在data.table包中使用.BY和.EACHI

问题描述

我想更好地了解 data.table 包中的一些特殊变量是如何工作的。其中之一是 .BY 语句。我没有看到很多人使用它的例子，但文档暗示这是有用的绘图。

I am trying to get a better grasp on how some of the special variables in the data.table package work. One of these is the .BY statement. I have not seen a lot of examples of people using it, but the documentation implies that is is useful in plotting.

例如，以下代码似乎工作得很好（显示每个物种的情节，并为每个情节指定正确的标题）in data.table 1.9.3 ：

For example, the following code seems to work quite well (showing a plot for each of the species and assigning the right title to each plot) in data.table 1.9.3:

iris <- data.table(iris)
iris[,plot(Sepal.Length~Sepal.Width,main=unlist(.BY)),by=Species]

虽然此代码无法按我的意图工作：

While this code does not work as intended by me:

iris[,plot(Sepal.Length~Sepal.Width,main=.BY),by=Species]

为什么这两种不同？从注释，它似乎不是一个问题在 data.table 1.9.2 。使用 .BY 语句还有什么其他方法可能有用吗？与 .EACHI 语句相比有何不同？

Why are these two different? From the comments, it does not seem to be an issue in data.table 1.9.2. In what other ways might it be useful to use the .BY statement? How is this different compared to the .EACHI statement?

推荐答案

code> .BY 是包含的值的 列表 变量。

.BY is a named list containing the values of the by variables.

将未命名的列表传递给 main >命名列表将失败（完全与 data.table

Passing an unnamed list to main will work, however a named list will fail (wholly unrelated to data.table

plot(1, main = list(1))
# works....
plot(1, main = list(s=1))
# Error in title(...) : invalid graphics parameter

最近提交了data.table 1.9.3修复了在.BY
中命名的错误。。

There is a recent commit to data.table 1.9.3 which fixed a bug to do with naming in `.BYCloses bug #5415. .BY gets names attribute set properly in april this year.

如果你有超过1个by变量，你可以连接一些如何

If you had more than 1 "by" variable, you would want to be able to concatenate some how

也许

iris[,plot(Sepal.Length~Sepal.Width,main=do.call(paste,.BY)),by=Species]

（除非您有 collapse ！）

EACHI 与此完全无关。请阅读的新闻以了解这一点。

EACHI is completely unrelated to this. Please read the NEWS for data.table 1.9.3 for an understanding of this.

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