# [R] Matrix multiplication

Steven Yen syen04 at gmail.com
Thu Jun 8 06:16:48 CEST 2017

```OK Thanks. Your response made me think. Here (the last line) is what I need:

set.seed(76543211)
w<-1:10; w
a<-matrix(rpois(20,2),nrow=10); a
t(w*a)%*%a

On 6/8/2017 12:09 PM, Jeff Newmiller wrote:
> Is this a question? You seem to have three possible calculations, have already implemented two of them (?) and it is unclear (to me) what you think the right answer for any of them is supposed to be.
> -- Sent from my phone. Please excuse my brevity. On June 7, 2017
> 8:50:55 PM PDT, Steven Yen <syen04 at gmail.com> wrote:
>> I need to have all elements of a matrix multiplied by a weight before
>> being post-multiplied by itself, as shown in the forst block of codes
>> below. I can also multiply the matrix by the square root of the weight
>> and then take the outer product.
>>
>> Actually, what I need is this. Denote each row of the matrix by a row
>> vector as xi and each element of the weighting vector as wi. Then, I
>> need the sum of wi * t(xi) %*% xi over i.
>>
>> Any suggestion for a compact approach would be appreciated.
>>
>> set.seed(76543211)
>> w<-1:10; w
>> a<-matrix(rpois(20,2),nrow=10); a
>> b<-a
>> a<-w*a
>> t(a)%*%b
>>
>> set.seed(76543211)
>> a<-matrix(rpois(20,2),nrow=10); a
>> a<-sqrt(w)*a; a
>> t(a)%*%a

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