find value closest to x by group in dplyr
<pre><code>library(dplyr)
a &lt;- data_frame(id = c("A","A","A","B","B","B"),
                b = c(1.2, 1.5, 1.8, 1.1, 1.6, 1.4))
</code></pre>

<p>Now, I´d like to retrieve the values closest to 1.43 for each of the catergories in id. 
I thought I could use:</p>

<pre><code>a %&gt;% group_by(id) %&gt;% nth(which.min(abs(.$b-1.43)))
</code></pre>

<p>but dplyr states</p>

<pre><code>Error: Don't know how to generate default for object of class grouped_df/tbl_df/tbl/data.frame
</code></pre>