R help - Jul 2011 - apply (or similar preferred) for multiple columns

Dear all, 

I would like to use the apply or a similar function belonging to this
family, but applying for each column (or row) but let say for each q
columns. For example I would like to apply a function FUN for the first q
columns of matrix X then for q+1:2*q and so on. If I do apply (X, 2, FUN) it
applies for each column and not for every q columns. Is that possible with
any similar function?

Thank you

Dimitris


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Probably not the most elegant, but a workable solution. Assume you have a
matrix x of dimensions 10 x 10. Assume further you want to calculate the
mean for each successive block of two columns. One way to do this is to
create a matrix that indicates the column numbers from/to which to apply the
function. Example:


#Random matrix
x<-rnorm(100)
dim(x)<-c(10,10)

#Create index for the first and last column in each block (your n would be
50)
n<-2

mim<-seq(from=1,to=dim(x)[2],by=n)
mox<-seq(from=2,to=dim(x)[2],by=n)

#Bind these values in matrix
mat<-cbind(mim,mox)

#Now run apply over the rows of mat, not over the data matrix
apply(mat,1,function(y){mean(x[,c(y[1]:y[2])])})



So you would replace mean(...) with whatever function you want to apply. The
argument to the function must be x[,c(y[1]:y[2])], where x is your data
matrix.

HTH,
Daniel



Dimitris.Kapetanakis wrote:
> 
> Dear all, 
> 
> I would like to use the apply or a similar function belonging to this
> family, but applying for each column (or row) but let say for each q
> columns. For example I would like to apply a function FUN for the first q
> columns of matrix X then for q+1:2*q and so on. If I do apply (X, 2, FUN)
> it applies for each column and not for every q columns. Is that possible
> with any similar function?
> 
> Thank you
> 
> Dimitris
> 
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Sent from the R help mailing list archive at Nabble.com.

On Tue, Jul 12, 2011 at 5:27 AM, Dimitris.Kapetanakis
<dimitrios.kapetanakis at gmail.com> wrote:
> Dear all,
>
> I would like to use the apply or a similar function belonging to this
> family, but applying for each column (or row) but let say for each q
> columns. For example I would like to apply a function FUN for the first q
> columns of matrix X then for q+1:2*q and so on. If I do apply (X, 2, FUN)
it
> applies for each column and not for every q columns. Is that possible with
> any similar function?
>
> Thank you
>
> Dimitris
Here is an example of using rollapply to sum all elements in the first
4 columns, then the next 4 columns, etc.   anscombe is a data set that
comes with R.  Be sure you are using zoo 1.7-0 which was just released
today.
> library(zoo)
> packageVersion("zoo")
[1] '1.7.0'
> rollapply(t(anscombe), 4, by = 4, sum, by.column = FALSE)
[1] 396.00 330.03
> c(sum(anscombe[, 1:4]), sum(anscombe[, 5:8]))
[1] 396.00 330.03


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