Re: identifying which column/row is passed in calls to apply()

["ONKELINX, Thierry" <Thierry.ONKELINX@inbo.be>]

I would recommend that you add a column to the dataset which indicates
what function needs to be evaluated in the apply loop.

innerFun <- function(row) {
        if (row[1] == 1){
                result <- mean(row[-1])
        } else {
                result <- median(row[-1])
        }
        return(result)
}
myDf <- cbind(action = rep(c(0, 1), dim(myDf)[2]/2), myDf)
apply(myDf, 1, innerFun)


Cheers,

Thierry
--------------------------------------------------------
ir. Thierry Onkelinx
Research Institute for Nature and Forest
Team Biometrics, methodology and quality assurance
Gaverstraat 4
B-9500 Geraardsbergen
Belgium
tel. + 32 54/436 185
Thierry.Onkelinx@inbo.be

-----Original Message-----
From: s-news-owner@lists.biostat.wustl.edu
[mailto:s-news-owner@lists.biostat.wustl.edu] On Behalf Of
Rich@Mango-Solutions.com
Sent: Saturday, February 11, 2006 1:00 PM
To: 'Schwarz,Paul'; s-news@lists.biostat.wustl.edu
Subject: Re: [S] identifying which column/row is passed in calls to
apply()

You could use sapply to pass in a vector of numbers (for column numbers)
or
characters (for column names) and index the columns of the input data.
Something like this:-

> innerFun <- function(i, df) {
        if (i == 1) mean(df[[i]])
        else median(df[[i]])
}
> sapply(1:length(myDf), innerFun, df=myDf)

Not as efficient as apply for larger data structures I'd imagine, since
you're passing the entire dataset in each time ...

Rich.

-----Original Message-----
From: s-news-owner@lists.biostat.wustl.edu
[mailto:s-news-owner@lists.biostat.wustl.edu] On Behalf Of Schwarz,Paul
Sent: 11 February 2006 02:19
To: s-news@lists.biostat.wustl.edu
Subject: [S] identifying which column/row is passed in calls to apply()

S-News readers,

How would I identify which column (or row) is being operated on in a
call to the function specified in apply()? That is, if I wanted to
perform some column-specific (or row-specific) operation, how do I
identify which column/row is being passed to the function that's
specified in the call to apply(), where the number of columns ranges
from 1 to ncol(mymat)?

apply( mymat, 2, function(x){ ? } )


I suspect that this is easy, but I'm drawing a blank on how to do this.

Thanks a lot everyone.

-Paul
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