On Fri, 20 Nov 2009, Andrew Barr wrote:
Hi R folks,
I have a massive array (object name "points") in the following form
[,1] [,2]
[1,] 1369 22
[2,] 1370 22
[3,] 1368 23
[4,] 1369 23
[5,] 1370 23
[6,] 1371 23
(10080 rows truncated)
These represent pixel coordinates of interest in a jpeg image. I need
to find the distance from each point to all other points of interest.
The only way I can see to do this is by pythagoras and nested for
loops.
distance<-function(x1,y1,x2,y2){sqrt((x2-x1)^2 + (y2-y1)^2)} #pythagoras
for(i in 1:nrow(points)){
for(j in 1:nrow(points)){
dist<-c(dist,distance(points[i,1],point.array.indices[i,2],points[j,1],points[j,2]))
}
}
This is obviously prohibitively slow with >10000 rows in the array.
Any thoughts on how to do this without for loops? I apologize in
advance if there is an obvious way around this.
Apology accepted.
The obvious way is to follow the posting guide:
?distance ## suggests trying ??distance
??distance ## lists stats::dist
?dist ## Mmmmm!
HTH,
Chuck
Thanks!
Andrew Barr
University of Texas at Austin
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Charles C. Berry (858) 534-2098
Dept of Family/Preventive Medicine
E mailto:cbe...@tajo.ucsd.edu UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
