# [R] Better way to find distances between points in a set?

cgw at witthoft.com cgw at witthoft.com
Wed Dec 10 17:58:05 CET 2008

```   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]]
Â  Â * Do help.search("keyword") and apropos("keyword") with different
keywords (type this at the R prompt).
[other homework items deleted]
help.search('distance')
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) Â 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? Â 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
>
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