Better way to find distances between points in a set?
Ok... then.. I didn't mean to start a flame war, so first off I apologize for firing off my second comment. I think a fair compromise is: I should have at least noted that I got as far as finding dist() (and which.max()), thus showing "some" effort on my part before posting my query. Anyway, I'm happy now because not only did I successfully navigate mapply, I got some great info on dist and which.* from this mailing list. Carl
Stavros Macrakis wrote:
On Tue, Dec 9, 2008 at 11:52 PM, Charles C. Berry <cberry at tajo.ucsd.edu>wrote:
[redacted]
Charles, I am distressed by the nasty tone of your note. Though I certainly
agree that poor queries on this list are an annoyance, and that the posting
guide gives useful advice on what to do before posting, I don't think that
poor queries are a reason to presume bad faith for any but the most flagrant
correspondents ("here is my problem set, solve it for me"). And though I
have not been on this list very long, I have been answering questions on
other lists like this for a long time.
What's more, it seems to me that Witthoft has a legitimate problem. I tried
to solve it myself, and here is what I come up with:
test <- matrix(rnorm(20),ncol=2)
which.max(dist(x),arr.ind=TRUE)
=> Error, though which supports arr.ind, which.max doesn't
Yes, that is documented, but it violates reasonable expectations.
which(dist(x)==max(dist(x)),arr.ind=TRUE)
=> 21 oops, that is a vector index; what about arr.ind?
Apparently the dist class (return class of dist fnc.)
doesn't act as an array here (and which/arr.ind is
happy to treat a vector as an array without warning).
Is this a problem in dist, or a problem in which?
So how do I map back from 21 to 3,7? Nothing obvious.
? `[`, ? array, and ? dist don't seem to help.
So let's convert to a matrix explicitly:
which(as.matrix(dist(x)==max(dist(x))),arr.ind=TRUE)
=> row col
[1,] 21 1
Hmm, that's not right; apparently the == flattened the
dist class into its base vector, which which/arr.ind
helpfully (?) treated as a matrix.
which(as.matrix(dist(x))==max(dist(x)),arr.ind=TRUE)
=> row col
7 7 3
3 3 7
Finally getting useful results; but converting to the array
added in the upper triangular part... oh, well, we need to
choose *which* maximum anyway.
So, as Witthoft said: what's a better, or cleaner way to do all this? And
why aren't R's operators more regular in their behavior? Why doesn't
arr.ind work for which.max? Why doesn't dist(x)==1 return an object of the
same shape as the original dist (as matrix(...)==1 would have), but instead
flatten it? Where does one find the function that maps from a list of array
indices to the corresponding vector index and vice versa?
-s