# Re: [R] multiplication question

From: N. Lapidus <n.lapidus_at_gmail.com>
Date: Wed, 02 Jul 2008 23:02:41 +0200

Hi Murali,

Just an idea, probably not the best :

```x<-1:4
y<-1:6
z<-matrix(1:(length(x)*length(y)),nrow=length(x))

```

I <- matrix(1,nrow=length(x),ncol=length(y)) I[row(I)==col(I)] <- 0

sum (outer (x, y, '*') * I)
sum (outer (x, y, '*') * z * I)

Hope this helps,

Nael

On Wed, Jul 2, 2008 at 6:30 PM, Murali Menon <feanor0_at_hotmail.com> wrote:

>
> folks,
>
> is there a clever way to compute the sum of the product of two vectors such
> that the common indices are not multiplied together?
>
> i.e. if i have vectors X, Y, how can i compute
>
> Sum (X[i] * Y[j])
> i != j
>
> where i != j
>
> also, what if i wanted
>
> Sum (X[i] * Y[j] * R[i, j])
> i != j
>
> where R is a matrix?
>
> thanks,
>
> murali
>
>
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