[R] Matrix multiplication - code problem

[Uwe Ligges]

MarcioRibeiro wrote:
> Hi listers,
> I am having some trouble in a matrix multiplication...
> I have already checked some posts, but I didn't find my problem...
> I have the following code...
> But I am not getting the right multiplication...
> I checked the dimension and they are fine...
> 
> id_y <- array(1:10,dim=c(2,1,5))
> id_yt<-aperm(id_y,c(2,1,3))
> m_id<-array(dim=c(dim(id_y)[1],dim(id_y)[1],dim(id_y)[3]))
> for (i in 1:dim(id_y)[3]){
> m_id[,,i]<-id_y[,,i]%*%id_yt[,,i]
> }
> m_id
> 
> 


Due to indexing, you are loosing dimensions.
The problem is that you expect to *loose exactly one* dimension in this 
case (hence drop = FALSE won't be of help right away). Hence it might be 
a good idea to know what you are going to have exactly at the end (as 
in- and output).

Uwe Ligges