data.table 列的类 [英] Class of data.table column

问题描述

我想知道如何在给定字符向量 w 的情况下确定 data.table dt 中列的类别.

I'd like to know how to ascertain the class of a column in a data.table dt given a character vector w.

可重现的例子:

dt <- data.table(matrix(1:10, 2))
w <- "V1"

当你直接按名称指定列时，它会返回向量，以便你获取它的类:

When you specify a column by name directly, it returns the vector so that you can get its class:

> dt[,V1]
[1] 1 2
> class(dt[,V1])
[1] "integer"

但是，将其指定为字符向量，它会返回一列 data.table:

Specify it as a character vector, however, and it instead returns a one-column data.table:

> dt[,w,with=FALSE]
   V1
1:  1
2:  2
> class(dt[,w,with=FALSE])
[1] "data.table" "data.frame"

我已经尝试了以下解决方案，但肯定有更好的方法:

I've sort of munged my way to the following solution, but surely there's a better way:

dt[,eval(parse(text=paste0("class(",w,")")))]

那么两个问题:

1. 是否有更好(更简洁)的方法来获取单列的类(不放弃上述解决方案通过在数据环境中评估对 class 的调用所获得的速度.桌子?
2. 有没有办法获取所有列的类的向量，类似于 sapply(myDataFrame, class) ?

1. Is there a better (more concise) to get the class of a single column (withoout giving up the speed that the above solution gains by evaluating the call to class in the environment of the data.table?
2. Is there a way to get a vector of the classes of all columns, analagous to sapply( myDataFrame, class) ?

推荐答案

这些似乎以你想要的方式工作:

These seem to work in the way you want:

1. 类(dt[[w]])
2. sapply(dt,class)

此外，执行 2 然后对 1 进行子集化:sapply(dt,class)[w].

Also, doing 2 and then subsetting works for 1: sapply(dt,class)[w].

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