[datatable-help] Help using a quoted eval statement

[Matthew Dowle]

Hi,
Welcome to the list.
Similar requests have been raised recently and there is now a .SDcols
argument in v1.6.3 on R-Forge. See NEWS and ?data.table in 1.6.3.
If that doesn't solve it, could you include a way to generate dt please,
include the actual error messages you're getting, and what the expected
output should be.
Thanks, Matthew


On Sun, 2011-07-24 at 18:10 +0100, Griffith Rees wrote:
> Hi,
> Sorry if there is a straightforward answer, been trying various
> solutions and I can't quite get what I want. What I need to do is find
> a weighted mean of a grouped set for most of the variables in a
> data.table. The sort of thing I wish I could write is:
> 
> dt[,sum(.SD[9:20]*Population/sum(Population)), by=ExchangeArea]
> 
> where [9:28] is the subset of the variables of data.table dt that I
> want a weighted sum of. Ideally, the resultant data.table would label
> the columns the same as those variables. Obviously this doesn't work,
> so I've tried to find a way to iterate over a list of variables.
> 
> Here's what I hoped would work:
> 
> dt.final <- dt #copy original data.table
> l <- names(dt)
> l <- l[9:28] #Get the right names
> wtavg <- function(variable) {dt.final <-
> dt.final[dt.final[,variable=sum(variable*Population/sum(Population)),by=ExchangeArea]]}
> lappy(l,wtavg)
> 
> That errored, so I tried using the eval(quote()) idiom from the documentation:
> wtavg.expression <-
> function(variable){paste(variable,"=sum(",variable,"*Population/sum(Population))",
> sep="")}
> 
> However, trying to just get one column out gives me this error:
> wtavg <- function(variable)
> {dt[,eval(quote(wtavg.expression(variable))),by=ExchangeArea]}
> wtavg('Technicians')
> 
>       ExchangeArea                                                      V1
>  [1,]        23826 Technicians=sum(Technicians*Population/sum(Population))
>  [2,]        23827 Technicians=sum(Technicians*Population/sum(Population))
>  [3,]        23828 Technicians=sum(Technicians*Population/sum(Population))
>  [4,]        23829 Technicians=sum(Technicians*Population/sum(Population))
>  [5,]        23830 Technicians=sum(Technicians*Population/sum(Population))
>  [6,]        23831 Technicians=sum(Technicians*Population/sum(Population))
>  [7,]        23832 Technicians=sum(Technicians*Population/sum(Population))
>  [8,]        23833 Technicians=sum(Technicians*Population/sum(Population))
>  [9,]        23834 Technicians=sum(Technicians*Population/sum(Population))
> [10,]        23835 Technicians=sum(Technicians*Population/sum(Population))
> First 10 rows of 16051 printed.
> 
> Now it appears that shouldn't work because I'm passing a string to
> quote() rather than an expression. However, I haven't been able to
> find a function that will do what I want. I've tried various
> combinations of:
> 
> expresssion()
> substitute()
> parse()
> eval()
> quote()
> 
> and most of them give me errors. I hope it's clear what I'm trying to
> do. I do realise that I could write a giant expression by hand, but
> part of me thinks there should be a better way. Any suggestions would
> be helpful, thanks very much.
> 
> --
> Griffith Rees
> Sociology DPhil Candidate
> Oxford University
> CABDyN Complexity Centre
> http://www.cabdyn.ox.ac.uk
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