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    <p><strong>Update Oct 2014</strong>. Now in data.table v1.9.5 :</p> <blockquote> <p><code>:=</code> no longer prints in <code>knitr</code> for consistency with behaviour at the prompt, <a href="https://github.com/Rdatatable/data.table/issues/505" rel="nofollow noreferrer">#505</a>. Output of a test <code>knit("knitr.Rmd")</code> is now in data.table's unit tests.</p> </blockquote> <p>and related :</p> <blockquote> <p><code>if (TRUE) DT[,LHS:=RHS]</code> now doesn't print (thanks to Jureiss, <a href="https://github.com/Rdatatable/data.table/issues/869" rel="nofollow noreferrer">#869</a>). Test added. To get this to work we've had to live with one downside: if a <code>:=</code> is used inside a function with no <code>DT[]</code> before the end of the function, then the next time <code>DT</code> is typed at the prompt, nothing will be printed. A repeated <code>DT</code> will print. To avoid this: include a <code>DT[]</code> after the last <code>:=</code> in your function. If that is not possible (e.g., it's not a function you can change) then <code>print(DT)</code> and <code>DT[]</code> at the prompt are guaranteed to print. As before, adding an extra <code>[]</code> on the end of a <code>:=</code> query is a recommended idiom to update and then print; e.g. <code>&gt; DT[,foo:=3L][]</code></p> </blockquote> <hr> <hr> <p>Previous answer kept for posterity (the <code>global$depthtrigger</code> business is no longer done as from data.table v1.9.5 so this is no longer true) ...</p> <p>Just to be clear I understand then: <code>knitr</code> is printing when you don't want it to.</p> <p>Try increasing <code>data.table:::.global$depthtrigger</code> a little bit at the start of the script.</p> <p>This will be 3 for you currently :</p> <pre><code>data.table:::.global$depthtrigger [1] 3 </code></pre> <p>I don't know how much eval depth <code>knitr</code> adds to the stack. But try changing the trigger to 4 first; i.e.</p> <pre><code>assign("depthtrigger", 4, data.table:::.global) </code></pre> <p>and at the end of the <code>knitr</code> script ensure to set it back to 3. If 4 doesn't work, try 5, then 6. If you get to 10 give up and I'll think again. ;-P</p> <p>Why might this work?</p> <p>See NEWS from v1.8.4 :</p> <blockquote> <p><code>DT[,LHS:=RHS,...]</code> no longer prints <code>DT</code>. This implements #2128 "Try again to get <code>DT[i,j:=value]</code> to return invisibly". Thanks to discussions here :<br> <a href="https://stackoverflow.com/questions/11359553/how-to-suppress-output-when-using-in-r-data-table">how to suppress output when using `:=` in R {data.table}, prior to v1.8.3?</a> <br> <a href="http://r.789695.n4.nabble.com/Avoiding-print-when-using-tp4643076.html" rel="nofollow noreferrer">http://r.789695.n4.nabble.com/Avoiding-print-when-using-tp4643076.html</a> <br> FAQs 2.21 and 2.22 have been updated.</p> <p><em>FAQ 2.21 Why does DT[i,col:=value] return the whole of DT? I expected either no visible value (consistent with &lt;-), or a message or return value containing how many rows were updated. It isn't obvious that the data has indeed been updated by reference.</em> <br> This has changed in v1.8.3 to meet your expectations. Please upgrade. The whole of DT is returned (now invisibly) so that compound syntax can work; e.g., DT[i,done:=TRUE][,sum(done)]. The number of rows updated is returned when verbosity is on, either on a per query basis or globally using options(datatable.verbose=TRUE).</p> <p><em>FAQ 2.22 Ok, thanks. What was so difficult about the result of DT[i,col:=value] being returned invisibly?</em> <br> R internally forces visibility on for [. The value of FunTab's eval column (see src/main/names.c) for [ is 0 meaning force R_Visible on (see R-Internals section 1.6). Therefore, when we tried invisible() or setting R_Visible to 0 directly ourselves, eval in src/main/eval.c would force it on again. To solve this problem, the key was to stop trying to stop the print method running after a :=. Instead, inside := we now (from v1.8.3) set a global flag which the print method uses to know whether to actually print or not.</p> </blockquote> <p>That global flag is <code>data.table:::.global$print</code>. At the top of <code>data.table:::print.data.table</code> you'll see it looking at it. That's because there is no known way to suppress printing from <code>[</code> (as FAQ 2.22 explains).</p> <p>So, inside <code>:=</code> inside <code>[.data.table</code> it looks to see how "deep" this call is :</p> <pre><code>if (Cstack_info()[["eval_depth"]] &lt;= .global$depthtrigger) { suppPrint = function(x) { .global$print=FALSE; x } # Suppress print when returns ok not on error, bug #2376. # Thanks to: https://stackoverflow.com/a/13606880/403310 # All appropriate returns following this point are # wrapped i.e. return(suppPrint(x)). } </code></pre> <p>Essential that's just saying: if <code>DT[,x:=y]</code> is running at the prompt, then I know the REPL is going to call the <code>print</code> method on my result, beyond my control. Ok, so given <code>print</code> method is going to run, I'm going to suppress it inside that <code>print</code> method by setting a flag (since the <code>print</code> method that runs (i.e. <code>print.data.table</code>) is something I can control).</p> <p>In <code>knitr</code>'s case it's simulating the REPL in a clever way. It isn't really a script, iiuc, otherwise <code>DT[,x:=y]</code> wouldn't print anyway for that reason. But because it's simulating REPL via an <code>eval</code> there is an extra level of <code>eval</code> depth for code run from <code>knitr</code>. Or something similar (I don't know <code>knitr</code>).</p> <p>Which is why I'm thinking increasing the <code>depthtrigger</code> might do the trick.</p> <p>Hacky/crufty, I agree. But if it works, and you let me know which value works, I can change <code>data.table</code> to be <code>knitr</code> aware and change the <code>depthtrigger</code> automatically. Or any better solutions are most welcome.</p>
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    1. COaargh! I just spent half an hour digging that out of the source code! Why is testing against C stack depth a good test for whether to invisibilise the result?
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    2. CO@Spacedman It's probably not _good_ per se. But it was the only way I found so far to know whether the result returned by this call to `[.data.table` is about to printed by the REPL. The internals of `invisible()` also set a global flag, btw. Basically, R ignores the invisibility of `[.class`'s result and prints it anyway (FAQ 2.22).
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    3. COJust done this, and it works with the caveat that `depthtrigger = 53` is the smallest number that prevents printing!
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