# The magic empty bracket

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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 of matrix . 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

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