## The magic empty bracket

January 30, 2013

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

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