Adding a value to each element of a matrix diagonal in R

Question

I have a matrix:

1 2 3
5 6 10
8 3 5

I want to add a value (say 2) to each element of the diagonal:

3 2 3    
5 8 10
8 3 7

How should I do it?

Answer 1

With your sample data

m<-matrix(scan(text="1 2 3
5 6 10
8 3 5"), ncol=3)

You can use the diag() function to both extract and update the diagonal elements of your matrix

diag(m) <- diag(m)+2
m

Answer 2

Although diag function is the best for this purpose, this is just another way around:

d <- row(m)-col(m)==0
m[d] <- m[d]+2

d is a logical matrix in which only diagonal elements are TRUE.