To: Submissions to R help
>>>> Subject: [R] Getting subsets of a data frame
>>>> 
>>>> I was reading in the Reference Manual about Extract.data.frame.
>>>> 
>>>> There is a list of examples of expressions using [ and [[, with the
>>>> outcomes. I was puzzled by the fact that, if sw is a data
>>> frame, then
>>>> 
>>>> sw[, 1:3]
>>>> 
>>>> is also a data frame,
>>>> 
>>>> but
>>>> 
>>>> sw[, 1]
>>>> 
>>>> is just a vector.
>>>> 
>>>> Since R has no scalars, it must be the case that 1 and 1:1
>>> are the same:
>>>> 
>>>>> 1 == 1:1
>>>> [1] TRUE
>>>> 
>>>> Then why isn't sw[,1] = sw[, 1:1] a data frame?
>>>> 
>>>> FS
>>>> 
>>>> ______________________________________________
>>>> R-help at stat.math.ethz.ch mailing list
>>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>>> PLEASE do read the posting guide!
>>>> http://www.R-project.org/posting-guide.html
>>>> 
>>> 
>>> ______________________________________________
>>> R-help at stat.math.ethz.ch mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide!
>>> http://www.R-project.org/posting-guide.html
>>> 
>>> 
>>> 
>> 
>> ______________________________________________
>> R-help at stat.math.ethz.ch mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide! 
>> http://www.R-project.org/posting-guide.html
>> 
>
> -- 
> 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
>

-- 
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