[R] data frame manipulation with zero rows

[arnaud Gaboury]

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
> >