R help - Dec 2008 - Better way to find distances between points in a set?

I was playing around a bit to see how I could find the two points in a 
set of points (or ordered pairs)  furthest from each other.

Here's what I did:
1) created a Nrow by 2col matrix, so each row contains an x,y coordinate 
pair.

2) fed the matrix to a nested mapply (cv is my matrix):

mapply(function(k,l) mapply(function(x,y,a,b)
+ sqrt((x-a)^2+(y-b)^2),cv[,1],cv[,2],k,l),cv[,1],cv[,2])->alldist

Then I just did which.max(alldist) and found the original two points by 
figuring out what row, col the results of which.max referred to.

So, what's a better, or cleaner way to do all this?  That is, is there a 
function in some package that will do anything like my nested mapply 
thing, and is there a better tool than which.max for locating a position 
in a matrix?

thanks
Carl

The 'better way' to do almost anything starts with a reading of the 
_posting guide_, which reminds you to

 	Do your homework before posting [Reasons whyfor deleted]]


 	    * Do help.search("keyword") and apropos("keyword")
with different
 			keywords (type this at the R prompt).

 	[other homework items deleted]

So, to start with you would have tried:

 	help.search('distance')

HA! This leads to

 	dist {stats}		Distance Matrix Computation

Well, doesn't that sound promising??
> alldist3 <- as.matrix( dist( cv ) )
> which( alldist3 == max( alldist3 ), arr.ind=TRUE )

Oh yes, if you are too lazy to look up the posting guide URL, the function 
help.request() will open it for you when you admit that you haven't yet 
read it (or lead you thru the further steps to prepare a question to this 
list if you say that you have read it).

HTH,

Chuck


On Tue, 9 Dec 2008, Carl Witthoft wrote:
> I was playing around a bit to see how I could find the two points in a set
of
> points (or ordered pairs)  furthest from each other.
>
> Here's what I did:
> 1) created a Nrow by 2col matrix, so each row contains an x,y coordinate 
> pair.
>
> 2) fed the matrix to a nested mapply (cv is my matrix):
>
> mapply(function(k,l) mapply(function(x,y,a,b)
> + sqrt((x-a)^2+(y-b)^2),cv[,1],cv[,2],k,l),cv[,1],cv[,2])->alldist
>
> Then I just did which.max(alldist) and found the original two points by 
> figuring out what row, col the results of which.max referred to.
>
> So, what's a better, or cleaner way to do all this?  That is, is there
a
> function in some package that will do anything like my nested mapply thing,
> and is there a better tool than which.max for locating a position in a 
> matrix?
>
> thanks
> Carl
>
> ______________________________________________
> 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.
>
>
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

Hah!
   I  DID do my homework first. From now on I guess I'll have to post the
   complete list of trails I followed before deciding
   I was lost. That appears to be the only way to stave off the Harpies.
   So, before I posted, and before I even wrote my toy script, I found and read
   thru  the  help file for dist(). If in fact dist() does what I want, I
   couldn't see that from either the description or the example. Or from an
   example I tried myself.
   Remember:  my Euclidean distance requires first finding the difference
   between the orthogonal components of the two points, i.e.
   (x1-x2) and (y1-y2) and then calculating the Pythagorean of those values.
   Just getting distances between the actual elements of my matrix (x and y
   components) isn't the same. And, yes, I tried 'euclidean' .
   So I would be greatful indeed if someone could confirm that, for a matrix as
   I described, where each row contains the coordinates of a single point, that
   dist() can be convinced to return the geometric euclidean distances.
   Or that it can't.
   Carl
   PS I also found which and which.max all by myself! ^_^
   Dec 9, 2008 11:52:46 PM, cberry at tajo.ucsd.edu wrote:
   ==========================================   The 'better way' to do
almost anything starts with a reading of the
   _posting guide_, which reminds you to
   Do your homework before posting [Reasons whyfor deleted]]
   ?  ? * Do help.search("keyword") and apropos("keyword")
with different
   keywords (type this at the R prompt).
   [other homework items deleted]
   So, to start with you would have tried:
   help.search('distance')
   HA! This leads to
   dist {stats} Distance Matrix Computation
   Well, doesn't that sound promising??
   > alldist3 <- as.matrix( dist( cv ) )
   > which( alldist3 == max( alldist3 ), arr.ind=TRUE )
   Oh yes, if you are too lazy to look up the posting guide URL, the function
   help.request() will open it for you when you admit that you haven't yet
   read it (or lead you thru the further steps to prepare a question to this
   list if you say that you have read it).
   HTH,
   Chuck
   On Tue, 9 Dec 2008, Carl Witthoft wrote:
   > I was playing around a bit to see how I could find the two points in a
set
   of
   > points (or ordered pairs) ? furthest from each other.
   >
   > Here's what I did:
   > 1) created a Nrow by 2col matrix, so each row contains an x,y coordinate
   > pair.
   >
   > 2) fed the matrix to a nested mapply (cv is my matrix):
   >
   > mapply(function(k,l) mapply(function(x,y,a,b)
   > + sqrt((x-a)^2+(y-b)^2),cv[,1],cv[,2],k,l),cv[,1],cv[,2])->alldist
   >
   > Then I just did which.max(alldist) and found the original two points by
   > figuring out what row, col the results of which.max referred to.
   >
   > So, what's a better, or cleaner way to do all this? ? That is, is
there a
   > function in some package that will do anything like my nested mapply
   thing,
   > and is there a better tool than which.max for locating a position in a
   > matrix?
   >
   > thanks
   > Carl
   >
   > ______________________________________________
   > 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.
   >
   >
   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