[R] data frame manipulation with zero rows

[arnaud Gaboury]

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?

TY




> -----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