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13922822
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2012-12-17T21:52:48.123
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2012-12-17T22:07:56.050
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text
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[ @mnel beat me to it as I was writing ...] Almost certainly, one column of <code>set1</code> happens to be called <code>"id"</code>; i.e., <pre><code>isTRUE("id" %in% names(set1)) </code></pre> causing <code>set1[J(id)]</code> to self join <code>set1$id</code> to <code>set1</code>, ignoring the <code>id</code> in calling scope. If so, there are several approaches to avoid scoping issues such as this : <pre><code>.id = <your 4M ids> set1[J(.id)] </code></pre> or use the fact that a single name <code>i</code> is evaluated in calling scope : <pre><code>JDT=J(id); set1[JDT] </code></pre> or that <code>eval</code> is <code>eval</code>'d in calling scope, too : <pre><code>set1[eval(J(id))] </code></pre> or, we do want to make this clearer, more robust and easier, so one thought is to add <code>..</code> : <pre><code>set1[..(J(id))] # .. alias for eval </code></pre> or perhaps : <pre><code>set1[J(..id)] </code></pre> where <code>..</code> borrows its meaning from the file system's <code>..</code>, meaning one-level-up. If the <code>..</code> was a prefix to symbols, you could then do something like : <pre><code>DT[colB==..id] </code></pre> where <code>==</code> is used there for illustration. In that example <code>colB</code> is expected to be a column name and <code>..id</code> will find <code>id</code> in calling scope (one level up). The thinking is that that would be quite clear to the reader of the code what the programmer intended.
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1. POHow to pass a parameter by variable into data.table[J()]
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1. PTAnswer
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1. USMatt Dowle
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1. USMatt Dowle
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1. POHow to pass a parameter by variable into data.table[J()]
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1. COHa! Yeah, that's exactly it. Thanks very much! Blast it, in retrospect that seems screechingly obvious.
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