[datatable-help] Unable to have expression for "by" criteria

[Harish]

Thanks.  The eval() did the trick in the simplified example.

Now I run into another hurdle when I make the code a little more complex.

# ==================

DT <- data.table( a=1:5, b=11:50, d=c("A","B","C","D") )
f <- quote( list(d) )
DT[ , mean(b), by=eval(f) ]   # Now this works; thanks
foo <- function( grp ) {
   DT[ , mean(b), by=eval( grp ) ]
}
foo( list(d) ) # Gives an error
foo( quote( list(d) ) )  # Also gives the same error

# ==================

The error I get is:
   Error in eval(grp) : object 'grp' not found


Conceptually, it looks like it should work.


Regards,
Harish


--- On Fri, 6/18/10, 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] Unable to have expression for "by" criteria
> To: "Harish" <harishv_99 at yahoo.com>
> Cc: datatable-help at lists.r-forge.r-project.org
> Date: Friday, June 18, 2010, 4:50 AM
> Try this (works for me) :
> 
> f <- quote( list(d) )
> DT[ , mean(b), by=eval(f) ]
> 
> If that works, you were very close, just needed to use eval
> in the by.  I
> _think_ this makes sense as syntax, something needs to
> signal to the
> reader of the query that f is not a column name but a
> pre-defined
> expression, for clarity.
> 
> I basically need to have another look at this and tidy up
> the
> documentation and examples. Might add a FAQ on it. There
> were big changes
> in this area internally when grouping was sped up e.g. its
> very recent
> that by can be list(), by used to be just a character
> string.
> 
> You can do the same thing for j btw. Kind of like a macro.
> There might
> already be a FAQ on that.
> 
> Its quite neat actually what R allows ... I don't believe
> in SQL you can
> as easily create expressions for criteria (select, group by
> and where) and
> re-use them like this.
> 
> I'll need an example for #2 as I don't quite follow
> that.  Maybe it drops
> out of answer above?
> 
> Matthew
> 
> 
> > I am trying to compute some values in a data.table by
> dynamically
> > generating the "by" criteria.  However, I am
> unable to figure out how to
> > do it.  (I had to resort to using the "plyr"
> package.)
> >
> > Questions:
> > 1) Why am I unable to pass a variable for the "by"
> criteria?  The comments
> > in the code indicate that it should be possible.
> > 2) Assuming the issue is a bug (and will be fixed),
> what is a "good" way
> > for me to accomplish dynamically creating a
> criteria?  (This is more of a
> > generic R question I suppose.)
> >
> > -----
> >
> > Question #1 -- Unable to pass variable for "by"
> criteria
> >
> > The comments in the code state: "The by expression
> also see variables in
> > the calling frame, just like j... but from v1.3 is
> e.g. bycriteria =
> > quote(list(colA,colB%%100)); DT[...,by=bycriteria]"
> >
> > Then the following code should work, but it does not.
> >
> > DT <- data.table( a=1:5, b=11:50,
> d=c("A","B","C","D"))
> > DT[ , mean(b), by=d ]     
>    # Works
> > DT[ , mean(b), by=list(d) ]   # Works
> > f <- quote( list(d) )
> > DT[ , mean(b), by=f ]     
>    # This does not work
> >
> > The response is:
> > Error in `[.data.table`(DT, , mean(b), by = f) :
> >   column 1 of 'by' list does not
> evaluate to integer e.g. the by should be
> > a list of expressions. Do not quote column names when
> using
> > by=list(...).
> >
> > What is going on?
> >
> > -----
> >
> > Question #2 -- Tips for dynamically generating
> criteria
> >
> > What are some tips and generally accepted approaches
> (in R) to dynamically
> > generate a criteria?
> >
> > The first step is to generate a list of columns to
> group by.  How should I
> > structure the function that gathers this info? 
> Is it better to get them
> > straight as strings, or should I get the variables
> directly and then use
> > either match.call() or deparse(substitute(x)) to
> convert to strings?  The
> > aes() function in ggplot2 uses the match.call()
> approach.  Or is the
> > conversion to strings even required?  (The plyr
> package accepted a vector
> > of strings, and a few other formats for its "by"
> criteria.)
> >
> > The next step is to generate the "language" or
> "symbol" object that I need
> > to create.  I would appreciate some guidance on
> how I can put together my
> > required columns into the by criteria dynamically.
> >
> > (I understand that it is a generic R question, but
> since it is so closely
> > related to my question #1, I am asking both of them
> here.)
> >
> > Thanks for your help.
> >
> >
> > Regards,
> > Harish
> >
> >
> >
> >
> > _______________________________________________
> > 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
> >
> 
> 
>