# Re: [R] multiplication question

From: Peter Alspach <PAlspach_at_hortresearch.co.nz>
Date: Thu, 03 Jul 2008 08:54:04 +1200

Murali

I don't know about 'clever', but does this do what you want?

v1 <- 1:3
v2 <- 4:6
sum(matrix(rep(v1, length(v1)), nrow=length(v1))%*%v2)-sum(v1*v2)

Peter Alspach

> -----Original Message-----
> From: r-help-bounces_at_r-project.org
> [mailto:r-help-bounces_at_r-project.org] On Behalf Of Murali Menon
> Sent: Thursday, 3 July 2008 4:31 a.m.
> To: r-help_at_stat.math.ethz.ch
> Subject: [R] multiplication question
>
>
> 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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