[R] create a factor variable from two numeric variables when order is irrelevant

[Daniel Malter]

Hi all,

I have two numeric variables that form combinations in a matched sample.
Let's say I have five levels of x and y. What I am seeking to create is a
factor variable that ignores the order of x and y, i.e., the factor should
indicate x=1, y=5, as the same factor as x=5, y=1. Obviously, this becomes
increasingly cumbersome to do by hand as the number of levels increases.

f<-1:5
x<-sample(f,100,replace=T)
y<-sample(f,100,replace=T)
d<-matrix(cbind(x,y),ncol=2)

#A working solution is to remove the order, multiply one column by a scaling
constant, add the second column, and create the factor for this numeric
value. However, I was wondering whether there is less awkward, more direct
way to do this.

i<-apply(t(apply(d,1,function(x) sort(x))),1,function(y) 10*y[1]+y[2])
i<-factor(i)
i

Thanks for your help,
Daniel




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