[R] Multiple operations on list

Liaw, Andy andy_liaw at merck.com
Tue Sep 21 01:39:25 CEST 2004

```Hope this does what you want:

> library(gregmisc)
> cmb <- combinations(length(aa), 2)
> apply(cmb, 1, function(i) sum(diag(aa[[i[1]]] %*% aa[[i[2]]])) /
+       (sum(diag(aa[[i[1]]])) * sum(diag(aa[[i[2]]]))))
[1] 0.5363973 0.3336318 0.6593545

Andy

> From: Stephane DRAY
>
> Hello,
>
> suppose I have a list with matrices:
>
> a=list(x1=matrix(rnorm(10),5,2),x2=matrix(rnorm(10),5,2),x3=ma
> trix(rnorm(10),5,2))
>
> I want to compute for all combination of xi and xj (x1,x2
> x1,x3 and x2,x3)
> a value.
> This value is given for the pair x1,x2 by
> trace(x1%*%t(x1)%*%x2%*%t(x2)) /
> trace(x1%*%t(x1))*trace(x2%*%t(x2))
>
> I know that product matrices t(xi)%*%xi can be obtained by:
>
> aa=lapply(a,crossprod)
>
> but I do not know how to "mix" the values in aa to obtain the
> desired values.
>
> Is there a way to do it without for loop ?
>
>
> sincerely
>
> --------------------------------------------------------------
> ------------------------------------
>
>
> Tel : (514) 343-6111 poste 1233         Fax : (514) 343-2293
> E-mail : stephane.dray at umontreal.ca
> --------------------------------------------------------------
> ------------------------------------
>
> Web
> http://www.steph280.freesurf.fr/
>
>
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