为什么knitr缓存失败data.table`：=`？ [英] why does knitr caching fail for data.table `:=`?

问题描述

这在精神上与此问题有关，但在机制上必须有所不同。

This is related in spirit to this question, but must be different in mechanism.

如果您尝试缓存包含 data.table <$>的 knitr c $ c>：= assignement，那么它的行为好像该块尚未运行，后来的块没有看到：= 。

If you try to cache a knitr chunk that contains a data.table := assignement then it acts as though that chunk has not been run, and later chunks do not see the affect of the :=.

任何想法为什么会是这样？ knitr 如何检测对象已更新，什么是 data.table 这样做会导致混淆？

Any idea why this is? How does knitr detect objects have updated, and what is data.table doing that confuses it?

看来你可以通过 DT = DT [，LHS：= RHS] 来解决这个问题。

It appears you can work around this by doing DT = DT[, LHS:=RHS].

：

Example:

```{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] 
```
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, 
from the second time onwards it will not.

第二次投放时输出

推荐答案

猜测：

这是看上去正在发生的事。

Speculation:

Here is what appears to be going on.

strong> knitr 相当明智地缓存对象，只要它们被创建。然后，当它检测到它们已经被改变时，它更新它们的缓存值。

knitr 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.

data.table 虽然绕过了R的正常的按值分配和替换机制，并使用 ：= 运算符，而不是 = ，< < - c $ c>< - 。因此， knitr 未收到 DT 已被 DT [，c：= 5]更改的信号] 。

data.table, though, bypasses R's normal copy-by-value assignment and replacement mechanisms, and uses a := operator rather than a =, <<-, or <-. As a result knitr isn't picking up the signals that DT has been changed by DT[, c:=5].

只需将此块添加到您的代码'd喜欢 DT 的当前值被重新缓存。它不会花你任何内存或时间（因为除了引用被复制 DT < - DT ），但它有效地发送一个（假的）信号到 knitr ， DT 已更新：

Just add this block to your code wherever you'd like the current value of DT to be re-cached. It won't cost you anything memory or time-wise (since nothing except a reference is copied by DT <- DT) but it does effectively send a (fake) signal to knitr that DT has been updated:

```{r, cache=TRUE, echo=FALSE}
DT <- DT 
```

---

示例doc的工作版本：

您的文档的已修改版本：

---

Working version of example doc:

Check that it works by running this edited version of your doc:

```{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 <- 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.

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