为什么rapply和lapply处理NULL的方式不同? [英] Why do rapply and lapply handle NULL differently?

问题描述

我知道列表中的NULL值有时可以

I'm aware that NULL values in lists can sometimes trip people up. I'm curious why in a specific instance lapply and rapply seem to treat NULL values differently.

l <- list(a = 1, c = NULL, d = 3)

lapply(l,is.null)
$a
[1] FALSE

$c
[1] TRUE

$d
[1] FALSE

到目前为止，一切都很好.如果我们用rapply做完全相同的事情怎么办?

So far so good. How about if we do the exact same thing with rapply?

rapply(l, is.null, how = "replace")
$a
[1] FALSE

$c
list()

$d
[1] FALSE

此示例非常简单且非递归，但您在带有嵌套列表的rapply中看到了相同的行为.

This example is very simple and non-recursive, but you see the same behavior in rapply with nested lists.

我的问题是为什么?如果如?rapply中所述，它是"lapply的递归版本"，为什么在这种情况下它们的行为如此不同?

My question is why? If, as advertised in ?rapply, it is a 'recursive version of lapply', why do they behave so differently in this case?

推荐答案

我认为您回答了自己的问题:因为它是递归的.

I think you answered your own question: because it's recursive.

您通常不会看到它，但是NULL实际上可以用来表示一个空序列，因为它是空的pairlist(类似于Scheme中()终止列表的方式.在内部，R是很有计划).

You don't often see this, but NULL can actually be used to indicate an empty sequence, because it is the empty pairlist (similar to how () in Scheme terminates a list. Internally, R is very scheme like).

因此，rapply递归到空列表中，但是完成后不会费心将其重新变成对列表；您会得到一个常规的空列表.

So, rapply recurses into the empty list, but doesn't bother turning it back into a pairlist when it's done; you get a regular empty list.

实际上，rapply和lapply对待NULL的区别并不大:

Actually, rapply and lapply don't really treat NULL that differently:

> lapply(NULL, identity)
list()

您可以在R源代码中看到(内存. c )，这正是配对表的工作方式:

And you can see in the R source code (memory.c) that this is exactly how pairlists are meant to work:

SEXP allocList(int n)
{
    int i;
    SEXP result;
    result = R_NilValue;
    for (i = 0; i < n; i++)
        result = CONS(R_NilValue, result);
    return result;
}

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