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    <h2>Speculation:</h2> <p>Here is what appears to be going on.</p> <p><strong>knitr</strong> quite sensibly caches objects as as soon as they are created. It then updates their cached value whenever it detects that they have been altered. </p> <p><strong>data.table</strong>, though, bypasses R's normal copy-by-value assignment and replacement mechanisms, and uses a <code>:=</code> operator rather than a <code>=</code>, <code>&lt;&lt;-</code>, or <code>&lt;-</code>. As a result <strong>knitr</strong> isn't picking up the signals that <code>DT</code> has been changed by <code>DT[, c:=5]</code>.</p> <h2>Solution:</h2> <p>Just add this block to your code wherever you'd like the current value of <code>DT</code> to be re-cached. It won't cost you anything memory or time-wise (since nothing except a reference is copied by <code>DT &lt;- DT</code>) but it does effectively send a (fake) signal to <strong>knitr</strong> that <code>DT</code> has been updated:</p> <pre><code>```{r, cache=TRUE, echo=FALSE} DT &lt;- DT ``` </code></pre> <hr> <h2>Working version of example doc:</h2> <p>Check that it works by running this edited version of your doc:</p> <pre><code>```{r} library(data.table) ``` Data.Table Markdown ======================================================== Suppose we make a `data.table` in **R Markdown** ```{r, cache=TRUE} DT = data.table(a = rnorm(10)) ``` Then add a column using `:=` ```{r, cache=TRUE} DT[, c:=5] ``` ```{r, cache=TRUE, echo=FALSE} DT &lt;- DT ``` Then we display that in a non-cached block ```{r, cache=FALSE} DT ``` The first time you run this, the above will show a `c` column. The second, third, and nth times, it will as well. </code></pre>
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    1. POwhy does knitr caching fail for data.table `:=`?
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    1. USJosh O'Brien
    1. USJosh O'Brien
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    1. POwhy does knitr caching fail for data.table `:=`?
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    1. COThanks for that - any guesses what would be involved in making `knitr` pick up on `:=`. Would it be a matter of making `data.table` give the correct signals, or making `knitr` watch out for `:=`?
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      1. USCorone
    2. CO@Corone in this case it sounds like a good idea to manually assign the object names to `knitr` to cache; I can consider it if you file a feature request: https://github.com/yihui/knitr/issues
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      1. USYihui Xie
    3. CO@Yihui Hi. If there's something I can change on the `data.table` side just let me know.
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      1. USMatt Dowle
 

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