[R] help me avoid nested for() loops!

[Charles C. Berry]

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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> and provide commented, minimal, self-contained, reproducible code.
>

Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901