[datatable-help] Expressions in "by" criteria (again)

[Harish]

Mike,

Matthew is right.  Here is a function that might help you transition from your current state to where you need to get to quickly.

I started creating a function for other purposes that might be useful to you.  I described the usage below.

========================

# Concatenate all given arguments into a quote of a list()
# Arguments can be any of:
#    1) an expression that returns a valid value when evaluated in calling
#       environment.
#    2) a character vector which will be treated as text inside list(...)
#    3) a quote of a list
#    4) a list() directly given in the argument
# Returns a quote of a list
c.listquote <- function( ... ) {
   
   args <- as.list( match.call()[ -1 ] )
   lstquote <- list( as.symbol( "list" ) );
   for ( i in args ) {
      # Evaluate expression in parent eviron to see what it refers to
      if ( class( i ) == "name" || ( class( i ) == "call" && i[[1]] != "list" ) ) {
         i <- eval( substitute( i ), sys.frame( sys.parent() ) )
      }
      if ( class( i ) == "call" && i[[1]] == "list" ) {
         lstquote <- c( lstquote, as.list( i )[ -1 ] )
      }
      else if ( class( i ) == "character" )
      {
         for ( chr in i ) {
            lstquote <- c( lstquote, list( parse( text=chr )[[1]] ) )
         }
      }
      else
         stop( paste( "[", deparse( substitute( i ) ), "] Unknown class [", class( i ), "] or is not a list()", sep="" ) )
   }
   return( as.call( lstquote ) )
}

========================

IMPORTANT: If you find any bugs in this or find ways to improve it, please let me know.

The usage is as follows:

my.fields <- c("Apples","Bananas","Coconuts","Dragonfruits","Pomelos")
q <- c.listquote( my.fields )
DT[ , Col1, by=eval( q ) ]
DT[ , q ]

The advantage of the function is that you can also easily add fields through a variety of ways...

foo <- function() {
   return( quote( list( Orange ) ) )
}
DT[ , eval( c.listquote( q, foo(), list( Pear ), "Peach", c( "New1", "New2=form" ) ) ) ]


Hope this helps.


Regards,
Harish


--- On Fri, 7/2/10, mdowle at mdowle.plus.com <mdowle at mdowle.plus.com> wrote:

> From: mdowle at mdowle.plus.com <mdowle at mdowle.plus.com>
> Subject: Re: [datatable-help] Expressions in "by" criteria (again)
> To: "Mike Sandfort" <cute_moniker at yahoo.com>
> Cc: datatable-help at lists.r-forge.r-project.org
> Date: Friday, July 2, 2010, 9:49 AM
> Quick answer is it needs to be this
> way :
> 
>    my.fields =
> quote(list(Apples,Bananas,...))
>    DT[,sum(NumericField),by=eval(my.fields)]
> 
> Also some bugs were just fixed in this area so you may need
> latest 1.5
> from r-forge for this.
> 
> Having said that its sometimes easier coding to use a flat
> format (i.e.
> have a single column 'fruit') then "[,...,by=fruit]". There
> was another
> thread showing examples of long to wide taking care of NAs
> etc, search for
> 'wide'.
> 
> HTH, thanks for the interest,
> 
> Matthew
> 
> 
> > Hi,
> >
> > I suspect my question is similar to Harish's "Question
> #2" from 6/18.
> > Suppose
> > I have a data.table with many fields and have a large
> subset of fields I
> > need to include
> > in several expressions. Ordinarily, I would create
> (once) a vector of
> > names of the fields
> > in my subset:
> > my.fields <-
> c("Apples","Bananas","Coconuts","Dragonfruits",...,"Pomelos")
> >     [where the whole data frame has many more
> fields, including
> > "Broccoli","Cabbages",...]
> >
> > Then I can re-use the my.fields vector when
> extracting subsets, creating
> > plots, aggregating with
> > ddply(), etc. The problem is that I can't figure out
> how to
> > (re)use my.fields to aggregate a
> > data.table.
> >
> >
> DT[,sum(NumericField),by=(Apples,Bananas,Coconuts,Dragonfruits,...,Pomelos)]
> > will work.
> > However,
> > DT[,sum(NumericField),by=my.fields]
> > won't work, nor will any combination of paste(),
> list(), eval(), quote(),
> > deparse(), etc. applied
> > to my.fields (at least I haven't found one yet).
> >
> > I know this is probably more an R-language issue, but
> since it's come up
> > in my work with
> > the (excellent) data.table package, I thought I would
> ask here.
> >
> > Thanks!
> > Mike S.
> >
> >
> >
> > _______________________________________________
> > datatable-help mailing list
> > datatable-help at lists.r-forge.r-project.org
> > https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
> >
> 
> 
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