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(This article was first published on <strong><a href="https://dahtah.wordpress.com/2016/09/18/vecpack-an-r-package-for-packing-stuff-into-vectors/"> R – dahtah</a></strong>, and kindly contributed to <a rel="nofollow" href="https://www.r-bloggers.com/">R-bloggers)</a> </div>
<p>Here’s a problem I’ve had again and again: let’s say you’ve defined a statistical model with several parameters. One of them is a scalar. Another is a matrix. The third one is a vector, and so on. When fitting the model the natural thing to do is to write a likelihood function that takes as many arguments as you have parameters in your model: i.e., lik(x,y,z) where x is a scalar, y a matrix and z a vector. The problem is that, while it’s the natural way of writing that function, that’s not what optimisers like “optim” want: they want a function with a single argument, and that argument should be a vector. So you have to pack everything into a vector, and write a whole lot of boilerplate code to unpack all the parameters out of that vector.</p>
<p><a target="_blank" href="https://github.com/dahtah/vecpack/">vecpack</a> saves you from having to write all that boilerplate:</p>
<pre>

devtools::install_github(&quot;dahtah/vecpack&quot;)

library(vecpack)

#A cost function in two arguments:
cost &lt;- function(a,b)  (3*a-b+2)^2

#Call optim via vpoptim
res &lt;- vpoptim(list(a=1,b=0),cost)
res$par

</pre>
<p>vecpack knows how to automatically pack and unpack scalars, vectors, matrices and images (from the <a target="_blank" href="https://github.com/dahtah/vecpack/">imager</a> package). It’s also very easy to extend.</p>
<p>The package is quite new, and not on CRAN yet. Feedback welcome, either here or on the issues page on <a target="_blank" href="https://github.com/dahtah/vecpack/issues">github</a>.</p>
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<div>To <strong>leave a comment</strong> for the author, please follow the link and comment on their blog: <strong><a href="https://dahtah.wordpress.com/2016/09/18/vecpack-an-r-package-for-packing-stuff-into-vectors/"> R – dahtah</a></strong>.</div>
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