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POhow to find the top N values by group or within category (groupwise) in an R data.frame
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  1. POhow to find the top N values by group or within category (groupwise) in an R data.frame
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    copied!<p>my apologies if this is a duplicate. this seems like a question that SO would've answered long ago, but i did quite a bit of searching and couldn't find anything specifically answering this. there are lots of related questions that could be used to answer this, but i figured it should be answered formally.</p> <p>this is in response to <a href="http://r.789695.n4.nabble.com/top-10-n-values-for-each-classes-tt4658074.html#a4658095" rel="noreferrer">this question asked on the r-help mailing list</a>.</p> <p><a href="https://stackoverflow.com/search?q=top+value+by+group">here are <em>lots</em> of examples</a> of how to do this using <code>sql</code>, so i imagine it's easy to convert that knowledge over using the R <code>sqldf</code> package.. but there are a few ways to do this with R, and i wanted to check if others had ideas.</p> <p>main question: using the example <code>mtcars</code> data.frame, how would someone find the top or bottom (maximum or minimum) N records within a stated category? the top or bottom N results, per group.</p> <p>if you open up R and type <code>mtcars</code> you get an example table with 32 records. when grouped by the cylinder column <code>cyl</code> - here are the top three records for each distinct value of <code>cyl</code>. note that ties are excluded in this case, but it'd be nice to show some different ways to treat ties.</p> <pre><code> mpg cyl disp hp drat wt qsec vs am gear carb ranks Toyota Corona 21.5 4 120.1 97 3.70 2.465 20.01 1 0 3 1 2.0 Volvo 142E 21.4 4 121.0 109 4.11 2.780 18.60 1 1 4 2 1.0 Valiant 18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1 2.0 Merc 280 19.2 6 167.6 123 3.92 3.440 18.30 1 0 4 4 3.0 Merc 280C 17.8 6 167.6 123 3.92 3.440 18.90 1 0 4 4 1.0 Cadillac Fleetwood 10.4 8 472.0 205 2.93 5.250 17.98 0 0 3 4 1.5 Lincoln Continental 10.4 8 460.0 215 3.00 5.424 17.82 0 0 3 4 1.5 Camaro Z28 13.3 8 350.0 245 3.73 3.840 15.41 0 0 3 4 3.0 </code></pre>
 

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