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POHow to pass a parameter by variable into data.table[J()]
 

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  1. POHow to pass a parameter by variable into data.table[J()]
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    <p>I'm brand new to the (completely marvelous) <code>data.table</code> package, and seem to have gotten stuck on a very basic, somewhat bizarre problem. I can't post the exact data set I'm working with, for which I apologize -- but I think the problem is simple enough to articulate that hopefully this will still be very clear.</p> <p>Let's say I have a data.table like so, with key x: </p> <pre><code>set1 x y 1: 1 a 2: 1 b 3: 1 c 4: 2 a </code></pre> <p>I want to return a subset of <code>set1</code> containing all rows where <code>x == 1</code>. This is wonderfully simple in data.table: <code>set1[J(1)]</code>. Bam. Done. I can also assign <code>z &lt;- 1</code>, and call <code>set1[J(z)]</code>. Again: works great. </p> <p>...except when I try to scale it up to my actual data set, which contains ~6M rows. When I call <code>set1[J(1674)]</code>, I get back a 78-row return that's exactly what I'm looking for. But I need to be able to look up (literally) 4M of these subsets. When I assign the value I'm searching for to a variable, <code>id &lt;- 1674</code>, and call <code>set1[J(id)]</code>... R nearly takes down my desktop. </p> <p>Clearly <em>something</em> I don't understand is going on under the data.table hood, but I haven't been able to figure out what. Googling and slogging through Stack Overflow suggest that this should work. Out of pure whimsey, I've tried:</p> <pre><code>id &lt;- quote(1674) set1[J(eval(id))] </code></pre> <p>...but that is far, far worse. What... what's going on? </p>
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    1. COCan you provide a bit more detail on what R is printing out?
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    2. COYes and no. Yes, I can provide more detail -- but R isn't printing out *anything*. According to `top`, when I call `set1[J(id)]`, rsession starts using up to 97% of system memory. The box becomes functionally unusable until I manage to kill the rsession process some time later. This is by contrast to `set1[J(1674)]`, which returns 78 rows as soon as I press `enter`.
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    3. COIs id a column in your data.table set1
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