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POReferring to data.table columns by names saved in variables
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  1. POReferring to data.table columns by names saved in variables
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    copied!<p><code>data.table</code> is a fantastic R package and I am using it in a library I am developing. So far all is going very well, except for one complication. It seems to be much more difficult (compared to the conventional data frames) to refer to <code>data.table</code> columns using names saved in variables (as for data frames would be, for example: <code>colname="col"; df[df[,colname]&lt;5,colname]=0</code>). </p> <p>Perhaps what complicates the things most is the apparent lack of consistency of syntax on this in <code>data.table</code>. In some cases, <code>eval(colname)</code> and <code>get(colname)</code>, or even <code>c(colname)</code> seem to work. In others, <code>DT[,colname, with=F]</code> is the solution. Yet in others, such as, for example, the <code>set()</code> and <code>subset()</code> functions, I haven't found a solution at all. Finally, an extreme, albeit also quite common use case was discussed earlier (<a href="https://stackoverflow.com/questions/15009669/passing-column-names-to-data-table-programmatically">passing column names to data.table programmatically</a>) and the proposed solutions, albeit apparently doing their job, did not seem particularly readable... </p> <p>Perhaps I am complicating things too much? If anyone could jot down a quick cheatsheet for referring to <code>data.table</code> column names using variables for different common scenarios, I would be very grateful. </p> <p><strong>UPDATE:</strong></p> <p>Some specific examples that work provided I can hard code column names:</p> <pre><code>x.short = subset(x, abs(dist)&lt;=100) set(x, which(x$val&lt;10), "val", 0) </code></pre> <p>Now assume <code>distcol="dist"</code>, <code>valcol="val"</code>. What is the best way to do the above using <code>distcol</code> and <code>valcol</code>, but not <code>dist</code> and <code>val</code>?</p>
 

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