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POR: When using data.table how do I get columns of y when I do x[y]?
 

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  1. POR: When using data.table how do I get columns of y when I do x[y]?
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    <p><strong><em>UPDATE</em></strong>: Old question ... it was resolved by data.table v1.5.3 in Feb 2011.</p> <p>I am trying to use the <code>data.table</code> package, and really like the speedups I am getting, but I am stumped by this error when I do <code>x[y, &lt;expr&gt;]</code> where <code>x</code> and <code>y</code> are "data-tables" with the same key, and <code>&lt;expr&gt;</code> contains column names of both <code>x</code> and <code>y</code>:</p> <pre><code>require(data.table) x &lt;- data.table( foo = 1:5, a = 5:1 ) y &lt;- data.table( foo = 1:5, boo = 10:14) setkey(x, foo) setkey(y, foo) &gt; x[y, foo*boo] Error in eval(expr, envir, enclos) : object 'boo' not found </code></pre> <p>UPDATE... To clarify the functionality I am looking for in the above example: I need to do the equivalent of the following:</p> <pre><code>with(merge(x,y), foo*boo) </code></pre> <p>However according to the below extract from the <code>data.table</code> FAQ, this should have worked:</p> <blockquote> <p>Finally, although it appears as though x[y] does not return the columns in y, you can actually use the columns from y in the j expression. This is what we mean by join inherited scope. Why not just return the union of all the columns from x and y and then run expressions on that? It boils down to eciency of code and what is quicker to program. When you write x[y,foo<em>boo], data.table automatically inspects the j expression to see which columns it uses. It will only subset, or group, those columns only. Memory is only created for the columns the j uses. Let's say foo is in x, and boo is in y (along with 20 other columns in y). Isn't x[y,foo</em>boo] quicker to program and quicker to run than a merge step followed by another subset step ?</p> </blockquote> <p>I am aware of <a href="https://stackoverflow.com/questions/2232699/r-how-to-do-a-data-table-merge-operation">this question</a> that addressed a similar issue, but it did not seem to have been resolved satisfactorily. Anyone know what I am missing or misunderstanding? Thanks.</p> <p>UPDATE: I asked on the data-table help mailing list and the package author (Matthew Dowle) <a href="http://r.789695.n4.nabble.com/Cannot-access-cols-of-y-when-doing-x-y-tt3231504.html" rel="nofollow noreferrer">replied</a> that indeed the FAQ quoted above is wrong, so the syntax I am using will not work currently, i.e. I cannot refer to the <code>y</code> columns in the <code>j</code> (i.e. second) argument when I do <code>x[y,...]</code>. </p>
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    1. USPrasad Chalasani
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    1. COBut you asked some time ago and it was addressed by v1.5.3 released to CRAN in Feb 2011. Please see it's NEWS, new ?data.table and corrected FAQ.
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      1. POR: When using data.table how do I get columns of y when I do x[y]?
      1. USMatt Dowle
    2. CO@Matthew thank you, yes I know it's been addressed by the latest release, and I'm glad you pointed it out here so it's clear to others.
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