Find the position of a cell in a matrix from the content

Question

Very basic question but I can't find an answer.

For a generic matrix (in my case I have an adjacency matrix like the following one, but much bigger):

   A B C D E 
A  0 1 0 2 1 
B  0 0 1 0 0 
C  0 1 0 1 0 
D  0 1 1 0 0 
E  0 0 0 1 0

I computed the frequency of values in the adjacency matrix

table <- data.frame(table(as.matrix(n)))

and I'd like now to know how to understand where those values come from.

Basically, I know the value of a cell, how do I find its position within the matrix?

I don't know how the output would look like, I just need number of row and column, or names of them.

Hallo · Accepted Answer · 2019-09-02T10:59:50.383

1

Arbitrary adjacency matrix n:

n <- as.matrix(rbind(c(1,0.2,0.8,0.6),
                c(0.3,1,0.8,0.2),
                c(0.8,0.1,1,0.3),
                c(0.8,0.2,0.3,1)))

Find position of 0.8's.

value = 0.8
which(n == value, arr.ind=T)

Output:

     row col
[1,]   3   1
[2,]   4   1
[3,]   1   3
[4,]   2   3

edited Sep 02 '19 at 10:59

answered Sep 02 '19 at 10:56

Hallo

1

`which` is certainly what OP is looking for. Nonetheless, I'd avoid, as an example, to use a matrix with floating values. We need to remember that exact comparison between floating points values must be avoided (see for instance https://stackoverflow.com/questions/9508518/why-are-these-numbers-not-equal). Better use integer number. – nicola Sep 02 '19 at 11:07

1 Answers1