# [Rd] relist, an inverse operator to unlist

From: Andrew Clausen <clausen_at_econ.upenn.edu>
Date: Sun, 13 May 2007 13:29:11 -0400

I wrote a function called relist, which is an inverse to the existing unlist function:

Some functions need many parameters, which are most easily represented in complex structures. Unfortunately, many mathematical functions in R, including optim, nlm, and grad can only operate on functions whose domain is a vector. R has a function to convert complex objects into a vector representation. This file provides an inverse operation called "unlist" to convert vectors back to the convenient structural representation. Together, these functions allow structured functions to have simple mathematical interfaces.

For example, a likelihood function for a multivariate normal model needs a variance-covariance matrix and a mean vector. It would be most convenient to represent it as a list containing a vector and a matrix. A typical parameter might look like

list(mean=c(0, 1), vcov=cbind(c(1, 1), c(1, 0)))

However, optim can't operate on functions that take lists as input; it only likes vectors. The solution is conversion:

initial.param <- list(mean=c(0, 1), vcov=cbind(c(1, 1), c(1, 0)))

```         ll <- function(param.vector)
{
param <- relist(initial.param, param.vector)
-sum(dnorm(x, mean=param\$mean, vcov=param\$vcov, log=TRUE))
# note: dnorm doesn't do vcov... but I hope you get the point
}

optim(unlist(initial.param), ll)

```

"relist" takes two parameters: skeleton and flesh. Skeleton is a sample object that has the right "shape" but the wrong content. "flesh" is a vector with the right content but the wrong shape. Invoking

relist(skeleton, flesh)

will put the content of flesh on the skeleton.

As long as "skeleton" has the right shape, it should be a precise inverse of unlist. These equalities hold:

```        relist(skeleton, unlist(x)) == x
unlist(relist(skeleton, y)) == y

```

Is there any easy way to do this without my new relist function? Is there any interest in including this in R's base package? (Or anywhere else?) Any comments on the implementation?

Cheers,
Andrew

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