从子表化数据表中检索特定值 [英] Retrieving specific values from subsetting a data.table

问题描述

我在R中有一个 data.table ：

my.dt <- data.table(x=seq(1:5),y=(c(TRUE, TRUE, FALSE, FALSE, FALSE)))

我想从中提取单个值或一个向量值：

I want to extract a single value, or a vector of values from this:

boolean.vector <- my.dt[x<4,"y",with=FALSE]
boolean.value <- my.dt[x<2,"y",with=FALSE]

$ b $ p

但是，这些返回值仍然属于 data.table 。因此，我不能，例如，做以下：

However, these return values are still of class data.table. As such, I cannot, for example, do the following:

> if(boolean.value) { print("Hello") }
Erro em if (boolean.value) { : argumento não é interpretável como lógico
# (Error in if (boolean.value) { : argument cannot be interpreted as logical)

如何检索原始值，方式？

How can I retrieve the raw values so I can use them in this manner?

推荐答案

而不是

my.dt[1:3,"y",with=FALSE]

只是做

my.dt$y[1:3]

或

my.dt[["y"]][1:3]

或

my.dt[x<2,"y",with=FALSE][[1]]

在基本R [[不复制向量（它是副本on-change），所以这是非常高效的，不需要为此使用 data.table 语法。

In base R [[ doesn't copy the vector (it's copy-on-change), so this is very efficient and there isn't any need to use data.table syntax for this.

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