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    <p>If you are going to be doing complicated operations inside your <code>j</code> expressions, you should probably use <code>eval</code> and <code>quote</code>. One problem with that in current version of <code>data.table</code> is that the environment of <code>eval</code> is not always correctly processed - <a href="https://stackoverflow.com/questions/15913832/eval-and-quote-in-data-table">eval and quote in data.table</a> (Note: There has been an update to that answer based on an update to the package.) - and the current fix for that is to add <code>.SD</code> to <code>eval</code>. As far as I can tell from a few tests that I've run this doesn't affect speed (the way e.g. having <code>.SD[1]</code> in <code>j</code> would).</p> <p>Interestingly this issue only plagues the <code>j</code> and you'll be fine using <code>eval</code> normally in <code>i</code> (where <code>.SD</code> is not available anyway).</p> <p>The other problem is assignment, and there you have to have strings. I know one way to extract the string name from a quoted expression - it's not pretty, but it works. Here's an example combining everything together:</p> <pre><code>x = data.table(dist = c(1:10), val = c(1:10)) distcol = quote(dist) valcol = quote(val) x[eval(valcol) &lt; 5, capture.output(str(distcol, give.head = F)) := eval(distcol)*sum(eval(distcol, .SD))] </code></pre> <p>Note how I was ok not adding <code>.SD</code> in one <code>eval(distcol)</code>, but won't be if I take it out of the other <code>eval</code>.</p> <p>Another option is to use <code>get</code>:</p> <pre><code>diststr = "dist" valstr = "val" x[get(valstr) &lt; 5, c(diststr) := get(diststr)*sum(get(diststr))] </code></pre>
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    1. COThanks a lot for the explanation. However as I mentioned in my comment to @Frank above, my problem is with using `quote(dist)` and `quote(val)`. In my case I don't have access to these, only to variables `distcol="dist"` and `valcol="val"`. Is there a solution for this?
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    2. COYou can go the other way, I guess, from string to `quote`-thing.
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    3. CO@msp added a version for that - it works for this simple example, but expect trouble ahead :)
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