list 如何从 R 中的列表列表中提取元素？
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How can I extract elements from lists of lists in R?
提问by Akos
I have a bunch of lists containing lists within them (generalised linear model output). I want to write a function which will extract several elements from each list and then combine the results into a data frame.
我有一堆列表，其中包含列表（广义线性模型输出）。我想编写一个函数，该函数将从每个列表中提取多个元素，然后将结果组合到一个数据框中。
I want to extract modelset[[1]]$likelihood& modelset[[1]]$fixef, modelset[[2]]$likelihood& modelset[[2]]$fixef, etc, and combine the results into a data frame.
我想提取modelset[[1]]$likelihood& modelset[[1]]$fixef、modelset[[2]]$likelihood&modelset[[2]]$fixef等，并将结果组合成一个数据框。
Can someone give me an idea of how to do this?
有人可以给我一个如何做到这一点的想法吗？
Apologies if my question is confusing: what I am trying to do is beyond my limited programming understanding.
如果我的问题令人困惑，我深表歉意：我想要做的超出了我有限的编程理解。
Further information about my list:
关于我的名单的更多信息：
modelset: Large list (16 elements, 7.3Mb)
:List of 29
..$ fixef : Named num [1:2] -1.236 -0.611
.. ..- attr(*, "names")= chr [1:2] "(Intercept)" "SMIstd"
..$ likelihood :List of 4
.. ..$ hlik: num 238
.. ..$ pvh : num 256
.. ..$ pbvh: num 260
.. ..$ cAIC: num 567
...etc
回答by Konrad Rudolph
In order to solve this elegantly you need to understand that you can use ['…']instead of $…to access list elements (but you will get a list back instead of an individual element).
为了优雅地解决这个问题，您需要了解您可以使用['…']而不是$…访问列表元素（但您将返回一个列表而不是单个元素）。
So if you want to get the elements likelihoodand fixef, you can write:
所以如果你想获取元素likelihoodand fixef，你可以写：
modelset[[1]][c('likelihood', 'fixef')]
Now you want to do that for each element in modelset. That's what lapplydoes:
现在您想对modelset. 这就是lapply它的作用：
lapply(modelset, function (x) x[c('likelihood', 'fixef')])
This works, but it's not very R-like.
这有效，但它不是很像 R。
You see, in R, almost everythingis a function. […]is calling a function named [(but since [is a special symbol for R, in needs to be quoted in backticks: `[`). So you can instead write this:
你看，在 R 中，几乎所有东西都是函数。[…]正在调用一个名为的函数[（但由于它[是 R 的特殊符号，因此需要在反引号中引用：）`[`。所以你可以这样写：
lapply(modelset, function (x) `[`(c('likelihood', 'fixef')])
Wow, that's not very readable at all. However, we can now remove the wrapping anonymous function (x), since inside we're just calling another function, and move the extra arguments to the last parameter of lapply:
哇，这根本不是很可读。但是，我们现在可以删除包装匿名function (x)，因为在内部我们只是调用另一个函数，并将额外的参数移动到的最后一个参数lapply：
lapply(modelset, `[`, c('likelihood', 'fixef'))
This works and is elegant R code.
这有效并且是优雅的 R 代码。
Let's step back and re-examine what we did here. In effect, we had an expression which looked like this:
让我们退后一步，重新审视我们在这里所做的事情。实际上，我们有一个如下所示的表达式：
lapply(some_list, function (x) f(x, y))
And this call can instead be written as
而这个调用可以写成
lapply(some_list, f, y)
We did exactly that, with somelist = modelset, f = `[`and y = c('likelihood', 'fixef').
我们正是这样做的，使用somelist = modelset,f = `[`和y = c('likelihood', 'fixef')。
