list 计算列表中对象的数量
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Count number of objects in list
提问by Karl
R function that will return the number of items in a list?
R函数将返回列表中的项目数？
回答by Joey
Get or set the length of vectors (including lists) and factors, and of any other R object for which a method has been defined.
获取或设置向量（包括列表）和因子的长度，以及已为其定义方法的任何其他 R 对象的长度。
Get the length of each element of a list or atomic vector (is.atomic) as an integer or numeric vector.
获取列表或原子向量 (is.atomic) 的每个元素的长度作为整数或数字向量。
回答by Skippy le Grand Gourou
Advice for Rnewcomers like me?: beware, the following is a list of a single object?:
给R像我这样的新手的建议？：当心，以下是单个对象的列表？：
> mylist <- list (1:10)
> length (mylist)
[1] 1
In such a case you are not looking for the length of the list, but of its first element?:
在这种情况下，您不是在寻找列表的长度，而是在寻找它的第一个元素的长度？：
> length (mylist[[1]])
[1] 10
This is a "true" list?:
这是一个“真实”列表？：
> mylist <- list(1:10, rnorm(25), letters[1:3])
> length (mylist)
[1] 3
Also, it seems that Rconsiders a data.frame as a list?:
另外，似乎将Rdata.frame 视为列表？：
> df <- data.frame (matrix(0, ncol = 30, nrow = 2))
> typeof (df)
[1] "list"
In such a case you may be interested in ncol()and nrow()rather than length()?:
在这种情况下，您可能对ncol()andnrow()而不是length()?感兴趣：
> ncol (df)
[1] 30
> nrow (df)
[1] 2
Though length()will also work (but it's a trick when your data.frame has only one column)?:
虽然length()也可以工作（但是当你的 data.frame 只有一列时这是一个技巧）？：
> length (df)
[1] 30
> length (df[[1]])
[1] 2
回答by anon
I spent ages trying to figure this out but it is simple! You can use length(·). length(mylist)will tell you the number of objects mylistcontains.
我花了很长时间试图弄清楚这一点，但这很简单！您可以使用length(·). length(mylist)会告诉你mylist包含的对象数量。
... and just realised someone had already answered this- sorry!
......刚刚意识到有人已经回答了这个 - 对不起！
回答by Jeff Kraus
Let's create an empty list (not required, but good to know):
让我们创建一个空列表（不是必需的，但很高兴知道）：
> mylist <- vector(mode="list")
Let's put some stuff in it - 3 components/indexes/tags (whatever you want to call it) each with differing amounts of elements:
让我们在其中放一些东西 - 3 个组件/索引/标签（无论你想怎么称呼它），每个组件/索引/标签都有不同数量的元素：
> mylist <- list(record1=c(1:10),record2=c(1:5),record3=c(1:2))
If you are interested in just the number of components in a list use:
如果您只对列表中的组件数量感兴趣，请使用：
> length(mylist)
[1] 3
If you are interested in the length of elements in a specific component of a list use: (both reference the same component here)
如果您对列表的特定组件中元素的长度感兴趣，请使用：（两者都在此处引用相同的组件）
length(mylist[[1]])
[1] 10
length(mylist[["record1"]]
[1] 10
If you are interested in the length of all elements in all components of the list use:
如果您对列表中所有组件中所有元素的长度感兴趣，请使用：
> sum(sapply(mylist,length))
[1] 17
回答by user6062513
You can also use unlist(), which is often useful for handling lists:
您还可以使用unlist()，这通常对处理列表很有用：
> mylist <- list(A = c(1:3), B = c(4:6), C = c(7:9))
> mylist
$A
[1] 1 2 3
$B
[1] 4 5 6
$C
[1] 7 8 9
> unlist(mylist)
A1 A2 A3 B1 B2 B3 C1 C2 C3
1 2 3 4 5 6 7 8 9
> length(unlist(mylist))
[1] 9
unlist() is a simple way of executing other functions on lists as well, such as:
unlist() 也是在列表上执行其他函数的一种简单方法，例如：
> sum(mylist)
Error in sum(mylist) : invalid 'type' (list) of argument
> sum(unlist(mylist))
[1] 45
