[R] data frame manipulation with zero rows

[Hadley Wickham]

Hi Arnaud,

I've added this case to the set of test cases in plyr and it will be
fixed in the next version.

Hadley

On Tue, Jun 1, 2010 at 2:33 PM, arnaud Gaboury <arnaud.gaboury at gmail.com> wrote:
> Maybe not the cleanest way, but I create a fake data frame with one row so
> ddply() is happy!!
>> if (nrow(futures)==0) futures<-data.frame(.......)
>
>
>
>
>
>> -----Original Message-----
>> From: Peter Ehlers [mailto:ehlers at ucalgary.ca]
>> Sent: Tuesday, June 01, 2010 12:07 PM
>> To: arnaud Gaboury
>> Cc: 'Prof Brian Ripley'; r-help at r-project.org
>> Subject: Re: [R] data frame manipulation with zero rows
>>
>> On 2010-06-01 1:53, arnaud Gaboury wrote:
>> > Brian,
>> >
>> > If I do understand correctly, I must use in my function something
>> else than
>> > ddply() if I want to avoid any error each time my df has zero rows?
>> > Am I correct?
>> >
>>
>> You could define a function to handle the zero-rows case:
>>
>> f <- function(x){
>>   if(nrow(x) < 1) out <- x[, c(1,3,2)]  # or whatever
>>   else
>>     out <- ddply(x, c("DESCRIPTION","SETTLEMENT"), summarise,
>>                      POSITION=sum(QUANTITY))[,c(1,3,2)]
>>   out
>> }
>> f(futures)
>>
>>   -Peter Ehlers
>>
>> >
>> >
>> >> -----Original Message-----
>> >> From: Prof Brian Ripley [mailto:ripley at stats.ox.ac.uk]
>> >> Sent: Tuesday, June 01, 2010 9:47 AM
>> >> To: arnaud Gaboury
>> >> Subject: Re: [R] data frame manipulation with zero rows
>> >>
>> >> On Tue, 1 Jun 2010, arnaud Gaboury wrote:
>> >>
>> >>> Dear group,
>> >>>
>> >>> Here is the kind of data.frame I obtain every day with my function
>> :
>> >>>
>> >>> futures<-
>> >>> structure(list(DESCRIPTION = c("CORN Jul/10", "CORN Jul/10",
>> >>> "CORN Jul/10", "CORN Jul/10", "CORN Jul/10", "LIVE CATTLE Aug/10",
>> >>> "LIVE CATTLE Aug/10", "SUGAR NO.11 Jul/10", "SUGAR NO.11 Jul/10",
>> >>> "SUGAR NO.11 Jul/10", "SUGAR NO.11 Jul/10", "SUGAR NO.11 Jul/10"
>> >>> ), CREATED.DATE = structure(c(18403, 18406, 18406, 18406, 18406,
>> >>> 18407, 18408, 18406, 18407, 18407, 18407, 18407), class = "Date"),
>> >>>     QUANTITY = c(1, 1, 1, 1, 1, 3, 1, 1, 1, 1, 1, 1), SETTLEMENT =
>> >>> c("373.2500",
>> >>>     "373.2500", "373.2500", "373.2500", "373.2500", "90.7750",
>> >>>     "90.7750", "14.9200", "14.9200", "14.9200", "14.9200",
>> "14.9200"
>> >>>     )), .Names = c("DESCRIPTION", "CREATED.DATE", "QUANTITY",
>> >>> "SETTLEMENT"), row.names = c(NA, 12L), class = "data.frame")
>> >>>
>> >>> I need then to apply to the df this following code line :
>> >>>
>> >>>> PosFut=ddply(futures, c("DESCRIPTION","SETTLEMENT"), summarise,
>> >> POSITION=
>> >>> sum(QUANTITY))[,c(1,3,2)]
>> >>>
>> >>> It works perfectly in most of case, BUT I have a new problem: it
>> can
>> >>> sometime occurs that my df "futures" is empty, with zero rows.
>> >>>
>> >>>
>> >>> futures<-
>> >>> structure(list(DESCRIPTION = character(0), CREATED.DATE =
>> >>> structure(numeric(0), class = "Date"),
>> >>>     QUANTITY = numeric(0), SETTLEMENT = character(0)), .Names =
>> >>> c("DESCRIPTION",
>> >>> "CREATED.DATE", "QUANTITY", "SETTLEMENT"), row.names = integer(0),
>> >> class =
>> >>> "data.frame")
>> >>>
>> >>> It is not the usual case, but it can happen. With this df, when I
>> >> pass the
>> >>> above mentione line, I get an error :
>> >>>
>> >>>> PosFut=ddply(futures, c("DESCRIPTION","SETTLEMENT"), summarise,
>> >> POSITION=
>> >>> sum(QUANTITY))[,c(1,3,2)]
>> >>> Error in tapply(1:nrow(data), splitv, list) :
>> >>>   arguments must have same length
>> >>>
>> >>>
>> >>> How can I avoid this when my df is empty?
>> >>
>> >> Ask the author of the (missing) function ddply() to correct the
>> error
>> >> of using 1:nrow(data) by replacing it by seq_len(nrow(data)).
>> >>
>> >> It's helpful to give example code, but much more helpful if you test
>> >> it: yours cannot work without the function ddply() -- this is what
>> >> 'self-contained' means in the footer here.
>> >>
>> >>
>> >>>
>> >>> Any help is appreciated
>> >>>
>> >>> ______________________________________________
>> >>> R-help at r-project.org mailing list
>> >>> https://stat.ethz.ch/mailman/listinfo/r-help
>> >>> PLEASE do read the posting guide http://www.R-project.org/posting-
>> >> guide.html
>> >>> and provide commented, minimal, self-contained, reproducible code.
>> >>
>> >>
>> >> --
>> >> 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
>> >
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>



-- 
Assistant Professor / Dobelman Family Junior Chair
Department of Statistics / Rice University
http://had.co.nz/