[R] apply (or similar preferred) for multiple columns

[Daniel Malter]

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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