R help - Mar 2011 - Building a matrix so that matrix(r, c)<-matrix(c, r) with No For Loops

Hello,

I would like to take advantage of the upper.tri() function here but I don't
know exactly. Here is some working code...
i<-5
fi<-matrix(0,nrow=i,ncol=i)
for(r in 1:i){
for(c in 1:i){
if(r==c){
 fi[r,c]<-1
}else if(r<c){
fi[r,c]<-1-runif(1)^.5
 }else{
fi[r,c]<-fi[c,r]
}
 }
}

So far I know I can simplify this code to 5 lines (no for loops):
i<-5
fi<-matrix(nrow=i,ncol=i)
fi[upper.tri(fi)]<-1-runif(length(fi[upper.tri(fi)]))^.5
diag(fi)<-1
fi[lower.tri(fi)]<-fi[upper.tri(fi)]#This entry is not correct. fi[r,c] !
=fi[c,r]

Any suggestions?

Sincerely,
Brian

	[[alternative HTML version deleted]]

On Mar 26, 2011, at 9:44 AM, Brian Pellerin wrote:
> Hello,
>
> I would like to take advantage of the upper.tri() function here but  
> I don't
> know exactly. Here is some working code...
> i<-5
> fi<-matrix(0,nrow=i,ncol=i)
> for(r in 1:i){
> for(c in 1:i){
> if(r==c){
> fi[r,c]<-1
> }else if(r<c){
> fi[r,c]<-1-runif(1)^.5
> }else{
> fi[r,c]<-fi[c,r]
> }
> }
> }
>
> So far I know I can simplify this code to 5 lines (no for loops):
> i<-5
> fi<-matrix(nrow=i,ncol=i)
> fi[upper.tri(fi)]<-1-runif(length(fi[upper.tri(fi)]))^.5
> diag(fi)<-1
> fi[lower.tri(fi)]<-fi[upper.tri(fi)]#This entry is not correct.  
> fi[r,c] ! => fi[c,r]
I've always found using the upper.tri and lower.tri functions error  
prone in my hands, because they are really logical matrices for  
selection rather than returning values as I naively expect. Try this:

i<-5
fi<-diag(1,i,i)
fi[upper.tri(fi)]<-1-runif(length(fi[upper.tri(fi)]))^.5
fi[lower.tri(fi)]<-t(fi)[lower.tri(fi)]
fi

It may seem odd to use lower.tri(fi) inside `[ ]` since the values of  
`fi` in the lower triangle are all zero, but you are really just using  
it to extract from `t(fi)`.

-- 
David.
>
> Any suggestions?
>
> Sincerely,
> Brian
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
David Winsemius, MD
West Hartford, CT

Thanks, David. This works splendidly. Thanks for your help.

Sincerely,
Brian
>>> David Winsemius <dwinsemius at comcast.net> 03/26/11 3:06 PM
>>>
On Mar 26, 2011, at 9:44 AM, Brian Pellerin wrote:
> Hello,
>
> I would like to take advantage of the upper.tri() function here but  
> I don't
> know exactly. Here is some working code...
> i<-5
> fi<-matrix(0,nrow=i,ncol=i)
> for(r in 1:i){
> for(c in 1:i){
> if(r==c){
> fi[r,c]<-1
> }else if(r<c){
> fi[r,c]<-1-runif(1)^.5
> }else{
> fi[r,c]<-fi[c,r]
> }
> }
> }
>
> So far I know I can simplify this code to 5 lines (no for loops):
> i<-5
> fi<-matrix(nrow=i,ncol=i)
> fi[upper.tri(fi)]<-1-runif(length(fi[upper.tri(fi)]))^.5
> diag(fi)<-1
> fi[lower.tri(fi)]<-fi[upper.tri(fi)]#This entry is not correct.  
> fi[r,c] ! => fi[c,r]
I've always found using the upper.tri and lower.tri functions error  
prone in my hands, because they are really logical matrices for  
selection rather than returning values as I naively expect. Try this:

i<-5
fi<-diag(1,i,i)
fi[upper.tri(fi)]<-1-runif(length(fi[upper.tri(fi)]))^.5
fi[lower.tri(fi)]<-t(fi)[lower.tri(fi)]
fi

It may seem odd to use lower.tri(fi) inside `[ ]` since the values of  
`fi` in the lower triangle are all zero, but you are really just using  
it to extract from `t(fi)`.

-- 
David.
>
> Any suggestions?
>
> Sincerely,
> Brian
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
David Winsemius, MD
West Hartford, CT