[R] Updating selected variables in a data frame

[Gabor Grothendieck]

Try this. It uses the fact that 2 column matrices can be used
as subscripts (as described in the Matrices and arrays section
of ?Extract):

with(recodes, replace(as.matrix(main), cbind(id, qst + 1), value))

It returns a matrix so use as.data.frame on that to turn it
back to a data frame, if need be.

On Mon, Mar 17, 2008 at 9:10 PM, Andrew C. Ward
<s195404 at student.uq.edu.au> wrote:
> Dear list,
>
> I'd like to update certain rows/columns in a data
> frame with new values. The application is that
> survey respondents may give an answer of "Other"
> to a categorical question and then provide some
> text describing what they mean. This text is then
> reviewed and placed into a category. These edits
> or recodes then need to be merged back into the
> main dataset.
>
> A small example follows.
>
> # Generate a main data set (10 people, 2 questions)
> main <- data.frame(id=1:10, q1=sample(1:5, size=10,
>                    replace=TRUE), q2=sample(1:5,
>                    size=10, replace=TRUE))
> # Generate a corresponding recodes dataset
> recodes <- reshape(main, idvar="id", direction="long",
>                    varying=list(2:3), v.names="value",
>                    timevar="qst")
> recodes <- recodes[sample(seq(nrow(recodes)), 10,
>                    replace=FALSE),]
> recodes$value <- recodes$value + sign(runif(1)-0.5)
>
> I could easily use a for() loop to update the main
> dataset. The recodes dataset can get quite large,
> however, so I would rather avoid this.
>
> The question:
> Is there some efficient way, using apply() perhaps,
> that I can update the main dataset with the new
> values from the recodes dataset?
>
> Thanks very much for your advice!
>
> Regards,
>
> Andrew C. Ward
>
> CAPE Centre
> Department of Chemical Engineering
> The University of Queensland
> Brisbane Qld 4072 Australia
>
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