R help - Aug 2008 - r function for calculating extreme spread in group

I'm trying to figure out how to write a r function that will calculate  
the extreme spread of a group of points given their (x,y)  
coordinates.  Extreme Spread is the maximal Euclidean distance between  
two points in a group

ex.spread = max{ sqrt [ (xi-xj)^2 - (yi-yj)^2 ] } for i not equal to j

I have 60 levels to apply this to.

There is the combination function in the dprep package but I wasn't  
sure how to use this or apply an index the points in each group.

Any ideas?



Steve





Steven Matthew Anderson
AdAstra69 at mac.com

Ad Astra per Aspera

Steven Matthew Anderson <adastra69 <at> mac.com> writes:
> I'm trying to figure out how to write a r function that will calculate
> the extreme spread of a group of points given their (x,y)  
> coordinates.  Extreme Spread is the maximal Euclidean distance between  
> two points in a group
> 
> ex.spread = max{ sqrt [ (xi-xj)^2 - (yi-yj)^2 ] } for i not equal to j
> 
> I have 60 levels to apply this to.
> 
   how about something like

max(dist(cbind(x,y))   ... ???

(or to do this in a data frame d with columns x, y, and g (group),

sapply(split(d,d$g),function(Z)max(dist(Z[,c("x","y")])))

  Ben Bolker

Thank you Jorge,

My talent in building functions is weak.  I think I'm close but I'm  
not doing something right.

res=dist(yourdata)
res[which.max(res)]
does provide me with the correct distance but how do I apply it across  
data with level such as ...

 >  
yourdata 
=as.data.frame(cbind(lvl=LETTERS[1:2],x=rpois(10,10),y=rnorm(10) ))
 > yourdata
    lvl  x                  y
1    A 10   0.14377148075807
2    B  5 -0.117753598165951
3    A 14 -0.912068366948338
4    B 10  -1.43758624082998
5    A 16 -0.797089525071965
6    B 11   1.25408310644997
7    A  7   0.77214218580453
8    B  9 -0.219515626753440
9    A 12 -0.424810283377287
10   B 13 -0.418980099421959

My attempt to use tapply blew up on me.

 > Extreme.Spread<-matrix(nrow=60,ncol=2)
 > ES<-function(Level) {
+ 	                 temp<- 
subset(x=ES.Data,Level==level,select=c(Horizontal,Vertical))
+ 	                 e<-dist(rbind(temp$Horizontal,temp 
$Vertical),method="euclidean")
+ 	                 m<-e[which.max(e)]
+ 	                 es<-cbind(level,m)
+ 	                 Extreme.Spread<-rbind(Extreme.Spread,es)
+ 	                 }
 > tapply(X=ES.Data,INDEX=Level,FUN=ES)
Error in tapply(X = ES.Data, INDEX = Level, FUN = ES) :
   arguments must have same length





On Aug 27, 2008, at 2:30 PM, Jorge Ivan Velez wrote:
> R-help@r-project.org
Steven Matthew Anderson
AdAstra69@mac.com

Ad Astra per Aspera




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