Hi casperyc,

Here is a suggestion:

# all at once
apply(combn(paste('x', 1:3, sep =""), 2), 2,
            function(v) get(v[1])*get(v[2]) )

# step by step
thex <- paste('x', 1:3, sep ="")
thex
combs <- combn(thex, 2)
combs
apply(combs, 2, function(v) get(v[1])*get(v[2]) )

x1, x2 and x3 correspond to the variables you sent.  See ?combn, ?apply,
?get and ?paste for more information.

HTH,
Jorge


On Fri, Mar 26, 2010 at 7:01 PM, casperyc <> wrote:

>
> Hi,
>
> I am tring to write a loop to compute this,
> ==========================
> x1=c(
>        rep(-1,4),
>        rep(1,4)
>        )
>
> x2=c(
>        rep(c(-1,-1,1,1),2)
>        )
>
> x3=c(
>        rep(c(-1,1),4)
>        )
>
> x1*x2
> x1*x3
> x2*x3
> ========================
>
> suppose i have x1,x2,x3
> i want to compute their ' two factor interactions', x1x2,x1x3 and x2x3,
> I wrote
>
> ========================
> for(i in 1:2){
>        for( j in i+1:3){
>        xij=c()
>        xij=xi*xj
>        }
> }
> ========================
> it did not seem to recognize xi and xj
>
> is there any suggestion?
> it would be wonderful if there exists a single command that i can use
>
> My ultimate aim is to find the 55 xixj s of the following data:
> http://n4.nabble.com/file/n1692945/test_pic.jpg test_pic.jpg
>
>
> Thanks.
> --
> View this message in context:
> http://n4.nabble.com/R-loop-help-tp1692945p1692945.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
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> PLEASE do read the posting guide
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>

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