方括号[]和双括号[[]]之间的区别是用于访问列表或数据框的元素 [英] The difference between bracket [ ] and double bracket [[ ]] for accessing the elements of a list or dataframe

问题描述

R提供了两种不同的方法来访问列表或data.frame的元素-[]和[[]]运算符.

两者之间有什么区别?在什么情况下我应该使用另一种?

解决方案

R语言定义可轻松回答以下类型的问题:

• http://cran.r-project.org/doc/manuals/R-lang.html#Indexing

R具有三个基本的索引运算符，其语法由以下示例显示

对于向量和矩阵，虽然[[形式与[形式在语义上有细微差别，但很少使用[[形式(例如，它删除了任何name或dimnames属性，并且部分匹配用于字符索引).用单个索引对多维结构进行索引时，x[[i]]或x[i]将返回x的第i个顺序元素.

对于列表，通常使用[[选择任何单个元素，而[返回所选元素的列表.

[[形式仅允许使用整数或字符索引选择单个元素，而[允许按向量进行索引.请注意，尽管对于列表，索引可以是一个向量，并且向量的每个元素依次应用于列表，所选组件，该组件的所选组件，依此类推.结果仍然是单个元素.

R provides two different methods for accessing the elements of a list or data.frame- the [] and [[]] operators.

What is the difference between the two? In what situations should I use one over the other?

解决方案

The R Language Definition is handy for answering these types of questions:

• http://cran.r-project.org/doc/manuals/R-lang.html#Indexing

R has three basic indexing operators, with syntax displayed by the following examples

    x[i]
    x[i, j]
    x[[i]]
    x[[i, j]]
    x$a
    x$"a"

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). When indexing multi-dimensional structures with a single index, x[[i]] or x[i] will return the ith sequential element of x.

For lists, one generally uses [[ to select any single element, whereas [ returns a list of the selected elements.

The [[ form allows only a single element to be selected using integer or character indices, whereas [ allows indexing by vectors. Note though that for a list, the index can be a vector and each element of the vector is applied in turn to the list, the selected component, the selected component of that component, and so on. The result is still a single element.

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