(至少对于一维向量)[ 似乎返回 NA 当 "i > length(x)" 而 [[ 返回错误.

(at least regarding 1D vectors) [ seems to return NA when "i > length(x)" whereas [[ returns an error.

x = runif(5)
x[6]
#[1] NA
x[[6]]
#Error in x[[6]] : subscript out of bounds

挖掘一下，do_subset_dflt(即[)调用ExtractSubset 我们注意到当一个想要的索引 ("ii") 是 ">length(x)" NA 被返回(稍微修改为干净):

Digging a bit, do_subset_dflt (i.e. [) calls ExtractSubset where we notice that when a wanted index ("ii") is "> length(x)" NA is returned (a bit modified to be clean):

if(0 <= ii && ii < nx && ii != NA_INTEGER)
    result[i] = x[ii];
else
    result[i] = NA_INTEGER;

另一方面do_subset2_dflt(即 [[) 如果想要的索引 ("offset") 是 ">length(x)" 则返回错误(稍微修改为干净):

On the other hand do_subset2_dflt (i.e. [[) returns an error if the wanted index ("offset") is "> length(x)" (modified a bit to be clean):

if(offset < 0 || offset >= xlength(x)) {
    if(offset < 0 && (isNewList(x)) ...
    else errorcall(call, R_MSG_subs_o_b);
}

where #define R_MSG_subs_o_b _("下标越界")

(我不确定上面的代码片段，但根据他们的回报，它们确实是相关的)

(I'm not sure about the above code snippets but they do seem relevant based on their returns)