Convert a column vector into a extended diagonal matrix

Question

Consider the following column vector:

vec <- rbind(c(0.5),c(0.6))

I want to convert it into the following 4x4 diagonal matrix:

0.5   0   0    0 
  0 0.6   0    0 
  0   0 0.5    0
  0   0   0  0.6

I know I can do it by the following code:

dia <- diag(c(vec,vec))

But what if I want to convert it into a 1000x1000 diagonal matrix. Then the code above is so efficient. Maybe I can use rep, but I am not totally sure how to do it. How can I do it more efficient?

score 2 · Accepted Answer · answered Oct 01 '19 at 09:37

2

Here is one other way using recycling:

diag(c(vec), length(vec)*2)

answered Oct 01 '19 at 09:37

Clemsang

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Ronak Shah · Answer 2 · 2019-10-01T09:20:03.837

1

I think your approach is already good enough, here is another way by initialising the matrix and using rep to fill diagonals.

n <- 4
mat <- matrix(0, ncol = n, nrow = n)
diag(mat) <- rep(vec, n/2)

mat
#     [,1] [,2] [,3] [,4]
#[1,]  0.5  0.0  0.0  0.0
#[2,]  0.0  0.6  0.0  0.0
#[3,]  0.0  0.0  0.5  0.0
#[4,]  0.0  0.0  0.0  0.6

and following your approach you could do

diag(rep(vec, n/2))

edited Oct 01 '19 at 09:20

answered Oct 01 '19 at 09:14

Ronak Shah

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Just a small note, but with `rep(vec, n/2)` it could be even nicer :) – tmfmnk Oct 01 '19 at 09:19
Yes, that is a great note :-) Thank you both! – Michael Oct 01 '19 at 09:19
Thanks @tmfmnk , I thought `vec` will always be of size `n/2`. – Ronak Shah Oct 01 '19 at 09:21

Convert a column vector into a extended diagonal matrix

2 Answers2