对于命名向量和矩阵，[[ 是否在不传递 exact=FALSE 参数的情况下使用部分匹配？

Question

我最近读了 this section of the R Language Definition，变得很困惑。它说：

For vectors and matrices the [[ forms are rarely used, although they have some slight semantic differences from the [ form (e.g. it drops any names or dimnames attribute, and that partial matching is used for character indices).

我看错了吗？似乎说 [[ 允许对命名向量和矩阵进行部分匹配（大概是默认情况下）。但是，我知道这显然是错误的：

> lett<-setNames(sample(26),paste0(letters,letters,letters))
> lett[["aaa"]]#No partial match
[1] 23
> lett[["a"]]#Error
Error in lett[["a"]] : subscript out of bounds

那么语言定义是什么意思？它可能是在谈论您可以设置的 exact=FALSE 标志，但这似乎是一种非常奇怪的方式。事实上，a later section 提出了以下声明，我上面的代码反驳了这一点，所以我更加困惑：

For [[ and $ partial matching is used if exact matching fails, so x$aa will match x$aabb if x does not contain a component named "aa" and "aabb" is the only name which has prefix "aa". For [[, partial matching can be controlled via the exact argument which defaults to NA indicating that partial matching is allowed, but should result in a warning when it occurs.

语言定义是否过时了？

Answer 1

实际上，我认为语言定义 - 至少部分 - 确实过时了。 help("[[") 关于 exact 参数的帮助页面指出

Controls possible partial matching of [[ when extracting by a character vector [...]. The default is no partial matching. Value NA allows partial matching but issues a warning when it occurs. Value FALSE allows partial matching without any warning.

用法支持这一说法：

x[[i, exact = TRUE]]
x[[i, j, ..., exact = TRUE]]

以下代码也证明了这些默认值。

set.seed(1)
lsub <- letters[1:3]
lett <- setNames(lapply(sample(3), c), paste0(lsub, lsub, lsub))
lett
#> $aaa
#> [1] 1
#> 
#> $bbb
#> [1] 3
#> 
#> $ccc
#> [1] 2

# partial matching
lett$a
#> [1] 1
lett[["aa", exact = FALSE]]
#> [1] 1

# no partial matching
lett[["aa"]]
#> NULL

# partial matching with warning
lett[["aa", exact = NA]]
#> Warning in lett[["aa", exact = NA]]: partial match of 'aa' to 'aaa'
#> [1] 1

对于命名向量和矩阵，[[ 是否在不传递 exact=FALSE 参数的情况下使用部分匹配？

For named vectors and matrices, does [[ ever use partial matching without passing the exact=FALSE argument?

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names

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