As part of my research, I find that I am often writing R code for solving specific convex optimization problems. In doing so, I often want to quickly check on small problems whether I am getting the right answer. CVX is a tool by Michael Grant and Stephen Boyd that makes it easy to solve general convex problems, and the syntax to input such problems is simple and straightforward. In this sense, CVX is the ideal tool for quickly confirming that my code is working. The trouble is that I work in R, but CVX is in Matlab.

**New:** If you prefer Julia to MATLAB, then consider using Convex.jl via this new R package: convexjulia.

I have written an R package `CVXfromR` with a function `CallCVX`, that lets you call CVX from R. You pass `CallCVX` four things:

a string containing what you would type between

`cvx_begin`and`cvx_end`if you were using CVX in Matlab (using semicolons in place of new line characters),a list of the non-optimization variables used in the CVX code,

a vector of the names of the optimization variables,

and the directory where

`cvx_setup.m`is located.

We begin by generating some data in R.

n <- 50 p <- 10 x <- matrix(rnorm(n * p), n, p) beta <- rnorm(p) y <- x %*% beta + 0.1 * rnorm(n)

Now suppose we would like to perform the Lasso using CVX. We begin by loading the package and giving the path of the directory containing the file `cvx_setup.m`:

library(CVXfromR) setup.dir <- "change/this/to/your/cvx/directory"

We are now ready to send off an optimization problem to CVX:

cvxcode <- paste("variables b(p)", "minimize(square_pos(norm(y - x * b, 2)) / 2 + lam * norm(b, 1))", sep=";") lasso <- CallCVX(cvxcode, const.vars=list(p=p, y=y, x=x, lam=2), opt.var.names="b", setup.dir=setup.dir)

The output contains an optimal point, the optimal value, the number of seconds elapsed for the call to CVX (not counting the time opening Matlab and transferring data between R and Matlab):

> names(lasso) [1] "b" "cvx_optval" "time" "command"

If there is an error in the Matlab code you have passed, it will leave you in Matlab (useful for diagnosing the problem). Typing `exit` brings you back into R.

**New feature:** To solve this problem for a sequence of values of the tuning parameter, one no longer needs to wrap `CallCVX` in a loop. Rather, use `CallCVX.varyparam`:

lasso <- CallCVX.varyparam(cvxcode, const.vars=list(p=p, y=y, x=x), tuning.param=list(lam=seq(0.1, 1, length=20)), opt.var.names="b", setup.dir=setup.dir)

The returned value `b` would be a list of length 20, giving optimal points for each value of the tuning parameter.

Matlab installed with alias

`matlab`(in OS X supply the argument`matlab.call`with path to Matlab)CVX installed (installation instructions are here)

Prior knowledge of CVX (and Matlab) is helpful (CVX user guide)

Here is a recent version of the package. The most up-to-date version is available on github. (Thanks to Stefan Avey for contributing an improvement.)

**New:** CVXfromR now available on Windows. Click here for Windows version.

My approach is very low-tech in the sense that it actually just forms a very long string that is passed to Matlab through a call to the command line. It passes data from R to Matlab and then back to R by saving temporary files.

I see this as a temporary solution until someone actually writes CVX for R. I look forward to that, but in the meantime, `CallCVX` works well for me.