# The magic empty bracket

January 30, 2013
By

(This article was first published on Rmazing, and kindly contributed to R-bloggers)

I have been working with R for some time now, but once in a while, basic functions catch my eye that I was not aware of…
For some project I wanted to transform a correlation matrix into a covariance matrix. Now, since cor2cov does not exist, I thought about “reversing” the cov2cor function (stats:::cov2cor).
Inside the code of this function, a specific line jumped into my retina:

r[] <- Is * V * rep(Is, each = p)


What’s this [ ]?

Well, it stands for every element $E_{ij}$ of matrix $E$. Consider this:

mat <- matrix(NA, nrow = 5, ncol = 5)

> mat
[,1] [,2] [,3] [,4] [,5]
[1,]   NA   NA   NA   NA   NA
[2,]   NA   NA   NA   NA   NA
[3,]   NA   NA   NA   NA   NA
[4,]   NA   NA   NA   NA   NA
[5,]   NA   NA   NA   NA   NA

With the empty bracket, we can now substitute ALL values by a new value:

mat[] <- 1

> mat
[,1] [,2] [,3] [,4] [,5]
[1,]    1    1    1    1    1
[2,]    1    1    1    1    1
[3,]    1    1    1    1    1
[4,]    1    1    1    1    1
[5,]    1    1    1    1    1

Interestingly, this also works with lists:

L <- list(a = 1, b = 2, c = 3)

>L
$a [1] 1$b
[1] 2
$c [1] 3 L[] <- 5 > L$a
[1] 5

$b [1] 5$c
[1] 5

Cheers,
Andrej

Filed under: R Internals Tagged: all elements, empty bracket, matrix