why dim of colMeans/rowMeans return null?

Question

I am new to R. I have the following code

sigma1  = matrix(c(2,0,0,1),2,2)
mu1     = matrix(c(0,10),2,1)

x1      = t(mvrnorm(n = 5, mu1, sigma1))

print(rowMeans(x1))
print(dim(colMeans(x1)))

and for some reason I get NULL for dimension of row/col means.

Answer 1

'as.matrix' turns the vector 'colMeans(x1)' into a matrix. Then the dimensions are 5 and 1, as expected:

library(rockchalk)

sigma1  = matrix(c(2,0,0,1),2,2)
mu1     = matrix(c(0,10),2,1)
x1      = t(mvrnorm(n = 5, mu1, sigma1))

print(rowMeans(x1))
print(dim(colMeans(x1)))
print(dim(as.matrix(colMeans(x1))))

Output:

> print(rowMeans(x1))
[1] -0.1518187  9.4232239

> print(dim(colMeans(x1)))
NULL

> print(dim(as.matrix(colMeans(x1))))
[1] 5 1
>