[R] Matrix multiplication - code problem

[Patrick Burns]

You need to put in calls to 'as.matrix'.  It's
a bit tricky though -- what you want depends
on whether it is the first or second subscript
that has length 1.

as.matrix(id_y[,,i])

if the second dimension has length 1.

t(as.matrix(id_y[,,i]))

if the first dimension has length 1.


Patrick Burns
patrick at burns-stat.com
+44 (0)20 8525 0696
http://www.burns-stat.com
(home of "The R Inferno" and "A Guide for the Unwilling S User")

MarcioRibeiro wrote:
> Hi again,
> I understood what you guys explained...
> But, there isn't a way to do a multiplication of matrix with a FOR command
> or otherelse where one o my dimension is ONE...
> Well, as my data file is small, I did the procedure at the excel... But,
> this is not the good procedure...
> Thanks,
> Marcio
>
>
> Uwe Ligges-3 wrote:
>   
>>
>> 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
>>
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>>
>>
>>     
>
>