How do I go from cell given by dist back to row and column numbers

Question

Say I have

> x<-1:5
> dist(x)
  1 2 3 4
2 1      
3 2 1    
4 3 2 1  
5 4 3 2 1
> which(dist(x)==max(dist(x)))
[1] 4

How do I get from the index 4 back to the row and column numbers (5,1)?

dist(x) returns an atomic vector. It only 'prints' a matrix. — asb, Jun 19 '13 at 12:54

Matthew Plourde · Accepted Answer · 2013-06-19T13:00:56.253

6

There might be a tidier way ...

dist.x <- dist(x)
which(as.matrix(dist.x) == max(dist.x) & lower.tri(dist.x), arr.ind=TRUE)
#   row col
# 5   5   1

edited Jun 19 '13 at 13:00

answered Jun 19 '13 at 12:55

Matthew Plourde

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+1 nice and simple, exactly what i'd do. – Simon O'Hanlon Jun 19 '13 at 13:02
+1 - also `which(lower.tri(dist.x), arr.ind=TRUE)[which.max(dist.x), ]` – flodel Jun 19 '13 at 21:45

score 2 · Answer 2 · answered Jun 19 '13 at 12:59

2

dist has a method to as.matrix which is useful. You can try this:

kk <- as.matrix(dist(x))
which(kk == max(kk), arr.ind=TRUE)

For your example,

  row col
5   5   1
1   1   5

answered Jun 19 '13 at 12:59

asb

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Matthew has already given this *exact* answer above (as had I, but I deleted mine because it was duplicate info and he answered first). – Simon O'Hanlon Jun 19 '13 at 13:05

score 1 · Answer 3 · answered Jun 19 '13 at 12:56

dist returns an object of class "dist." You should start by reading the help file, which says:

Value

dist returns an object of class "dist".

The lower triangle of the distance matrix stored by columns in a vector, say do. If n is the number of observations, i.e., n <- attr(do, "Size"), then for i < j ≤ n, the dissimilarity between (row) i and j is do[n*(i-1) - i*(i-1)/2 + j-i]. The length of the vector is n*(n-1)/2, i.e., of order n^2.

The other answers posted modify the "dist" object in useful ways for you.

How do I go from cell given by dist back to row and column numbers

3 Answers3

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