选择列序列:`:` 有效，但 `seq` 无效 [英] Select a sequence of columns: `:` works but not `seq`

问题描述

我正在尝试通过从 data.table 中选择一些列来对数据集进行子集化.但是，我的代码不适用于某些变体.

I'm trying to subset a dataset by selecting some columns from a data.table. However, my code does not work with some variations.

这是一个示例数据表

library(data.table)
DT <- data.table( ID = 1:50,
            Capacity = sample(100:1000, size = 50, replace = F),
            Code = sample(LETTERS[1:4], 50, replace = T),
            State = rep(c("Alabama","Indiana","Texas","Nevada"), 50))

这是一个工作子集代码，其中列的数字序列使用 ::

Here is a working subset code, where a numeric sequence of columns is specified using ::

DT[ , 1:2]

但是，使用 seq 指定 same 列序列不起作用:

However, specifying the same sequence of columns using seq does not work:

DT[ , seq(1:2)]

请注意，这适用于数据框，但不适用于 data.table.

Note that this works with a dataframe but not with a data.table.

我需要类似于第二种格式的东西，因为我正在根据 grep() 的输出进行子集化，它提供与第二种格式相同的输出.我做错了什么?

I need something along the lines of the second format because I'm subsetting based on the output of grep() and it gives the same output as the second format. What am I doing incorrectly?

谢谢！

推荐答案

在最新版本的data.table中，可以在j中使用数字来指定列.此行为包括诸如 DT[,1:2] 之类的格式来指定列的数字范围.(请注意，此语法不适用于旧版本的 data.table).

On recent versions of data.table, numbers can be used in j to specify columns. This behaviour includes formats such as DT[,1:2] to specify a numeric range of columns. (Note that this syntax does not work on older versions of data.table).

那么为什么 DT[ , 1:2] 有效，而 DT[ , seq(1:2)] 无效?答案隐藏在 data.table:::[.data.table 的代码中，其中包括以下几行:

So why does DT[ , 1:2] work, but DT[ , seq(1:2)] does not? The answer is buried in the code for data.table:::[.data.table, which includes the lines:

  if (!missing(j)) {
    jsub = replace_dot_alias(substitute(j))
    root = if (is.call(jsub)) 
      as.character(jsub[[1L]])[1L]
    else ""
    if (root == ":" || (root %chin% c("-", "!") && is.call(jsub[[2L]]) && 
        jsub[[2L]][[1L]] == "(" && is.call(jsub[[2L]][[2L]]) && 
        jsub[[2L]][[2L]][[1L]] == ":") || (!length(all.vars(jsub)) && 
            root %chin% c("", "c", "paste", "paste0", "-", "!") && 
            missing(by))) {
      with = FALSE
    }

我们可以看到，data.table在检测到使用:函数时，会自动为你设置with = FALSE参数在 j 中.它没有为 seq 内置相同的功能，所以如果我们想使用 seq，我们必须自己指定 with = FALSE语法.

We can see here that data.table is automatically setting the with = FALSE parameter for you when it detects the use of function : in j. It doesn't have the same functionality built in for seq, so we have to specify with = FALSE ourselves if we want to use the seq syntax.

DT[ , seq(1:2), with = FALSE]

这篇关于选择列序列:`:` 有效，但 `seq` 无效的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！