以不同方式用数字索引设置data.table列的结果不同 [英] Different results when subsetting data.table columns with numeric indices in different ways

问题描述

请参阅最小示例：

library(data.table)
DT <- data.table(x = 2, y = 3, z = 4)

DT[, c(1:2)]  # first way
#    x y
# 1: 2 3

DT[, (1:2)]  # second way
# [1] 1 2

DT[, 1:2]  # third way
#    x y
# 1: 2 3

此发布，现在可以使用数字索引对列进行子设置。但是，我想知道为什么索引以第二种方式而不是列索引来计算为向量？

As described in this post, subsetting columns with numeric indices is possible now. However, I would like to known why indices are evaluated to a vector in the second way rather than column indices?

此外，我更新了 data.table 刚刚：

> sessionInfo()
R version 3.4.4 (2018-03-15)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 16.04.4 LTS

Matrix products: default
BLAS: /usr/lib/atlas-base/atlas/libblas.so.3.0
LAPACK: /usr/lib/atlas-base/atlas/liblapack.so.3.0

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
 [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8    LC_PAPER=en_US.UTF-8       LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C             LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] data.table_1.11.2

loaded via a namespace (and not attached):
[1] compiler_3.4.4 tools_3.4.4    yaml_2.1.17

推荐答案

通过查看在源代码中，我们可以模拟不同输入的data.tables行为

By looking at the source code we can simulate data.tables behaviour for different inputs

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(av<-all.vars(jsub)) || all(substring(av,1L,2L)=="..")) &&
          root %chin% c("","c","paste","paste0","-","!") &&
          missing(by) )) {   # test 763. TODO: likely that !missing(by) iff with==TRUE (so, with can be removed)
      # When no variable names (i.e. symbols) occur in j, scope doesn't matter because there are no symbols to find.
      # If variable names do occur, but they are all prefixed with .., then that means look up in calling scope.
      # Automatically set with=FALSE in this case so that DT[,1], DT[,2:3], DT[,"someCol"] and DT[,c("colB","colD")]
      # work as expected.  As before, a vector will never be returned, but a single column data.table
      # for type consistency with >1 cases. To return a single vector use DT[["someCol"]] or DT[[3]].
      # The root==":" is to allow DT[,colC:colH] even though that contains two variable names.
      # root == "-" or "!" is for tests 1504.11 and 1504.13 (a : with a ! or - modifier root)
      # We don't want to evaluate j at all in making this decision because i) evaluating could itself
      # increment some variable and not intended to be evaluated a 2nd time later on and ii) we don't
      # want decisions like this to depend on the data or vector lengths since that can introduce
      # inconistency reminiscent of drop=TRUE in [.data.frame that we seek to avoid.
      with=FALSE

基本上， [。data.table 捕获传递给 j 的表达式，并根据一些预定义的规则来决定如何处理它。如果满足其中一个规则，它将设置 with = FALSE ，这基本上意味着列名已传递给 j ，

Basically, "[.data.table" catches the expression passed to j and decides how to treat it based on some predefined rules. If one of the rules is satisfied, it sets with=FALSE which basically means that column names were passed to j, using standard evaluation.

规则大致如下：

1. 设置 with = FALSE ，

1.1。如果 j 表达式是一个调用并且该调用是：或

1.1. if j expression is a call and the call is : or

1.2。如果呼叫是 c（-，！）和（和：或

1.2. if the call is a combination of c("-","!") and ( and : or

1.3。如果某些值（字符，整数，数字等）或。 传递给 j ，调用位于 c（， c， paste， paste0 ，-，！），并且没有 by 呼叫

1.3. if some value (character, integer, numeric, etc.) or .. was passed to j and the call is in c("","c","paste","paste0","-","!") and there is no a by call

否则将 with = TRUE

设置为将其转换为函数，然后查看是否满足任何条件（我已跳过了将。转换为 list 功能，因为它在这里无关紧要。我们将直接使用 list 进行测试）

So we can convert this into a function and see if any of the conditions were satisfied (I've skipped the converting the . to list function as it is irrelevant here. We will just test with list directly)

is_satisfied <- function(...) {
  jsub <- substitute(...)
  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(av<-all.vars(jsub)) || all(substring(av,1L,2L)=="..")) &&
      root %chin% c("","c","paste","paste0","-","!"))) TRUE else FALSE
}

is_satisfied("x")
# [1] TRUE
is_satisfied(c("x", "y"))
# [1] TRUE
is_satisfied(..x)
# [1] TRUE
is_satisfied(1:2)
# [1] TRUE
is_satisfied(c(1:2))
# [1] TRUE
is_satisfied((1:2))
# [1] FALSE
is_satisfied(y)
# [1] FALSE
is_satisfied(list(x, y))
# [1] FALSE

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